L10 Nucleic Acids and DNA Replication

Question 1

__?__ are the fundamental biochemical building block of DNA and RNA

Answer 1

Nucleotides

Question 2

Nucleotides are composed of three components:

• __1__ base: pyrimidine (C, U, T) or purine (G, A)
• __2__ : deoxyribose in DNA, ribose in RNA
• __3__ group: acts as a bridge between adjacent ribose/deoxyribose groups

Answer 2

1) Nitrogenous
2) Pentose sugar
3) Phosphate

Question 3

Adenosine is the __?__ (sugar + base) component of ATP

Answer 3

nucleoside

Question 4

True or false: in DNA a pyrimidine always pairs with a purine?

Answer 4

True

C (pyrimidine) and G (purine)

T (pyrimidine) and A (purine)

Question 5

Nucleotides are always added to the __?__ end of a polynucleotide chain

Answer 5

3’

Question 6

When adding a new nucleotide to a polynucleotide chain, the __1__ of the new nucleotide reacts with the __2__ of the polymer to a __3__ bond. The chain grows in a __4__ to __5__ direction.

Answer 6

1) alpha-phosphate
2) 3’-OH group
3) 3’-5’ phosphodiester
4) 5’
5) 3’

Question 7

A nucleic acid chain of any length has a __1__ at the start and terminates with a __2__. By convention, the base sequence of a DNA chain is written in the __3__ to __4__ direction.

Answer 7

1) 5’ phosphate
2) 3’ OH
3) 5’
4) 3’

Question 8

How would you write the complement to this DNA strand?

5’-AGTCT-3’

Answer 8

5’-AGACT-3’

Although it would be oriented TCAGA, you always write it from 5’ to 3’

Question 9

What is a nucleosome?

Answer 9

DNA wrapped around 8 histone molecules - the basic unit of compacted DNA.

Contains high amounts of arginine and lysine (+ve charge) to bind DNA backbone (-ve charge).

Roughly 146bp of ‘core DNA’ wraps around the histone core, and 60-80bp of ‘linker DNA’ leads to adjacent nucleosome.

Question 10

How many molecules of DNA make up a chromosome?

Answer 10

2 - each chromatid is made up of one identical strand of DNA

Question 11

What are the p and q arms of a chromosome?

Answer 11

The binding of the chromatids at the centromere results in four ‘arms’. There are 2 short p arms and 2 long q arms.

Question 12

What are ‘chromosome territories’?

Answer 12

Regions in the nucleus in which specific chromosomes are localised.

Chromosomes don’t just mix within the nucleus, they are organised into these territories.

Question 13

DNA replication occurs in the __1__ phase of interphase during the cell cycle. Replication follows several steps that involve multiple proteins called __2__ enzymes and __3__.

DNA replication is vital for cell growth, repair, and reproduction.

Answer 13

1) S
2) replication
3) RNA

Question 14

Step 1 of DNA replication is __?__.

Answer 14

Replication Fork Formation

Double stranded DNA is ‘unzipped’ into 2 single strands. Base pairs broken by DNA helicase.

Strands form a Y shaped replication fork, which is the template for replication to begin.

Proteins bind to stabilise unwound strands of DNA.

Question 15

A DNA replication fork has 2 strands in opposing directions. The 3’ to 5’ strand is used to form the __1__ strand, while the 5’-3’ strand is is used to form the __2__ strand.

Answer 15

1) leading

2) lagging

Question 16

The 3’ to 5’ arm of a DNA replication fork produces a [(1) continuous/ discontinuous] strand of complementary DNA in the [(2) 3’ to 5’/5’ to 3’] direction. This is the __3__ strand.

Answer 16

(1) continuous
(2) 5’ to 3’
(3) leading

Question 17

The 5’ to 3’ arm of a DNA replication fork produces a [(1) continuous/ discontinuous] strand of complementary DNA in the [(2) 3’ to 5’/5’ to 3’] direction. This is the __3__ strand.

Answer 17

(1) discontinuous (Okazaki fragments)
(2) 3’ to 5’
(3) lagging

Question 18

Which DNA Polymerase produces Okazaki fragments?

Answer 18

DNA Polymerase-delta

the delta symbol is the lowercase form and looks like an ‘s’

Question 19

__1__ forms the first 20 base pairs of the leading strand during DNA replication. After that, __2__ takes over. Both enzymes have proofreading 3’ to 5’ exonuclease activity.

Answer 19

1) DNA polymerase-alpha

2) DNA polymerase-epsilon

Question 20

What is the main physical difference between the leading and lagging strand prior to termination?

Answer 20

The leading strand is continuous and oriented 3’ to 5’; the lagging strand is discontinuous and oriented 5’ to 3’.

Question 21

Step four (termination) of DNA replication applies to both strands equally. __1__ are degraded and filled by action of __2__ and __3__.

__4__ joins any breaks in the leading and lagging strands to generate continuous double stranded DNA.

Answer 21

1) RNA primers
2) RNAase-H
3) DNA polymerase-delta
4) DNA ligase

Question 22

True or false: there is only ever one origin of DNA replication on any one DNA strand.

Answer 22

False. It would take far too long, and we know the who genome within a cell replicates in 8 hours.

There are multiple origins of replication with replication forks proceeding in opposite directions (replication bubbles).

Question 23

What is epigenetics?

Answer 23

Heritable changes to DNA that do not include a change in the DNA sequence.

i.e. changes to gene expression (which genes are switched on and off) which can be passed on to descendants.

Question 24

What are some examples of epigenetic changes?

Answer 24

HISTONE MODIFICATIONS:
acetylation of Lys, methylation of Lys and Arg

DNA MODIFICATIONS:
Methylation of cytosine

Question 25

How can our environment (or ‘nurture’) become imprinted within our genes?

Answer 25

Epigenetic marks can be altered by environmental stimuli. These stimuli can make changes such as methylating cytosine to switch a gene off.

e.g. smoking, nutrition

Question 26

True or false: Altered epigenetic marks can be inherited.

Answer 26

True.

A study showed that children born in famine had altered gene expression, and that these genes were also seen in their children even though their children were not born in famine.

Question 27

What is ‘precision medicine’?

Answer 27

By sequencing an individual’s genome, you can accurately determine the efficacy of different drugs, drug combinations, and side-effects - allowing doctors to personalise treatments for optimal performance.