1.3 Nature of the Gene and the Genome

Question 1

Ohno thought what?

Answer 1

•proposed the 2R hypothesis (1971)
•Two rounds of genome duplication during vertebrate evolution
Evidence from multiple copies of genes in derived vertebrates
E.g. Derived vertebrates possess four times the gene number found in amphioxous (protochordate organism)
Conserved blocks of sequence provide evidence of large scale duplications

Question 2

Orthologues and Paralogues

Answer 2

• Two segments of DNA can have shared ancestry because of either a speciation event (orthologs) or a duplication event (paralogs)
• Homologous sequences are orthologous if they are inferred to be descended from the same ancestral sequence separated by a speciation event
• Paralogous genes, simply put, are genes that are related via duplication events in the last common ancestor

Question 3

Evolution of Globin Genes

Answer 3

• All modern globin genes (e.g. hemoglobin) contain three exons (coding) and two introns
• Ancestral globin-like genes contain four axons
• the Fusion of Exons 2 and 3 created the modern goblin

Question 4

Steps in the Globin gene evolution

Answer 4

• the genes fused into 3 axons
• then they dupicatied into å ß these were in a single chromo
• came separated and moved to dif chromo where they were again duplicated
• this shoes how the duplication can lead to a family of genes with specialized functions

Question 5

Mobile Genetic Elements “Jumping Genes”

Answer 5

• movement of genetic elements from one place in chromo to different site is transposition and the mobile genetic elements are trsposable elements
• transposable elements were called transposons (most code a protein or a tramsposase that single handedly catayse the excision of a traspospon from a donor DNA site and its insertion at a target DNA site
• Done by 2 separate transposes subunits that bind to septic seweundes at the two ends of the transposon. the subunits come other and form active dimer, that catalyze a series of rxns that lead to excision of the trasposson.
• Binds to target DNA where transpoosase catalyses rxns needs to integrate the transospon into the DNA. •This prices a small duplication (the green ends that flank either side of the transposed elementn(these are footprints to identify whats been transposed)

Question 6

Creation of Direct Repeats?

Answer 6

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Question 7

The three Pathways of Transposon Movement

Answer 7

1. DNA Transposon
   Cut and Paste Pathway
   Non replicative Transposition (this is what was first described)
2. Replicative Transposition (a new copy of transposable element is introduced at a new site and the old copy is left behind)
3. Retrotransposon
   Copy and Paste Pathway (DNA is transcribed then reverse transcribed by an reverse transcriptase enzyme producing complementary DNA, copy turn double stand and ingrates into target DNA)

Question 8

P-Elements

Answer 8

P elements are transposable elements that were discovered in Drosophila as the causative agents of genetic traits called hybrid dysgenesis. The transposon is responsible for P trait of P element and it is found only in wild flies

• Laboratory fruit flies descended from Morgan’s original stocks do not have P-elements
• Flies descended from wild caught strains have P-elements
• Mediated by parasitic organisms (viruses)
• P element carrying male strain mates with non p-element female. P-elements mobilized in embryo  Hybrid dysgenesis
• Dysgenesis is an abnormal organ development during embryonic growth and development.

Question 9

Moderately repetitive DNA

Answer 9

Moderately repetitive DNA arose by transposition (e.g. rRNA, tRNA)

most common moderately repeated sequences ALU and L1 resulted from Retrotransposon

Question 10

L1 Element

Answer 10

• 6000 bp
• Encodes reverse transcriptase and endonuclease (cleaves target)
• L1 encodes protein with 2 catalytic activities: reverse transcriptase that makes a DNA copy and endonuclease that mivld target DNA prior to insertion

Question 11

Alu

Answer 11

• sequences in humans derived from 7S RNA (signal recog. particle)
• Originated 60 mya in primates
• Increasing in copy number (1 copy per 100 years)
• Alu is short related sequences about 300bp

Question 12

Function of Transposable Elements?

Answer 12

1. Form part of some gene regulatory regions
2. Can combine genes to form novel genes or novel functions
3. Sometimes evolve into functional genes (e.g. telomerase gene)