molecular genetics

Question 1

properties of DNA

Answer 1

• store complex information
• replicate faithfully
• relatively stable
• mutate occasionally to produce variation
• be translated into proteins to produce phenotype

Question 2

PCR

Answer 2

• polymerase chain reaction
• en vitro amplification of specific DNA/RNA fragments
• used in DNA profiling, diagnostics, analysis of ancient DNA and genetic engineering
• cycle repeats 20-40 times, producing millions of copies

Question 3

PCR setup

Answer 3

• synthetic primer = short strands of complementary RNA (oglionucleotide) specific to sample. Anneals to unwound DNA to indicate to Taq polymerase where to start, the DNA that is amplified is the sequence between primers
• sample
• Taq polymerase
• free nucleotides (dNTPs)
• sterile water

Question 4

process of PCR

Answer 4

1. denaturation, DNA heated to ~95C, breaking H bonds
2. annealing, temperature drops to ~55C, primer bonds to DNA section to be replicated
3. extension, 70-75C, Taq polymerase binds at primer, free nucleotides align, Taq creates phosphodiester bonds

Question 5

structure of DNA

Answer 5

• nucleotide subunits containing phosphate, deoxyribose and base A,T,C,G
• antiparallel strands, molecule has 5’ end and 3’ end

Question 6

nucleotide base pairs

Answer 6

• A and T bind with 2 H bonds
• C and G bind with 3 H bonds, so are more stable
• a higher proportion of CG base pairs in DNA means that more energy is required to split it

Question 7

semi-conservative DNA replication

Answer 7

1. DNA helicase breaks H bonds between complementary base pairs, topoisomerase unwinds helix
2. both strands used as template, complementary base pairing occurs
3. DNA polymerase forms phosphodiester bonds between adjacent nucleotides (condensation reaction)

Question 8

replication bubbles

Answer 8

• DNA synthesis happens at multiple places on the chromosome at once in replication bubbles
• bubbles merge as replication forks meet each other

Question 9

discontinuous replication

Answer 9

• DNA polymerase can only add nucleotides to 3’ end
• strands are antiparallel
• leading strand continuously replicated (direction or DNA synthesis same as replication)
• lagging strand discontinuously replicated, direction of DNA synthesis is opposite to replication
• produces Okazaki fragments 100-200 nucleotides long, DNA ligase seals gaps between fragments after synthesis

Question 10

primers

Answer 10

• short strands of single stranded complementary RNA produced by enzyme primase
• anneal to unwound DNA to initiate synthesis, as DNA polymerase can only add nucleotides to an existing strand
• removed after synthesis and replaced by extension of adjacent replicated strand

Question 11

telomeres

Answer 11

• at chromosome ends there is a gap where primer is removed, so the chromosome is shortened every time it is replicated
• telomeres are a cap on the end of chromosomes made up of repeated DNA sequences that don’t code for anything, so can be lost in replication

Question 12

telomerase

Answer 12

• enzyme that extends telomeres
• only active in early embryonic cells

Question 13

Hafflick limit

Answer 13

amount of divisions adult somatic cells can undergo before DNA that code for proteins are lost

Question 14

double strand breaks

Answer 14

• can be caused by radiation or chemical mutagens
• occurs in prophase I of meiosis
• homologous chromosome used as template for repair

Question 15

homologous recombination

Answer 15

1. homologous chromosomes line up during prophase I
2. enzyme (Spo11) creates double strand break in one of the chromosomes
3. nuclease enzyme (MRX) digests sections either side of breaks, creating 3’ overhang
4. strand invasion (enzymes Dmc1 and Rad51), free 3’ end invades intact chromosome
5. DNA polymerase adds nucleotides back on using homologous chromosome as a template for repair
6. 2 Holliday junctions formed between DNA molecules, direction that Holliday junctions are cut determine whether recombination occurs

Question 16

recombination and evolution

Answer 16

• shuffles alleles at different genetic loci creating genotypic diversity
• decouples beneficial alleles from harmful ones, preventing genetic hitchiking (deleterious genes accumulating as they are linked to a beneficial mutation)

Question 17

recombination between sex chromosomes

Answer 17

• XX chromosomes undergo recombination normally
• XY chromosomes do not undergo crossing over except for a very small region near tip
• Y chromosomes never recombine, so has far fewer functional genes probably due to accumulation of deleterious mutations