16. Structure and Replication of DNA

Question 1

What are the components of a nucleotide

Answer 1

1. deoxyribose sugar
2. a phosphate group
3. a nitrogenous base

Question 2

Purines and pyrimides

Answer 2

Purines - Adenine, Guanine

Pyrimidines - Cytosine, Thymine

Question 3

What type of bonds hold bases together?

Answer 3

Hydrogen bonds

Question 4

How many bonds between C and G

Answer 4

3 hydrogen bonds

Question 5

How many bonds between A and T

Answer 5

2 hydrogen bonds

Question 6

locus

Answer 6

The position of a gene on a chromosome

Question 7

When are chromosomes visible

Answer 7

during cell division

Question 8

Chromatin

Answer 8

DNA + protein = chromatin

Question 9

What is a nucleosome made up of?

Answer 9

• histone molecule (protein) - dna double helix coils around this
• linker histone clamps DNA to the core

The whole thing makes up a nucleosome

Question 10

Euchromatin

Answer 10

• “switched on”
• is a lightly packed form of chromatin (DNA, RNA, and protein) that is enriched in genes, and is often (but not always) under active transcription.

Question 11

Heterochromatin

Answer 11

• “Switched off”
• chromosome material of different density from normal (usually greater), in which the activity of the genes is modified or suppressed.

Question 12

What is meant by ‘DNA is semi-conservative’

Answer 12

New molecule of DNA has one strand which comes from the parent molecule and one strand which is newly
synthesised.

Question 13

Why do eukaryotes have multiple replication origins?

Answer 13

•having one replication origin means it would take years to replicate DNA
- so humans have thousands

Question 14

Replication origin in mammals

Answer 14

• chromatin structure rather than a DNA sequence

- a complex of proteins forms ORC (origin recognition complex)

Question 15

Replication origin in yeast

Answer 15

• specific sequence of DNA at the replication origin with 120 bases

Question 16

What is the sliding clamp?

Answer 16

Complex of proteins in the shape of a donut which keeps the DNA polymerase in place and increases the efficiency of replication

Question 17

Describe the steps of DNA replication in E.coli

Answer 17

• Helicases separate strands of DNA
• DNA topoismerase (gyrase) relieves supercoiling
• single stranded binding proteins keeps the strands apart
• DNA polymerase III adds bases 3’ -> 5’

Question 18

What does DNA polymerase III need to start replication

Answer 18

• needs a short double-stranded region to start DNA replication
• short primer RNA

Question 19

In what direction can bases be added

Answer 19

From 3’ prime to 5’ end

Question 20

What are okazaki fragments?

Answer 20

One of the RNA-primed short segments of DNA synthesised during replication
of the lagging strand of the double helix.

Question 21

Laggin strand replication

Answer 21

• primers are added to start replication
• primers are removed and the gaps are filled with complementary bases
• okazaki fragments are joined

(Bases added from 5’ to 3’ end)

Question 22

Speed of replication bw eukaryotes and prokaryotes

Answer 22

Prokaryotic DNA replication is a lot faster

Question 23

Leading strand vs the lagging strand

Answer 23

•The leading strand is that which is synthesised continuously
•The lagging strand is synthesised in a discontinuous fashion during DNA
replication.

Question 24

What 3 elements are needed for a chromosome to function and why?

Answer 24

• telomeres - structural stability, replication, positioning
• centromeres - essential for segregation at cell division (acentric fragments get lost on the spindle)
• origins of replication - DNA replication starting point

Question 25

What type of bond links adjacent nucleotides together?

Answer 25

Phosphodiester bond

Question 26

5’ (PO4) end

Answer 26

The end of the polynucleotide that terminates with a phosphate attached to the
5’- carbon of the sugar.

Question 27

3’ (OH) end

Answer 27

The end of a polynucleotide which terminates with a hydroxyl group attached
to the 3’- carbon of the sugar.

Question 28

DNA heliCase

Answer 28

An enzyme that uses energy from ATP to unwind the DNA

Question 29

What is Topoisomerase

gyrase

Answer 29

An enzyme which can relax DNA by cutting and reforming the polynucleotide
backbones

Question 30

Single-strand binding protein function

Answer 30

One of the proteins that attaches to single stranded regions of DNA and
prevents the reformation of base pairs, particularly in the region of the
replication fork.

Question 31

primase

Answer 31

an enzyme which synthesises the RNA primer

Question 32

DNA polymerase I

Answer 32

Enzyme which adds nucleotides to gaps when primers removed

Question 33

DNA polymerase III

Answer 33

Main polymerase which adds nucleotIdes to template in a complementary way

Question 34

RNA primer

Answer 34

Sequence of RNA to make a stretch of double stranded DNA on which the
DNA pol III can work

Question 35

Ligase

Answer 35

Enzyme which joins sequences of DNA

Question 36

What are telomeres

Answer 36

• A compound structure at the end of a chromosome.

* protect ends of chromosome by looping or recruiting protective proteins

Question 37

Relation bw telomeres and ageing

Answer 37

• problems with replication of lagging strand
• unreplicated DNA is degraded so telomeres shorten with age
• lose bases after each round of replication
• genes on the ends of chromosomes are lost so cell dies

Question 38

Telomere replication

Answer 38

• leading strand replication is continuous from 3’ to 5’ end
• lagging strand a problem since there is not enough DNA template for primer
- there is an overhang which isn’t replicated (not enough sequence for primer)
•enzyme telomerase extends the lagging strand and primer attaches so DNA polymerase can complete the replication

Question 39

What are the effects of having shorter telomeres?

Answer 39

• premature aging

* shorter telomeres