#exon shuffling

[biology-geology-beaches-india]

TSRNOSS, page 209.

[tipsycad147]

Winter Animals: Meet 20+ Merry, Magical Christmas Creatures

posted by : kitty fields

When the Winter Solstice knocks at the door, we hunker down and prepare for the cold days ahead. As does the wildlife. But before we completely go into hibernation, let’s meet some of the Winter Animals, the magical Christmas creatures that share some of our most beloved Yuletide legends and lore. So even when these mystical creatures are hibernating over the Winter, their spirits are still with us…sharing the very essence of the holiday season.

First, What do we consider Winter Animals?

I’ve noticed quite a few sites stick to one “theme” of Winter Animals. I’m here to tell you, I’m not going to do that. What I consider Winter Animals are animals that are associated with the snow, with ice, or with the season Winter itself. In addition, I’m including Winter Animals from Biblical lore, as well as ancient and Medieval pagan lore. Lastly, I will also be including Christmas creatures from modern times.

Wintry Feathered Friends: Birds of the Winter Skies and Icebergs

Many birds migrate for the Winter. Some stick around the North Pole because it’s their job. ALL are adorable and mystical in their own special ways. Let’s meet the Birds of Winter and Christmas.

1. Robin “Red-Breast”: A Traditional Christmas Animal

2. Penguin: Our Icy Wintry Friend

Who doesn’t love the penguin? These feathered friends are completely different than the others for one reason: they don’t fly! But they DO swim and shuffle and glide across the icy plains of Antarctica and the Arctic. Since these Winter birds actually live in the Polar regions on Earth, they are often associated with Winter, Christmas, and all things Yuletide. The penguin is featured in quite a few movies, books, and TV shows with a Winter or holiday feel. Take Topper the Penguin, for example. One of the cutest animals on the classic Santa Claus is Comin’ to Town.

3. The MYSTIC Snowy Owl

4. Cardinal

5. Raven, the Yulefather’s Familiar

6. And a Partridge in a Pear Tree

7. Two Turtle Doves

8. Wren: The Beloved Christmas Bird of Ireland

Wren Day on December 26th is a holiday celebrated in Ireland and on the Isle of Man. It was likely celebrated in Britain and in other countries in Europe in older times. On the Isle of Man, Wren Day is celebrated by the “Hunting of the Wren”, which is more of a parade or procession of dancers and musicians than it is a hunt. This tradition dates back centuries and likely thousands of years to ancient Celtic times. The Wren is much beloved in these countries and associated with Christmas for this reason.

Christmas Creatures from the Snowy Woodlands

While many animals are hibernating during Christmas, we still get small, magical glimpses of wildlife from time to time. And especially in our beloved stories, movies, and plays. Here are some of our favorite Christmas Animals from the woodlands.

9. Stag

I don’t know when the Stag became such a popular animal in legend and lore, but I’m betting it was in ancient times. The Hunting of the White Stag is an ancient ritual in Celtic lore in which whomever caught the white stag would be king and would prosper. White animals, in general, have long been linked to the fairy world. In recent years, the Stag is taking a precedence in Christmas decor…a nod to the older ways of pagan Yuletide and the wild woods.

10. Wolf: Christmas Werewolves and the Devil?

The Wolf is an interesting Christmas animal for a few reasons. A. the white wolf is sometimes featured in Winter and Christmas decor. And B. it’s history is intertwined with Christmas, it’s just that most people wouldn’t realize it. In a sixteenth century Catholic Spanish poem, a lamb is featured and represents the Virgin Mother Mary. While the wolf’s character is a shoe-in for the Devil himself. In addition, old Medieval lore claimed that a child born near or ON Christmas Day was more likely to become a werewolf. My honest opinion? The white wolf represents Winter and therefore should be exonerated of its old reputation and added to MORE Yuletide decor and literature!

11. Arctic Fox: The Furriest, Cutest Christmas Animal There Is!

I can’t help it. I’m partial to the Arctic Fox. In fact, I think he is the cutest of all the Christmas and Winter animals on this list! Also known as the Snow Fox or Polar Fox, this fuzzy little predator occupies some of the coldest places on earth and doesn’t even start shivering until it’s nearly one hundred degrees below fahrenheit! Another cute fact about this Winter animal: during the mating season, they are monogamous and choose one mate with whom to raise their pups. The fox, in general, has been making its way into modern Christmas decor and stories. And we hope it continues!

12. Snowshoe Hare

13. The Christmas Mouse

14. Santa’s Reindeer

I don’t think there’s another animal in modern Christmas history that’s as prominent and beloved as the reindeer. Besides the elves, who helps Santa Claus carry on the spirit of Christmas? Santa’s Reindeer! Rudolph, Comet, Cupid, Donder, Blitzen, Dasher, Dancer, Prancer, and Vixen. Reindeer, also called Caribou, are a species of deer native to the Arctic and subarctic areas of Northern Europe, Siberia, and North America. There is some speculation that Santa’s eight reindeer originate in Odin’s eight-legged horse Sleipnir. However they got their Wintry start, they aren’t going away any time soon.

15. The Yule Boar

Every year on Christmas Eve, my family and I sit down to a delicious Christmas ham. This is a tradition that’s shared among many American and Northern European families. There is a theory that this tradition is carried on from ancient times, when the Norse sacrificed the Yule Boar to the god Freyr during the Winter Nights. From the Sagas:

“And they would sacrifice a boar in the sonarblót. On Yule Eve the sonar-boar was led into the hall before the king; then people laid their hands on its bristles and made vows.”

While this theory makes sense to me, there are scholars who disbelieve it. Either way, we can say the Boar is indeed a Christmas animal because of its ancient association to the pagan Yule festival. So go get you that Honey Baked Ham, y’all!

Down On the Farm Yuletide Animals

Christmas is downright cozy when its down on the farm. And so are these Yuletide farm animals!

16. Horse

We have to give an honorable mention to the Horse. Horses are the leaders of horse-drawn carriages, obviously, and are mentioned frequently in Christmas carols and stories. For example in Jingle Bells, “dashing through the snow, in a one-horse open sleigh”. And who could forget the cheery horse sounds we hear in Sleigh Ride? On the creepier side of Christmas, the Mari Lwyd of Welsh Christmas tradition is disturbing to some. Yet its popularity is growing in modern times. What is it, you ask? The Mari Lwyd is the skull of a horse that’s put on a stick and paraded around town, singing carols and asking for entrance into people’s homes. Upon giving admittance, they would be fed with food and ale.

17. The Yule Goat

18. The Yule Cat

In our recent article Gryla the Christmas Witch, the Yule Lads, and Yule Cat, we are introduced to the terrifying feline of Icelandic lore. Otherwise known as the Jolakotturinn or Yule Cat.��He is not cute. He is not fluffy. And he will gobble up any of those who aren’t wearing new clothes on Christmas OR who have misbehaved during the year! On a softer note, in the U.S. kittens are frequently given as gifts during the holiday season. And in Netflix’s Christmas Chronicles 2, we get an animated glimpse of the Yule Cat at the North Pole.

19. The Shepherds and their Sheep

Because of the biblical reference to shepherds and their sheep, the sheep have been a common part of the nativity scene for centuries. From Luke Chapter 2 of the New Testament:

“The angels left and the shepherds decided to go to Bethlehem. They found Mary and Joseph and saw the baby lying in the manger. The shepherds returned, praising God for all they had heard and seen.”

20. The Christmas Rooster

Yes, believe it or not the rooster is considered a Christmas animal in Spanish and Latin American communities. In Roman Catholic lore, the rooster crowed at midnight to announce Jesus’ birth. For this reason, Roman Catholics in these communities call their midnight mass the “Misa de Gallo” or Rooster Mass.

Other Animals Linked to Christmas Around the World

There are other animals around the world that remind us of the spirit of Christmas. We may not mention every single one on our list but we did try our best. Here are a few more:

21. The Camel

This might seem unorthodox or strange to some, but the camel is considered a Christmas animal because of the story of the birth of Jesus. The wisemen who came to Jesus bearing gifts are often depicted as riding camels.

22. The Yule Dragon

23. Kangaroo

[lifescienceconcepts]

#Molecular Biology Lecture 33 | Alternate Splicing | Exon Shuffling | CSIR NET | GATE | DBT | Important For CSIR, ICAR, ICMR, DBT, and GATE Click on the below link for the complete video 👇 https://youtu.be/mWwjSD1_s1w Like, Share & Subscribe to the channel for more upcoming videos #lifescience_concepts https://www.instagram.com/p/CgdtdB9Px4F/?igshid=NGJjMDIxMWI=

[chromoscience]

The Importance Of Introns

Correspondence between exons and protein domains. Source: Urry, Lisa A.. Campbell Biology (p. 347). Pearson Education. Kindle Edition. Campbell Biology Whether or not RNA splicing and the presence of introns have provided selective advantages during evolutionary history is a matter of some debate. In any case, it is informative to consider their possible adaptive benefits. Specific functions have not been identified for most introns, but at least some contain sequences that regulate gene expression, and many affect gene products. One important consequence of the presence of introns in genes is that a single gene can encode more than one kind of polypeptide. Many genes are known to give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA processing; this is called alternative RNA splicing. Results from the Human Genome Project suggest that alternative RNA splicing is one reason humans can get along with about the same number of genes as a nematode (roundworm). Because of alternative splicing, the number of different protein products an organism produces can be much greater than its number of genes. Proteins often have a modular architecture consisting of discrete structural and functional regions called domains. One domain of an enzyme, for example, might include the active site, while another might allow the enzyme to bind to a cellular membrane. In quite a few cases, different exons code for the different domains of a protein. The presence of introns in a gene may facilitate the evolution of new and potentially beneficial proteins as a result of a process known as exon shuffling. Introns increase the probability of crossing over between the exons of alleles of a gene—simply by providing more terrain for crossovers without interrupting coding sequences. This might result in new combinations of exons and proteins with altered structure and function. We can also imagine the occasional mixing and matching of exons between completely different (nonallelic) genes. Exon shuffling of either sort could lead to new proteins with novel combinations of functions. While most of the shuffling would result in non-beneficial changes, occasionally a beneficial variant might arise. Source: Urry, Lisa A.. Campbell Biology (p. 346). Pearson Education. Kindle Edition. Read the full article

[mythopoeticreality]

Beginings

((written as part of this challenge))

Sunlight. Catherine frowned, staring out the window, her eyes narrowing. Brilliant, blindingly bright afternoon sunlight catching off of the white towers and walls of the courtyard outside, and green turf being crushed beneath people's feet as they darted back and forth calling to one another. She shuffled onto her knees, pulling herself up to the sill. If she reached, just stretched...There! The tips of her fingers just touched the latch enough that she could push it up. She cracked the window open. And there it was! Yes, she could smell warm stone and sweet grass and horse and sheep and all those other daytime scents.

Her frown deepened. It all made sence, of course, she thought as she slid back down to the floor again, turning her back on the window. They had arrived, her family and herself, no more than  a few hours ago, and it had been daylight. She had just slipped away from her father and mother, and it had been daylight. Only moments ago, when she'd been standing outside this room, it had been daylight.

A sharp sigh escaped her, sounding all the louder in the heavy silence of the room. Floorboards creaked beneath her feet as she paced a step back, then turned.  She stared at the ground, crossing her arms, then sighed again. Across all of England, she was sure, every person and every place were in agreement: It was day. All of England that was, save for this singular room.

The patch of light that filtered through the window was pale and cold, silvery in color against the black void of the floor. Catherine knelt down, waving a hand and watching her shadow follow her movements. “Hrmmm...” She sat up, glancing around.

The room itself seemed to be made of nothing but silhouettes and shadows, cut out in silver: walls and floors, the chests and bookcases, the fireplace on the far wall and the silver basin set in a place of honor upon the desk. Catherine shivered, a chill raising bumps across her arms as she pushed herself to her feet. She felt, if she just listened hear enough, that somewhere on the edge of her hearing she would hear the chirping of crickets.

It wasn't crickets that reached her ears and pulled her from her thoughts, however, but voices. Footsteps, drifting down the corridor, coming this way. Oh...oh, no. Catherine froze, feeling her throat draw. She was not supposed to be here, she was not supposed to be here at all... She stumbled back, eyes darting across the room for somewhere to hide.

“....And really, John, I do have to wonder to what purpose you intend --” Light flooded into the room, framing the two men who stood there. They froze, the one who was speaking fell silent, turned and blinked at Catherine.Catherine blinked back at him.

A moment passed where the man merely sighed and shook his head. Though built like a knight, there was nothing he reminded Catherine so much of as one of her tutors. He was holding a candlestick, and he ran his free hand up through his mousey brown hair. From the look of things he did that often. Turning back to his companion, his brows climbed up his forehead as if somehow blaming the other man for her presence.

The other man – oh, God above! – The Raven King, for who else could he be? Willow-reed slender and pale and tall, all dark hair and sharply carved featues and handsomeness. He looked just like the statues on the city steets, He looked just like the stories said, down to the boots of soft black leather.

The Raven King's gaze scanned over Catherine for a moment, in a way that made her heart thud, and felt as if he were looking through her, rather than at her, into her mind and into her soul. Then he shrugged, waving his companion – servant? – after him.

Catherine backed out of his way, her eyes never leaving him.

“Place it on the desk, William.” The Raven King murmured, sweeping past her, towards a silver-framed mirror hanging on the wall. His lips pressed together and his eyes narrowed  as he ran his hand along the edge of the glass.

Then he turned, glancing over his shoulder at Catherine. Another long pause as her looked at her in that way of his again. He turned more fully to face her, leaning back against the wall, his arms crossed over his chest.

“Where did you come from, then?”

Catherine felt her face color, despite herself, and found herself greatful it was so dark. Shuffling from foot to foot, she glanced for a moment over back towards the door. “Ummm...Winchester?” She ventured, turning her eyes back up towards the King.

“Ah, she's a bold one.” The servant – William – said, standing in the candlelight. Catherine glanced back towards him, and he bowed his head towards her.

Even the corner of the Raven King's lips twitched upwards. “You came a rather long way.” He drawled, “How did you get in here?”

“The door was opened.” She said, pausing a moment, before adding, “It's night in here.” as though that should somehow exoneate her.

That was met with another shrug. Another silence as the Raven King eyed her for a bit longer, and then, with a whimsical suddenness, he turned away, pacing back towards the desk and William.  “Tell me, Catherine of Winchester.” He said, “What is it that brings you all the way to Newcastle?”

Under the combined gazes of both men, she felt the uge to shift from foot to foot again. No. She was better than that. She was twelve years old and soon to be apprenticed to a magician herself. She was not afraid, and if she was, she would not show it. Taking in a deep breath she streghtened her back and steadied herself. “My father saw I had magical tallent. He wants to apprentice me to a magician he knows here...a, a member of your court, Your Majesty.”

That did not quite seem to settle them. The Raven King's brows shot up, as though prompting her on, and William was muttering something that sounded like, “And there are no Magicians in the south?”  

“No...no one will take me at home, sir. Not after my last tutor, he...” Catherine pressed her lips together, and absently reached back and tugging at her hair, “He was moving too slowly for my tastes and I was getting on in my studies well enough without him you see...he was more of a distraction than anything...umm....” She sighed, “He became lost. In a labyrinth I'd made of my father's house.” a pause before she quickly added, “He did find his way out. Eventually...”

The two men shared a look.

“Did you, now?”

Catherine nodded.

“John...” There was a warning in William's tone, but The Raven King merely reached back, pulling out a chair from beneath the desk and dropping himself down onto it.

There was a glint in The King's eye as he turned back towards his servant. Something in his look that Catherine recognized from the way she acted towards her tutors. Like he was daring the other man to go on.

When Wiliam said nothing more (beyond an exasperated sigh and a roll of his eyes) The Raven King turned back to her.

“Show me.”

Catherine's heart stopped for an instant.“Pardon?”

“Let me see what you can do, Catherine of Winchester.”

And what else was she to do? The Magician King of Northern England was standing before her, asking her to preform some magic for him. As her eyes scanned the room she felt almost as though she were watching herself from a distance. Her skin was buzzing and she felt lightheaded. Finally her eyes landed back upon the Mirror.

She sliped infront of it, waving for the King's attention (Though in truth, she had no need to, she could feel his gaze intent and focused, upon her even as her back was turned.). Standing there, Catherine took in a deep breath and closed her eyes, pressing the tips of her fingers against the glass. The moments passed. And then something shifted, the air changed, as it has when she'd opened the window letting in the sweet air from outside.

Catherine opened her eyes, and grinning she stepped back, gesturing towards the mirror.

Her refection turned, and curtsied to the King.

The Raven King nodded, his expression unreadable. He stood then, and it was William who spoke next, clearing his throat.

“My lord,” He said, “We should probably find the girl's parents, yes?”

“Hrmm?” the Raven King glanced back,  gave a vauge nod, and gestured for Catherine to follow.

In a daze, unsure, exactly of what just was happening, Catherine did all she could do and followed along.

“Who was the Magician you were to be apprenticed to?” It was William who asked this as he walked along beside the other two. Catherine glanced up at him, her brows drawing together.

“Thomas of Manchester.” I said. “Why?”

“Because,” William said, casting a glance back over to his King, “I have a feeling he is about to learn very soon that his apprentice has been stolen away from him.”

[eurekamag--com]

Exon shuffling generates an immunoglobulin heavy chain gene

http://dlvr.it/Pcjl6C

[emmagreen1220-blog]

New Post has been published on Biology Dictionary

New Post has been published on https://biologydictionary.net/intron/

Intron

Intron Definition

An intron is a long stretch of noncoding DNA found between exons (or coding regions) in a gene. Genes that contain introns are known as discontinuous or split genes as the coding regions are not continuous. Introns are found only in eukaryotic organisms.

Here we see the structure of a pre-mRNA (or hrRNA) and a mature mRNA following mRNA processing (splicing, the addition of a 5′-cap and a poly-A tail).

Intron Discovery

Introns were discovered in 1977 with the introduction of DNA sequencing. While it was known that mature eukaryotic mRNA molecules were shorter than the initial transcripts, it was believed that the transcripts were simply trimmed at the ends. When the two molecule types were sequenced it was revealed that this was not the case; much of the removed transcript came from internal regions rather than the extreme ends. This prompted extensive research into how introns were removed from transcripts, and what their role might be.

Intron Structure

In general, introns are much longer than exons; they can make up as much as 90% of a gene and can be over 10,000 nucleotides long. Introns are prevalent in genes; over 90% of human genes contain introns with an average of nine introns per gene.

An intron is a stretch of DNA that begins and ends with a specific series of nucleotides. These sequences act as the boundary between introns and exons and are known as splice sites. The recognition of the boundary between coding and non-coding DNA is crucial for the creation of functioning genes. In humans and most other vertebrates introns begin with 5′ GUA and end in CAG 3′. There are other conserved sequences found in introns of both vertebrates and invertebrates including a branch point involved in lariat (loop) formation.

Here we see a consensus sequence for a vertebrate intron. The intron begins with GUR and ends in a polypyrimidine tract followed by YAG.

Intron Function

While introns were initially – and to an extent still are – considered “junk DNA”, it has been shown that introns likely play an important role in regulation and gene expression. As introns cause an increase in gene length, this increases the likelihood of crossing over and recombination between sister chromosomes. This increases genetic variation and can result in new gene variants through duplications, deletions, and exon shuffling. Introns also allow for alternative splicing. This allows a single gene to encode multiple proteins as the exons can be assembled in multiple ways.

Splicing

During transcription RNA polymerase copies the entire gene, both introns and exons, into the initial mRNA transcript known as pre-mRNA or heterogeneous nuclear RNA (hrRNA). As introns are not transcribed, they must then be removed before translation can occur. The excision of introns and the connection of exons into a mature mRNA molecule occurs in the nucleus and is known as splicing.

Introns contain a number of sequences that are involved in splicing including spliceosome recognition sites. These sites allow the spliceosome to recognise the boundary between the introns and exons. The sites themselves are recognised by small nucleolar ribonucleoproteins (snRNPs). There are a number of snRNPs involved in mRNA splicing which combined create a spliceosome.

Splicing occurs in three steps:

Cleavage of the phosphodiester bond between the exon and the GU at the 5′ end of the intron. One snRNP (U1) contains a complementary sequence to the 5′ splice site and binds there to initiate splicing.

Formation of a lariat or loop structure. The free 5′ end of the intron connects to a branch site, a conserved sequence near the 3′ end of the intron. A second snRNP (U2) binds to the branch site and attracts U1 to initiate the lariat. The lariat is then formed by a phosphodiester bond between the free 5′ G and an A at the branch site.

Cleavage of the phosphodiester bond between the second exon and the 3′ AG of the intron.

This figure shows the splicing of an intron through formation of a lariat. The intron is then removed leaving the two exons connected.

It is unknown how the snRNPs and the spliceosome identify which recognition sites to bind to given the that the introns can be thousands of base pairs long and there are many cryptic splice sites where the recognition sequences are found elsewhere in the gene. It is believed that certain proteins (for example, SR proteins), enhancers, and silencers are involved. Splicing silencers have also been implicated in human diseases.

Alternative splicing

Introns and the splicing mechanism also allow for alternative gene products in a process known as alternative splicing. Each discontinuous gene is made up of two or more exons, allowing for multiple ways in which the exons can be assembled. Alternative splicing can result in two to hundreds of different mRNAs. Alternative splicing is common in some species but rare in others; it is found in over 80% of human genes but there are only three known cases in Saccharomyces cerevisiae (yeast).

Alternative splicing can occur in a number of ways:

Exon skipping: one (or more) exons are not included in the final mRNA

Intron retention: part of the intron is not properly spliced and remains in the final mRNA

Alternative splice site: the spliceosome removes part of one (or more) exon as well as the intron

Different alternative splicing mechanisms

rRNAs and tRNAs

Introns can also be found in both pre-rRNAs and pre-tRNAs. Introns in rRNAs are rare, with examples so far found only in lower eukaryotes. Unlike introns in other molecules, some rRNA introns have a unique characteristic – they are self-splicing. Self-splicing introns fall into a category known as Group I introns. Rather than relying on an external enzyme to perform the excision the introns themselves act as an enzyme known as a ribozyme. Ribozymes were discovered in the ciliate Tetrahymena in 1982 and revolutionized the way scientists viewed enzymes.

Introns in tRNAs are more common than those in rRNAs but much less prevalent than in mRNAs, particularly in vertebrates (i.e., 6% of human tRNAs). Introns in tRNAs are relatively short, ranging from 14 to 60 base pairs in length. The introns form part of the stem and loop structure of the tRNA, binding to a section of the anticodon arm. Removal of pre-tRNA introns is done by a single endonuclease.

Quiz

1. Which organisms do not have introns? A. bacteria B. fungi C. protozoa D. plants

Answer to Question #1

A is correct. Introns are only found in eukaryotic organisms. While the prevalence of introns varies between taxa, they can be found in all eukaryotic phyla.

2. Where does splicing occur? A. cytosol B. ribosomes C. nucleus D. chloroplasts

Answer to Question #2

C is correct. Splicing, or removal of the introns from a pre-mRNA, occurs in the nucleus. Splicing is a component of mRNA processing along with addition of a 5′ cap and 3′ poly-A tail. Once processed the mature mRNA is transported out of the nucleus for translation.

3. What molecules contain introns? A. pre-mRNA B. pre-rRNA C. pre-tRNA D. all of the above

Answer to Question #3

D is correct. While introns are rare in pre-rRNA, and uncommon in tRNA, they can be found in both these and pre-mRNA molecules. In many organisms introns are common in pre-mRNA, being found in over 90% of human genes and in a similar proportion of other vertebrate genomes.

References

Brown, T. (2012).Introduction to genetics: a molecular approach Chs. 3, 5, and 6. New York, NY: Garland Science, Taylor & Francis Group, LLC. ISBN: 978-0-8153-6509-9.

Wong, G. K.-S., Passey, D. A., Huang, Y.-Z., Yang, Z., & Yu, J. (2000). “Is “junk” DNA mostly intron DNA?”. Genome Research 10: 1672-1678.

Wong, J. J.-L., Au, A. Y. M., Ritchie, W., & Rasko, J. E. J. (2015). Intron retention in mRNA: no longer nonsense. Bioessays 38: 41-49.

[naivelocus]

Novelty by Furcation and Fusion: How tree-like is evolution? [CONFIRMATORY RESULTS]

Novelty and innovation are fundamental yet relatively understudied concepts in evolution. We may study novelty phylogenetically, with a key question of whether evolution occurs by tree-like branching, or through exchange of distantly related parts in processes akin to horizontal transfer. Here, I argue that except at the lowest levels of biological organization, evolution is not usually tree-like. Perfectly vertical inheritance, an assumption of evolutionary trees, requires simultaneous co-duplication of all the parts of a duplicating or speciating (which I collectively call furcating) biological feature. However, simultaneous co-duplication of many parts usually requires variational processes that are rare. Therefore, instead of being perfectly tree-like, evolution often involves events that incorporate or fuse more distantly related parts into new units during evolution, which herein I call fusion. Exon shuffling, horizontal gene transfer, introgression and co-option are such fusion processes at different levels of organization. In addition to co-duplication, units under phylogenetic study must individuate (gaining evolutionary independence) before they can diverge. A lack of individuation erases evolutionary history, and provides another challenge to tree-like evolution. In particular, biological units in the same organism that are the products of development always share the same genome, perhaps making full individuation difficult. The ubiquity of processes that fuse distantly related parts or oppose individuation has wide ranging implications for the study of macroevolution. For one, the central metaphor of a tree of life will often be violated, to the point where we may need a different metaphor, such as economic public goods, or a "web of life". Secondly, we may need to expand current models. For example, even under the prevailing model of cell-type evolution, the sister-cell-type model, a lack of complete individuation and evolution by co-option will often be involved in forming new cell-types. Finally, these processes highlight a need for an expansive toolkit for studying evolutionary history. Multivariate methods are particularly critical to discover co-variation, the hallmark of an absence of complete individuation. In addition to studying phylogenetic trees, we may often need to analyze and visualize phylogenetic networks. Even though furcation - the splitting and individuation of biological features - does happen, fusion of distant events is just as critical for the evolution of novelties, and must formally be incorporated into the metaphors, models, and visualization of evolutionary history. — bioRxiv Subject Collection: Evolutionary Biology

[chromoscience]

The Importance Of Introns

Correspondence between exons and protein domains. Source: Urry, Lisa A.. Campbell Biology (p. 347). Pearson Education. Kindle Edition. Campbell Biology Whether or not RNA splicing and the presence of introns have provided selective advantages during evolutionary history is a matter of some debate. In any case, it is informative to consider their possible adaptive benefits. Specific functions have not been identified for most introns, but at least some contain sequences that regulate gene expression, and many affect gene products. One important consequence of the presence of introns in genes is that a single gene can encode more than one kind of polypeptide. Many genes are known to give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA processing; this is called alternative RNA splicing. Results from the Human Genome Project suggest that alternative RNA splicing is one reason humans can get along with about the same number of genes as a nematode (roundworm). Because of alternative splicing, the number of different protein products an organism produces can be much greater than its number of genes. Proteins often have a modular architecture consisting of discrete structural and functional regions called domains. One domain of an enzyme, for example, might include the active site, while another might allow the enzyme to bind to a cellular membrane. In quite a few cases, different exons code for the different domains of a protein. The presence of introns in a gene may facilitate the evolution of new and potentially beneficial proteins as a result of a process known as exon shuffling. Introns increase the probability of crossing over between the exons of alleles of a gene—simply by providing more terrain for crossovers without interrupting coding sequences. This might result in new combinations of exons and proteins with altered structure and function. We can also imagine the occasional mixing and matching of exons between completely different (nonallelic) genes. Exon shuffling of either sort could lead to new proteins with novel combinations of functions. While most of the shuffling would result in non-beneficial changes, occasionally a beneficial variant might arise. Source: Read the full article

[emmagreen1220-blog]

New Post has been published on Biology Dictionary

New Post has been published on https://biologydictionary.net/exon/

Exon

Exon Definition

An exon is a coding region of a gene that contains the information required to encode a protein. In eukaryotes, genes are made up of coding exons interspersed with non-coding introns. These introns are then removed to make a functioning messenger RNA (mRNA) that can be translated into a protein.

Exon Structure

Exons are made up of stretches of DNA that will ultimately be translated into amino acids and proteins. In the DNA of eukaryotic organisms, exons can be together in a continuous gene or separated by introns in a discontinuous gene. When the gene is transcribed into pre-mRNA the transcript contains both introns and exons. The pre-mRNA is then processed and the introns are spliced out of the molecule. Mature mRNAs can be a few hundred to several thousand nucleotides long.

The mature mRNA consists of exons and short untranslated regions (UTRs) on either end. The exons make up the final reading frame which consists of nucleotides arranged in triplets. The reading frame begins with a start codon (usually AUG) and ends in a termination codon. The nucleotides are arranged in triplets as each amino acid is coded for by a three-nucleotide sequence.

The figure depicts a gene made up of three exons. The resulting gene is 1317 bp in length despite an initial gene length of over 13,000 bp.

Exon Function

Exons are pieces of coding DNA that encode proteins. Different exons code for different domains of a protein. The domains may be encoded by a single exon or multiple exons spliced together. The presence of exons and introns allows for greater molecular evolution through the process of exon shuffling. Exon shuffling occurs when exons on sister chromosomes are exchanged during recombination. This allows for the formation of new genes.

Exons also allow for multiple proteins to be translated from the same gene through alternative splicing. This process allows the exons to be arranged in different combinations when the introns are removed. The different configurations can include the complete removal of an exon, the inclusion of part of an exon, or the inclusion of part of an intron. Alternative splicing can occur in the same location to produce different variants of a gene with a similar role, such as the human slo gene, or it can occur in different cell or tissue types, such as the mouse alpha-amylase gene. Alternative splicing, and defects in alternative splicing, can result in a number of diseases including alcoholism and cancer.

The figure depicts possible alternative splicing mechanisms which can result in alternative proteins being translated.

Human slo gene

An example of extreme alternative splicing is the human slo gene which encodes a transmembrane protein involved in regulation of potassium entry in the hair cells of the inner ear, resulting in frequency perception. The gene consists of 35 exons which can combine to form over 500 mRNAs through the excision of one to eight exons. The different mRNAs control which sound frequencies can be heard.

Mouse alpha-amylase gene

The mouse alpha-amylase gene encodes two different mRNAs – one in the salivary glands and one in the liver. Which of the mRNA transcripts is formed is controlled by different promoters specific to the tissue type. In this case the processed mRNA contains the same two exons, resulting in the same protein, but it is regulated by a tissue-specific promoter.

Quiz

1. A protein is coded for by how many exons? A. 1 B. 2 C. 10 D. All of the above

Answer to Question #1

D is correct. Proteins are coded for by exons. A simple protein may be coded for by a single exon, while complex proteins may be coded for by tens of exons.

2. How can new genes be formed? A. Alternative splicing B. Exon shuffling C. Splicing D. None of the above

Answer to Question #2

B is correct. Alternative gene variants can be formed by exon shuffling. This occurs when two sister chromatids exchange one or more exons resulting in a new gene form. Alternative splicing is the formation of multiple proteins from the same gene.

3. What sequence is commonly found at the beginning of an exon? A. AUG B. UAG C. UAA D. UGA

Answer to Question #3

A is correct. Exons begin with start codons. The vertebrate start codon is AUG, while UAG, UAA, and UGA are all termination codes. The genetic codes vary slightly among groups.

References

Brown, T. (2012). Introduction to genetics: a molecular approach Ch. 5. New York, NY: Garland Science, Taylor & Francis Group, LLC. ISBN: 978-0-8153-6509-9.

Chen, J., Crutchley, J., Zhang, D. D., Owzar, K. & Kastan, M. B. (2017). Identification of a DNA damage-induced alternative splicing pathway that regulates p53 and cellular senescence markers. Cancer discovery 7: 766-781.

Klug, W. S., & Cummings, M. R. (1994). Concepts of genetics, 4th ed. Ch. 17. Englewood Cliffs, NJ: Prentice Hall, Inc. ISBN: 0-02-364801-5.

Warden, A. S., & Mayfield, R. D. (2017). Gene expression profiling in the human alcoholic brain. Neuropharmacology 122: 161-174.