1-12 Dynamic Genomes and the Creation of Genetic Diversity

Question 1

When do unequal crossovers occur?

more info needed

Answer 1

when transpons insert themselves into introns

Question 2

How do unequal crossovers create new gene families?

Answer 2

Transposons create sites for miss-alignment during recombination and thereby an unequal crossover. Misalignment of homologues from transposons inserting into introns, during recombination, causes both exon deletions and duplications
Led to the creation of the dystrophin gene

Once a chromosome has multiple gene copies, these paralogs can become specialized through genetic drift (fetal hemoglobin)

Question 3

How do unequal crossovers cause exon duplication and deletion?

Answer 3

during unequal crossover, and exchange of exons can occur instead of an entire gene.

Leaves one homolog with extra exons, other with far fewer. (deletion/duplication happen in tandem)

Question 4

How do unequal crossovers lead to exon amplification?

Answer 4

unequal crossovers over several generations can lead to one homologue accumulating many copies of an exon.

Question 5

How do unequal crossovers contribute to exon shuffling?

Answer 5

exons flanked by transposons may be mistakenly excised and moved to other regions of genone. Bringing together different exons in this way can give rise to genes that code for proteins with new domains/functions.

Question 6

Describe the transposable elements found in humans. How are they formed? When Induced?

Answer 6

Transposable (here retrotranspons) elements work via an RNA intermediate, where the element is first transcribed into an RNA (from DNA), the reverse transcriptase (from LINEs) synthesizes a dsDNA copy which then inserts itself into a new site in the genome

Transposition is induced during meiosis

LINE1 -long interspersed elements. Code own reverse transcriptases, allowing them to transpose on their own.

SINE - short interspersed elements. Do not code own reverse transcriptases. mucst use those encoded on line to transpose

Alu sequence – Genetic artifacts. Very few are fit to transpose.

Question 7

How do transposons contribute to genetic diversity?

Answer 7

disrupt gene function by inserting within exon of gene

affect expression of genes (presence of transposons within genes generally decreases expression)

Create sites for illigitimate homo recombination (unequal crossovers) - this can cause gene amp or exon amp/deletion

Question 8

how do meiosis and sexual reproduction lead to the creation of genetically unique individuals?

Answer 8

combination of mendels law of independent assortment and the average of 3 crossing over events per homologous pair per Meiosis I - number of unique individuals that can be created from a single conception of two gametes is in the billions

Question 9

Describe the creation of new genes by exon shuffling

Answer 9

New genes with unique combinations of functions can be created by bringing together exons that code for functional protein motifs

Whole exons can be moved by the transposition of flanking transposable elements

Insertion of the excised exon at a new location, in the intron of another gene, can lead to the formation of a chimera with a unique combination of activity

Question 10

What is homologous recombination?

Answer 10

the reciprocal exchange of genetic information (DNA)

Question 11

How/When does Homologlous recombination occur?

Answer 11

when two homologous combinations combine so they are in register. both strands of each double helix are broken and rejoined to the other homologue.

-Can occur anywhere with a region of homology (can lead to integration of exogenous dna fragments, plasmids containing short regions of homology can also integrate,

Question 12

both _____ and _____ mechanisms are used by bacteria to transfer antibiotic resistance genes into the bacterial chromosome

Answer 12

crossover and plasmid mechanisms

Question 13

Enzymes that catalyze homogous recombination…

Result?

Answer 13

are induced during meiosis and the result is even greater shuffling of maternal and paternal alleles and therefore a greater diversity in offspring

Question 14

what are the rec enzymes?

Answer 14

homologous recombination is exchanged through a process called recombination. these reactions are catalyzed in bacteria by a set of enzymes called rec enzymes.
-specialized to mediate genetic exchange between homologous sequences

Question 15

Describe the steps in bacterial homologous recombination

Answer 15

• initiates with alignment in register of two homologous, double stranded dna sequences
• nick made in one strand, allowing it to invade and anneal the complementary strand of the homologue.
• the displaced strand is nicked and anneals to the other homologue
• ends are ligated to form holliday junction
• gymnastics involving outside ends to cross each other
• crossing strands are cut and ligated

Question 16

Steps in eukayotic homologous recombination

Answer 16

• induced by double strand break
• RAD50 complex resects the 5’ ends due to 5’ to 3’ exonuc activity
• exposed 3’ overhangs, with help from RecA/rad51 homologe, can find it’s homo sequence
• gaps from resection are filled by DNA synthesis, creating holliday junction
• gymnastics leads to strands crossing. cut and ligated

Question 17

when does homo recom occur?

Answer 17

during meiosis after DNA replication.

Question 18

cohensins?

Answer 18

helps replicated homologues pair in register.

Cohesin is a protein complex that regulates the separation of sister chromatids during cell division, either mitosis or meiosis.

Question 19

Meiosis I vs II

Answer 19

1 - homologues separate (XX -> X X)

2. sister separate (X -> ] [)

Question 20

how do retroviruses contribute to genetic instability?

Answer 20

1. disrupt gene function by inserting into exon
2. viral promoters very active, inserting can inappropriately turn on neighboring genes
3. upon packaging of new virus, neighboring genes can be incorporated into viral genome and past to next host or cell infected

Question 21

(3) contributes to genetic diversity during gamete formation in humans

Answer 21

1) independant assortment
2) homo recom
3) newly arising mutations due to errors in DNA replication and repair

Question 22

unequal crossovers have contributed to (4)

Answer 22

creation of gene families (human globin genes)
creation of new gene via exon shuffling
deletion of exons to create disease alleles (dystropin in MD)
creation of gene products with repeated domains

Question 23

Describe the process of retrotranspons

Answer 23

1. the retrotransposon within the gene gets transcribed into ss RNA.
2. Portion of RNA gets translated into reverse transcriptase
3. Reverse transcriptase takes the RNA strand and reverse transcribes the RNA into a strand of DNA
4. The synthesis of the second DNA strand occurs
   5.

Question 24

steps of exon shuffling

Answer 24

1. two similar transposable elements flank an exon
2. transposase recognizes the distal ends of the two flanking transposable elements (it clips the wrong end, thinks that the one is a transposable element, like incorrect parentheses)
3. the transposase inserts the exon into another gene