Genetics IV

Question 1

DNA Helicase

Answer 1

Unwinds the DNA

Question 2

DNA Polymerase

Answer 2

Synthesis DNA 5’-3’ direction

Question 3

DNA topoisomerase

Answer 3

Relieves the tension in DNA

By creating and rejoining temporary breaks in the DNA backbone

Tension is caused by super coiling during the process of DNA replication, transcription, recombination, and chromosome segregation.
By cutting and rejoining DNA strands, they allow the helix to unwind or coil as needed, preventing knots and tangles and maintaining DNA stability

Question 4

DNA acts as a template for its own replication. This is known as ________ ________.

Answer 4

Semi-conservative replication

Question 5

Function of DNA primase

Answer 5

Synthesises short RNA primers on a DNA template

Question 6

Ribonucleases (aka exonuclease)

Answer 6

Regulates cellular processes such as RNA metabolism

By breaking down RNA by degrading the RNA primers

Question 7

DNA ligase

Answer 7

Joins DNA fragments

Question 8

Telomerase

Answer 8

Replicates the ends of the chromosomes

By adding DNA repeats

Question 9

Explain why DNA replication occurs differently on the two DNA strands (leading and lagging strand synthesis)

Answer 9

*) A new strand of DNA is always synthesised in a 5’ to 3’ direction
*) It elongates from a free 3’ -OH
*) DNA synthesis takes place inside a replication fork
*) Leading strand is already in 5’-3’ direction, so replication is continuous
*) In the lagging strand, it is positioned 3’-5’, replication cannot occur in the opposite direction, so replication is discontinuous, forming Okazaki fragments

Question 10

The short DNA sequences on the lagging strand are called a)_____ ______ and are composed of b)__-__ nucleotides.

Answer 10

a) Okazaki fragments
b) 100-1000 nucleotides

Question 11

What are repetitive regions at the very ends of chromosomes called?

Answer 11

Telomeres

Question 12

What is the purpose of telomeres?

Answer 12

Telomeres act as caps that protect the internal regions of the chromosomes, and they’re worn down a small amount in each round of DNA replication

Question 13

What is the repeated sequence of nucleotides that makes up telomeres in mammals?

Answer 13

TTAGGG

Question 14

What is the solution to the single-stranded overhand in the lagging strand, which prevents the chromosome getting shorter with each round of replication and cell division?

Answer 14

Enzyme telomerase (an RNA-dependent DNA polymerase)
a) Binds to a special RNA molecule, complementary to telomeric repeat (AAUCCC - TTAGGG)
b) Telomerase recognises the end of a repeat sequence, and uses the RNA template within the enzyme to add additional repeats
c) When the overhand is long enough, a matching strand can be made by DNA polymerase (which has its own primase subunit, therefore no primer needed)

Result = telomere cap is added to the end, made up of repeated DNA

Therefore, protecting the inside of the telomere DNA

Question 15

DNA can be damaged as a result of;

Answer 15

-UV light
-Ionising radiation exposure
-Toxic chemical agents
-Reactive oxygen species
-Mutations in genes

Question 16

Name and explain the two classes of mutations

Answer 16

1) Gain of function - a DNA sequence change that leads to increased or alternative activity
2) Loss of function - a DNA sequence change that leads to decreased activity

Question 17

Substituting a nucleutide can alter the codon in 3 ways:

Answer 17

1) Silent (different code, same amino acid)
2) Missense (different code, different amino acid)
3) Nonsense (Results in a STOP codon, protein truncated/shortened, may not function properly or at all)

Question 18

What is Nonsense Mediated Decay (NMD)?

Answer 18

A process during translation that detects transcripts with premature stop codons and degrades them

Question 19

Types of mutation

Answer 19

Insertion
Deletion
Frameshift
Missense
Nonsense
Splice-site
Promoter

Question 20

What is a promoter mutation?

Answer 20

A change in the DNA sequence of a gene’s promoter region, which can alter how the gene is expressed

By altering the binding of transcription factors, which can either increase or decrease the rate of transcription, leading to more or fewer protein copies

Question 21

What is a splice

Answer 21

A genetic alteration that occurs at the boundaries between exons and introns, disrupting the natural process of RNA splicing

Question 22

What mechanism is used to repair DNA?

Answer 22

1) Excision
-Recognition and removal of damage using EXONUCLEASE
2) Repair
-Re-synthesis of missing DNA using DNA POLYMERASE
3) Joining
-Sealing the nick using DNA LIGASE

Question 23

How does a thymine dimer occur?

Answer 23

Occurs when two adjacent thymine bases on the same DNA strand become abnormally linked together by a covalent bond
Due to exposure to ultraviolet (UV) radiation from the sun

Question 24

How is a thymine dimer repaired?

Answer 24

1) Detection
2) The surrounding DNA is opened to form a bubble
3) Enzymes cut the damaged region out
4) DNA polymerase replases excised DNA and ligase seals the backbone

Question 25

A double-strand break can occur as a result of high energy radiation. What two ways can this be fixed?

Answer 25

a) Non-homologous end joining b) Homologous end joining

Question 26

Explain non-homologous end joining

Answer 26

-Ends are 'polished' to produce 'blunt ends' -Ends are joined by DNA ligase It does result in the loss of nucleotides, but is very quick (minutes)

Question 27

Explain homologous end joining

Answer 27

Recombination between two corresponding regions of the two alleles = homologous recombination It is a complete repair, however is slower (hours)