Genomes - Lecture 6

Question 1

How many base pairs are in the human genome?

Answer 1

3,000,000 base pairs

Question 2

How many genes code for proteins?

Answer 2

19600 genes, 3%

Question 3

What percentage of genes are regulatory?

Answer 3

10%

Question 4

What percentage of genes are junk?

Answer 4

85% - no effect on fitness if deleted

Question 5

What percentage of the human genome is repetitive?

Answer 5

45% (10% of that contains traces of Alu element)

Question 6

What idea did Barbara McClintock come up with?

Answer 6

That genes can move - based on her studies of maize (corn)

Question 7

What are transposons?

Answer 7

○ Sequences of DNA that can move in the genome
○ Simplest transposon: produce transposase - enables ‘ungluing’ and ‘jump’
○ Retrotransposons: produce RNA, which is then reverse transcribed into DNA, then inserts into genome
○ Remnants of retroviruses in our DNA

Question 8

What are transposons involved in?

Answer 8

○ Can produce mutations
○ May be involved in some cancers and developmental disorders
○ Some transposons produce small interfering RNAs which act to prevent transposition

Question 9

What can prevent transposition?

Answer 9

Methylation

Question 10

What is an example of a new gene?

Answer 10

○ Fishes living in polar seas have ‘anti-freeze- glycoprotein
○ Arctic cod have different form to that found in Antarctic fish

Question 11

How are new genes formed?

Answer 11

○ Duplication of non-coding sequence
○ Mutations led to it being tagged as secretory
○ Translocation brought sequence near promoter region

Question 12

What is an example of new structures?

Answer 12

○ Mammilian placenta requires syncitins - genes that once encoded the envelope of a retrovirus
○ Even involved in evolution of placenta in some reptiles

Question 13

What did comparison of animal genome find?

Answer 13

○ The BovB elements in cows (25% of their genome) are closely related to sheep and snakes
○ Cows got 25% of their genome from snakes
○ How: Tick bit snake -> Same tick bit ancestor of sheeps and cows -> Transfer of blood into ancestor -> Found its way to sex cells

Question 14

What is horizontal gene transfer?

Answer 14

○ Widespread amongst prokaryotes
○ Minor source of genetic variation
○ In multicellular organisms e.g. cows: involves parasite or symbiote
○ Can lead to adaptations

Question 15

What did reconstruction of extinct genomes show?

Answer 15

○ There are 25 groups of genes that are necessary to be an animal
○ These were new genes that appeared at the origin of animals
○ Examples: homeobox developmental genes, signalling pathways, cell adhesion, receptors
○ Some insects (hexapods) are crustaceans

Question 16

How many giraffe species are there?

Answer 16

○ 4 - groups diverged 1-2 million years ago
○ No gene flow between them

Question 17

What is environmental DNA?

Answer 17

○ DNA of an organism left in the environment
○ Make it possible to extract sequences without an organism
○ Mainly from soil, water, air
○ Non-invasive observational technique
○ Molecules behave differently in different situations
○ Requires careful amplification and then bioinformatic identification

Question 18

What are the key facts of humans genomes?

Answer 18

○ We differ from each other by 0.1% (3 million bases)
○ We have 70 new nuclear mutations that were not present in our parents
○ 910 individuals of recent African descent had 10% more DNA that in reference genome

Question 19

Evidence of evolution in humans

Answer 19

Tibetans:
○ Live at high altitude
○ Selection on EPASI gene - controls RBC production - mutation in 87% of Tibetans
Bajau:
○ In SE Asia and are able to dive for long periods of time
○ Selection on PDEI0A gene - led to larger spleens so they have more RBCs
○ Selection on BDKRB2 gene - involved in diving reflex

Question 20

More evidence for evolution in humans

Answer 20

Selection in the UK:
○ Detect changes occurring 2-3000 years
○ Positive effect on height and infant head circumference
○ Negative effect on cholesterol
Skin Colour:
○ Darker skin protects against UV, lighter skin enables vitamin D biosynthesis where sun is weaker