Name: how genome size shapes new genes
Uploaded: 2020-11-23T11:47:33.000Z
Duration: 3 min 21 s
Channel: JoVE

K12 Standards

north-carolina-k-12-science-standard-course-of-study

NC K-12 Science Standard Course of Study

9-12

Biology

Biological Evolution- Unity and Diversity

LS.Bio.9.2

genome-size-and-the-evolution-of-new-genes

Genome Size and the Evolution of New Genes

Malgré la nature simple du code génétique, il y a une variation considérable en termes de taille génomique, entre le génome le plus petit connu, soit la protéobactérie Candidatus Carsonella ruddii qui a moins de 160 000 paires de base, au plus grand connu soit la plante à fleurs japonaise, le Paris japonica qui en a environ 150 milliards. Malgré ces extrêmes, en général les bactéries et archées ont environ 3, 000 gènes dans leurs génomes. Car les procaryotes n'ont presque aucunes séquences non-codantes, ce qui signifie que leurs génomes peuvent être relativement petits par rapport à ceux des eucaryotes.Un génome plus petit signifie également moins de réplicats à chaque tour de division cellulaire, ce qui fait sens logique pour une reproduction rapide. Typiquement, les eucaryotes ont aux environs de 20 000 gènes mais leurs génomes sont ponctués par de longues étendues de séquences non-codantes, ce qui signifie que la taille de leurs génomes n'est pas nécessairement liée à la complexité. Le génome du Paris japonica peut faire plus de 50 fois la taille du génome humain, mais cela est dû au moins en partie, à de grandes quantités de séquences non-codantes et probablement des niveaux élevés de duplication, et pas nécessairement plus de gènes nouveaux.Alors, comment les organismes peuvent-ils développer de nouveaux gènes Typiquement la réponse est en modifiant la séquence qui existe déjà. L'une des principales ressources pour l'apparition de nouveaux gènes est la duplication génétique. Imaginez une section d'ADN contenant un gène qui est accidentellement dupliqué.Maintenant, l'organisme a une deuxième copie d'un gène existant. Ces nouvelles copies de gènes sont exemptes des contraintes imposées à l'original pour maintenir la fonction de base. Et ainsi, elles peuvent diverger, potentiellement en développant un nouveau rôle ou en modifiant la fonction d'origine.Un autre moyen de créer de nouveaux gènes est la recombinaison aléatoire d'ADN où les segments d'un gène ou d'une copie de gène existant sont séparés et se déplacent pour rejoindre ceux d'un gène différent, créant ainsi un gène hybride qui peut assurer une nouvelle fonction. La mutation intragénique, changements dans une séquence génique introduits par des mutations au fil du temps, explique de nombreux nouveaux gènes. Cette divergence est la plus notable quand l'on compare des espèces ou lignées qui divergent indépendamment.Une fois que cette divergence va au-delà d'un certain point ou qu'un gène prend une nouvelle fonction. Ils peuvent être classés comme des gènes entièrement différents. Enfin, le transfert horizontal de gènes apporte de nouveaux gènes et de nouvelles séquences au génome depuis des sources externes, comme d'autres individus et même d'autres espèces.Ce type d'acquisition de nouveaux gènes est plus courant chez les procaryotes et les archées avec en exemple bien connu le transfert des gènes de résistance aux antibiotiques. Bien que rare chez les eucaryotes, il est toujours considéré comme une source essentielle de nouveauté génétique. Et le matériel génétique peut même venir d'espèces éloignées telles que les bactéries et les champignons, dans cet exemple.

taille du génome et évolution de nouveaux gènes

Bien que chaque organisme vivant possède un génome quelconque (qu’il s’agisse d’ARN ou d’ADN), il existe des variations considérables dans la taille de ces modèles. Un facteur majeur qui influe sur la taille du génome est de savoir si l’organisme est procaryote ou eucaryote. Chez les procaryotes, le génome contient peu ou pas de séquences non codantes, de sorte que les gènes sont étroitement rassemblés en groupes ou encore en opérons, de manière séquentielle le long du chromosome. À l’inverse, chez les eucaryotes, les gènes sont ponctués de longues séquences non codantes. Dans l’ensemble, cela contribue au phénomène selon lequel les génomes des procaryotes ont tendance à être plus petits (c’est-à-dire qu’il contient moins de bases) en moyenne que ceux des eucaryotes.

Sans surprise, compte tenu de cette observation, les plus petits génomes connus sont pour la plupart des procaryotes. Candidatus Carsonella ruddii, par exemple, est une protéobactérie hautement simplifiée dont le génome ne compte que 160 000 paires de bases. Ayant perdu de nombreux gènes nécessaires à la synthèse des protéines essentielles à la vie, il a évolué pour devenir un symbiote intracellulaire obligatoire. À l’opposé du spectre, la plante à fleurs eucaryote japonaise Paris japonica est l’un des plus grands génomes connus, avec environ 150 milliards de paires de bases. Bien que le nombre de gènes codés ne soit pas connu, le génome présente de grandes quantités de duplication et de séquences non codantes.

Dans le génome d’un procaryote moyen, il existe environ 3 000 gènes. L’eucaryote moyen en compte environ 20 000. Mais la taille du génome, en particulier chez les eucaryotes, est extrêmement variable – en grande partie à cause de la quantité de séquences non codantes.

La création de nouveaux gènes

Afin de faire évoluer de nouveaux gènes, les organismes disposent de quelques options principales. La plupart d’entre eux ont un point commun : ils modifient des séquences déjà existantes.

La duplication joue un rôle important dans la création de nouveaux gènes, et plusieurs types de duplication peuvent aboutir à ces nouvelles séquences. Lors de la duplication d'un gène, une section d'ADN contenant un gène est dupliquée. Cette seconde copie ne subit pas la pression de sélection qui contraint la première et peut donc diverger. Avec le temps, cela peut conduire à l’évolution de nouveaux gènes, dotés de nouveaux rôles.

Un autre type de duplication – le brassage d’ADN – peut entraîner la duplication d’une seule section d’un gène et sa jonction avec un autre gène. Cela peut aboutir à la création de nouveaux gènes, avec de nouveaux produits.

Parfois, de nouveaux gènes évoluent simplement à partir de mutations accumulées au fil du temps. Ceci est connu sous le nom de mutation intragénique et est particulièrement visible lors de comparaisons entre espèces ou populations divergentes.

Enfin, il est également possible d’obtenir de nouveaux gènes à partir de sources externes, selon un processus appelé transfert horizontal de gènes. Cela signifie que le matériel génétique peut être incorporé à partir d’autres individus, parfois de la même espèce, mais aussi potentiellement d’une autre espèce. Il s’agit d’une source fréquente de nouveaux gènes chez les procaryotes et les archées. C'est moins courant chez les eucaryotes, mais il a été démontré que cela se produit, et les eucaryotes peuvent même capter des informations génétiques provenant de sources aussi éloignées que des bactéries ou des champignons.

Taille du génome et évolution de nouveaux gènes

Bien que chaque organisme vivant ait un génome d’une sorte ou d’une autre (qu’il s’agisse d’ARN ou d’ADN), la taille de ces plans varie considérablement. Un ...

Education

Enseignement

JoVE Core

Molecular Biology

Biologie moléculaire

Unit 1

Unité 1

DNA, Cells, and Evolution

L'ADN, les cellules et l'évolution

KEY TERMS AND CONCEPTS

TERMES ET CONCEPTS CLÉS

how genome size shapes new genes

Genome size can vary widely, and it also helps show how new genes form. Every living organism has a genome, whether it uses RNA or DNA. Yet the size of that genome can be very different from one organism to another.

One major difference is whether the organism is prokaryotic or eukaryotic. Prokaryotic genomes usually have little to no non-coding sequence, which means genes are packed closely together in operons along the chromosome. Eukaryotic genomes contain long stretches of non-coding DNA between genes. This helps explain why prokaryotic genomes are often smaller than eukaryotic genomes.

The smallest known genomes are mostly prokaryotic. One example is Candidatus Carsonella rudii, a highly simplified proteobacterium with a genome of only 160 thousand base-pairs. It has lost many genes needed to make life-essential proteins, so it now lives as an obligate intracellular symbiont. On the other end of the range, the Japanese flowering plant Paris japonica has one of the largest known genomes, at about 150 billion base-pairs. Its genome contains extensive duplication and non-coding sequence, and the number of genes it encodes is not known.

Gene counts also differ between these groups. An average prokaryote has roughly 3,000 genes. An average eukaryote has around 20,000 genes. Even so, genome size in eukaryotes can vary a great deal because of how much non-coding DNA they contain.

New genes usually arise by changing sequences that already exist. Gene duplication copies a DNA segment that contains a gene. The extra copy is not under the same selection pressure as the original, so it can change over time and take on a new role. DNA shuffling can duplicate only part of a gene and join it to another gene, which can create a new gene product.

New genes can also appear through intragenic mutation, which means mutations build up within a gene over time. This is often seen when comparing different species or populations that have diverged. Another source is horizontal gene transfer, where genetic material comes from another individual or even another species. This is a common way for prokaryotes and archaea to gain novel genes, although it can also happen in eukaryotes, including gene transfers from bacteria or fungi.

How Genome Size Shapes New Genes

the dna helix

DNA Structure and Base Pairing

DNA has a helical structure that helps store genetic information. The double helix is the shape formed by two strands that twist around each other. This structure is a key feature of DNA in cells. The two strands are held together by base pairing. Bases on one strand match with bases on the other strand in a regular way. This pairing helps keep the DNA molecule stable and organized. The helix shape also helps explain how DNA fits into chromosomes and carries instructions for living things. By...

the central dogma

DNA to Protein: Central Dogma Steps

The central dogma explains how genetic information moves from DNA to RNA to protein. This flow is a key idea in molecular biology. It helps show how cells use genes to make the molecules that carry out work. DNA stores the instructions for making proteins. During transcription, a gene in DNA is copied into RNA. The RNA then carries that message to the next stage. In translation, the RNA message is used to build a protein. Proteins do much of the cell’s work, so this process connects genes to...

prokaryotic cells

Inside Prokaryotic Cell Structure

Prokaryotic cells have a simple internal structure compared with many other cells. They are usually divided into two groups: bacteria and archaea. These cells have no nucleus, so their DNA is not enclosed in a membrane-bound compartment. Instead, the genetic material is found in the nucleoid, a region where the DNA is located in the cell. Some prokaryotic cells also contain plasmids, which are small extra DNA molecules. A capsule may surround the cell and provide an additional outer layer. <br...

eukaryotic compartmentalization

Cell Compartments in Eukaryotes

Eukaryotic cells use compartments to organize many different jobs inside one cell. These compartments help separate processes so they can happen in the right place. The compartment called the cytosol is the fluid part of the cell. It surrounds the other internal structures and helps support many cell processes. Inside the cell, membrane-bound organelles create distinct microenvironments. A microenvironment is a small internal space with conditions that fit a specific task. This separation...

the tree of life - bacteria, archaea, eukaryotes

the-tree-of-life-bacteria-archaea-eukaryotes

How Life Is Classified Into Three Domains

The tree of life shows how organisms are related through evolution. Its root represents the common ancestor of all life on Earth. From that point, branches spread out like a tree. The tips of the branches represent living, or extant, species. Species that no longer exist, called extinct species, appear closer to the center of the tree. Today, these organisms are grouped into three domains: bacteria, archaea, and eukaryotes. Scientists still debate these divisions, so the tree of life keeps...

mutations

How Mutations Change DNA

Mutations are changes in DNA. They can alter the genetic code and affect how genes work. In high school biology, students often study mutations to understand how DNA changes can influence traits. The transcript focuses on two common mutation types: point mutations and frameshift mutations. A point mutation changes one nucleotide, which is one DNA building block. A frameshift mutation happens when nucleotides are added or removed and the reading frame shifts. These changes matter because they...

gene evolution - fast or slow?

Why Some Gene Regions Evolve Faster

Gene regions do not all change at the same rate. In eukaryotes, long stretches of DNA do not code for proteins or RNAs. Some of these non-coding regions include important regulatory sequences, but most have no known function. Because little or no selection pressure acts on them, they often change the fastest over evolutionary time. Protein-coding regions usually evolve more slowly. A change in a coding sequence can alter a protein and make it less effective at doing its job. That creates...

genome size and the evolution of new genes

Genome size can vary widely, and it also helps show how new genes form. Every living organism has a genome, whether it uses RNA or DNA. Yet the size of that genome can be very different from one organism to another. One major difference is whether the organism is prokaryotic or eukaryotic. Prokaryotic genomes usually have little to no non-coding sequence, which means genes are packed closely together in operons along the chromosome. Eukaryotic genomes contain long stretches of non-coding DNA...

gene families

How Gene Families Evolve and Diversify

Gene families are groups of genes that are thought to come from a shared ancestor. They usually form when a gene is copied by mistake during cell division. The original gene stays under selection pressure to keep its job, while the extra copy can change more quickly over time. Some copied genes gain a new role in the organism. When that happens, they are called paralogs, which are two genes in the same species that evolved from one common ancestral gene. Gene families also include orthologs,...

types of genetic transfer between organisms

types-of-genetic-transfer-between-organisms

How Genes Move Within and Across Generations

Genetic transfer is the movement of genetic information from one organism to another. It happens in two main ways: vertical gene transfer and horizontal gene transfer. Vertical gene transfer moves genes from one generation to the next. It is much more common than horizontal gene transfer. Vertical gene transfer happens during both sexual reproduction and asexual reproduction. In both cases, one or more organisms pass some or all of their genome to their offspring. This kind of transfer occurs...

key terms and concepts

Compare prokaryotic and eukaryotic genome sizes, then see how gene duplication, DNA shuffling, mutation, and horizontal gene transfer create new genes.

How Genome Size Shapes New Genes | JoVE K12

molecular biology

JoVE Core Biologie Moléculaire illustre les processus moléculaires au sein de la cellule grâce à des leçons vidéos claires et concises. Les applications de ces concepts sont ensuite démontrées grâce à des expériences concrètes en laboratoire.

new-jersey-student-learning-standards-for-science

New Jersey Student Learning Standards for Science

High School

Life Science

heredity-inheritance-and-variation-of-traits

Heredity: Inheritance and Variation of Traits

HS-LS3-1

Florida Department of Education

SC.912.L.15

SC.912.L.15.2

Development of Antibiotic Resistance

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Understanding Species and Reproductive Barriers

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Speciation Rates

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Genetics of Speciation

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Hybrid Zones

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What is Natural Selection?

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Types of Selection

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Frequency-dependent Selection

When the fitness of a trait is influenced by how common it is (i.e., its frequency) relative to different traits within a population, this is referred to ...

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Natural Selection

Fitness
Widespread variation of phenotypes in natural populations provides the raw material for evolution, which is the change in the inherited traits of ...

Simulating Natural Selection
ExpandTo prepare for the lab, set out workstations for each student pair containing 6 white, 6 black, and 13 gray 12 inch ...

Simulating Natural Selection
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Artificial Selection

Natural Selection and Adaptive Evolution
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Observing Artificial Selection in Plants
ExpandTwo weeks before beginning the pollination experiments, collect a sufficient amount of small pots with a ...

Observing Artificial Selection in Plants 
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Analyze and interpret data to summarize how various factors such as geographic isolation, pesticide resistance, antibiotic resistance can influence natural selection.

LS.Bio.9.1

Phylogenetic Trees

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Around 4 billion years ago, oceans began to condense on earth while volcanic eruptions released nitrogen, carbon dioxide, methane, ammonia, and hydrogen ...

The Colonization of Land

Changes in the environment of the early Earth drove the evolution of organisms. As prokaryotic organisms in the oceans began to photosynthesize, they ...

What is Evolutionary History?

Scientists record evolutionary history by analyzing fossil, morphological, and genetic data. The fossil record documents the history of life on Earth and ...

The Evidence for Evolution

Genetic variations accumulating within populations over generations give rise to biological evolution. Evolutionary changes can result in the formation of ...

The Fossil Record

The fossil record documents only a small fraction of all organisms that have ever inhabited Earth. Fossilization is a rare process, and most organisms ...

Convergent Evolution

Evolution shapes the features of organisms over time, ensuring that they are suited for the environments in which they live. Sometimes, selection pressure ...

Gene Evolution - Fast or Slow?

The genomes of eukaryotes are punctuated by long stretches of sequence which do not code for proteins or RNAs. Although some of these regions do contain ...

While every living organism has a genome of some kind (be it RNA, or DNA), there is considerable variation in the sizes of these blueprints. One major ...

Gene Families

Gene families consist of groups of genes proposed to have originated from a common ancestor. Typically these arise through events in which a gene or genes ...

Types of Genetic Transfer Between Organisms

Genetic transfer occurs when genetic information is passed from one organism to another. It occurs via two mechanisms: vertical gene transfer and ...

Ribozymes

The term ribozyme is used for RNA that can act as an enzyme. Ribozymes are mainly found in selected viruses, bacteria, plant organelles, and lower ...

Construct an explanation to illustrate how common ancestry and biological evolution are supported by multiple lines of empirical evidence.

Use models to illustrate the conditions required for natural selection, including the overproduction of offspring, inherited variation, and the struggle to survive.

LS.Bio.9.3

Construct an explanation to explain how natural selection leads to adaptations within populations.

LS.Bio.9.4

Extinction

The History of Life on Earth
Fossils of organisms that were different from anything alive today had been documented as early as the 17th century, but it ...

Diversity and Extinction during the Paleogene Epoch
ExpandFirst, familiarize yourself with the extinction reading supplement for the background on what ...

Diversity and Extinction during the Paleogene Epoch
ExpandBegin this procedure by reading the extinction reading supplement that has been printed out for ...

Construct explanations to illustrate how varying environmental conditions may result in: changes in the number of individuals of a species, the emergence of new species over time, or the extinction of other species.

LS.Bio.10.1

Synteny and Evolution

John H. Renwick first coined the term &#8220;synteny&#8221; in 1971, which refers to the genes present on the same chromosomes, even if they are not ...

Multi-species Conserved Sequences

Next-generation sequencing technologies have created large genomic databases of a variety of animals and plants. Ever since the human genome project was ...

Taxonomy

Taxonomy is the science of defining and naming groups of biological organisms based on shared characteristics. It uses a hierarchy of increasingly ...

Phylogeny

Phylogeny is concerned with the evolutionary diversification of organisms or groups of organisms. A group of organisms with a name is called a taxon ...

Use models (including dichotomous keys, scientific nomenclature, cladograms, phylogenetic trees) to identify organisms and exemplify relationships.

LS.Bio.10.2

Understand natural selection as a mechanism for biological evolution.

How Genome Size Shapes New Genes

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