Maintanence and Use of Genetic Information

Question 1

What are the letters of the X4 stages of the cell cycle?

Answer 1

G1 (Growth 1)
S (DNA Synthesis)
G2 (Growth 2)
M (Mitosis)

Question 2

At which part of the cell cycle is a signal received to replicate DNA?

Answer 2

G1.

Question 3

What happens if no signal is received in G1 instructing the cell to replicate its DNA?

Answer 3

The cell enters a quiescent state (G0) where it exists but does not divide.

Question 4

Which phases of the cell cycle are interphase?

Answer 4

G1, S, and G2.

Question 5

Explain in basic terms the process of semi-conservative DNA replication.

Answer 5

The DNA unwinds to expose its base pairs, which are used as a template for the daughter strands to be synthesised from. This utilises complementary base pairing.

Question 6

What is the name of the specific points on genomes at which replication begins?

Answer 6

Origins of replication .

Question 7

What is the name of the area of unwound DNA which is the site of DNA replication?

Answer 7

Replication bubble.

Question 8

What is the prong like structure at the replication bubble called?

Answer 8

Replication fork (X2 formed).

Question 9

What is the name of the enzyme which unwinds DNA ready for replication?

Answer 9

Helicase.

Question 10

A broken double helix may instantly reform, if it wasn’t for which structures?

Answer 10

Single strand binding proteins.

Question 11

What is the name of the process of intense chromatin coiling of a remaining DNA helix following the separating of its strands via DNA helicase?

Answer 11

Positive Supercoiling.

Question 12

What does topoisomerase do in DNA replication?

Answer 12

Acts above the replication fork to break some phosphodiester bonds, providing the helix with a degree of freedom to avoid positive supercoiling.

Question 13

From which end/directions does DNA polymerase work to synthesise new daughter DNA strands?

Answer 13

ONLY in the 5-prime to 3-prime direction.

Question 14

What is the name of the strand of DNA replication that is synthesised continuously?

Answer 14

The leading strand.

Question 15

What is the name of the strand that is synthesised discontinuously?

Answer 15

The lagging strand.

Question 16

Out of DNA polymerase and RNA polymerase, which can initiate synthesis of (DNA/RNA) and which can only add nucleotides to a pre-existing chain?

Answer 16

DNA polymerase = can only add nucleotides to existing chains

RNA polymerase = can initiate RNA synthesis

Question 17

What is the name of the multiple sections of daughter strand DNA created by the lagging strand of DNA replication?

Answer 17

Okazaki fragments.

Question 18

DNA synthesis starts with the synthesis of a primer of what?

Which enzyme is involved in this process?

Answer 18

A short RNA primer.

Primase (a type of RNA polymerase)

Question 19

Each lagging strand fragment will have an RNA primer. These need to be degraded, but by which enzyme?

Answer 19

Exonuclease.

Question 20

Which enzyme joins the two fragments of lagging strand DNA once their RNA primers have been removed?

Answer 20

DNA ligase.

Question 21

What process sees the lagging strand bend around to co-ordinate DNA synthesis in the same direction despite opposing topological directions.

Answer 21

The formation of a lagging strand loop.

Question 22

What is the role of HELICASE?

Answer 22

Unwinds the DNA strands ready for replication.

Question 23

What is the role of TOPOISOMERASE?

Answer 23

Releases the positive supercoiling of DNA.

Question 24

What is the role of SINGLE STRAND BINDING PROTEIN?

Answer 24

Stops unwound DNA from reforming a double helix.

Question 25

What is the role of PRIMASE?

Answer 25

Forms a RNA primer which allows DNA synthesis to begin.

Question 26

What is the role of EXONUCLEASE?

Answer 26

Removes RNA primer from each Okazaki fragment.

Question 27

What is the role of DNA polymerase?

Answer 27

Synthesised DNA.

Question 28

What is the role of DNA ligase?

Answer 28

Links adjacent Okazaki fragments once Exonuclease has removed their RNA primers.

Question 29

In which direction does DNA polymerase degrade DNA?

Answer 29

3-prime to 5-prime, as it contains an exonuclease.

Question 30

What does DNA polymerase do when a wrong base is added during replication?

Answer 30

Rate of synthesis will slow, activating the DNA polymerase exonuclease which will undo the fault, allowing DNA synthesis to continue correctly.

Question 31

What is the end replication paradox?

Answer 31

This is the name given to the loss of the ends of linear chromosomes after each round of replication due to DNA polymerase ability to only extend from the 5-prime to 3-prime direction.

Question 32

Which structures prevent the end replication paradox?

Answer 32

Telomeres.

Question 33

Which enzyme catalyses the synthesis of telomeres?

Answer 33

Telomerase.

Question 34

How do telomeres work?

Answer 34

Extend the DNA without a chromosomal template to prevent loss of information due to the end replication paradox.

Question 35

What combination of bases is repeated to form the composition of telomeres?

Answer 35

(5-prime) TTAGGG (3-prime)

Question 36

In which cells are telomeres active, and in which are they not?

Answer 36

Active within gametes.

Inactive within somatic cells.

Question 37

What type of mutation involves a single base being changed, and how many types of this mutation are there?

Answer 37

Point mutations

X3 types

Question 38

What are the X3 types of point mutation?

Answer 38

Silent mutations (synonymous)

Missense mutations (non-synonymous)

Nonsense mutations

Question 39

What is a silent mutation?

Answer 39

A form of point mutation which does NOT result in a different amino acid being coded for.

E.g. CGA (arg) — CGG (arg)

Question 40

What is a missense mutation?

Answer 40

A form of point mutation which results in a DIFFERENT amino acid being coded for.

E.g. GAG (Glu) — GTG (val)

Question 41

What is a nonsense mutation?

Answer 41

A form of point mutation which results in a STOP codon being coded for.

E.g. TGC (cys) — TGA (stop)

Question 42

What is an INDEL?

Answer 42

A small insertion or deletion of bases in a DNA sequence.

Question 43

How many base pairs would be needed in an INDEL to maintain a reading frame?

Answer 43

A multiple of X3.

Question 44

Name X4 other types of mutations that involve larger chunks of chromosome DNA and describe what they do.

Answer 44

Inversions (a section of DNA flips itself around)

Deletions (a section of DNA deletes itself)

Duplications (a section of DNA duplicates)

Translocations (a section of DNA moves to a different place along the chromosome)

Question 45

Name X3 ways spontaneous mutations can occur.

Answer 45

Errors in DNA replication
- these have escaped proof reading and repair mechanisms

Replication slippage
- the gain or loss of repeated triplets of bases in areas of many repeats due to the repeating unit’s ability to ‘slide’ over each other and still retain complementary base pairing. Can occur in the forwards (occurs to parents, strand and leads to n and n-1 strands on second replication) or backwards (occurs in daughter strand and leads to n and n+1 strands on second replication) direction.

Spontaneous deamination
- cysteine loses an amino group to become uracil. This changes a C-G pairing to an A-T pairing.

Question 46

Name X2 ways in which induced mutations occur.

Answer 46

Physical
- UV light (UV-B) indices a chemical bond between adjacent thymines (making thymine dimers) which distorts DNA and interferes with replication.

• ionising radiation causes strand breaks.

Chemical
Some agents react with bases:
- nitrous acid = cytosine - uracil 
- alkylating agents = guanine modifiers 
- free radicals = break strands

Question 47

What is the process by which nucleosomes are altered to enable access to DNA that is highly condensed as chromatin?

Answer 47

Nucleosome remodelling.