16. Structure and replication of DNA

Question 1

What is a gene?

Answer 1

The fundamental physical and functional unit of heredity which arrives information fro one generation to the next
- Segment of DNA

Question 2

What are the three components of a nucleotide?

Answer 2

• Deoxyribose sugar
• Phosphate group
• Nitrogenous base

Question 3

Which bases are purines?

Answer 3

A and G

Question 4

Which are pyrimidines?

Answer 4

C and T

Question 5

What are the bases joined by?

Answer 5

Hydrogen bonds

Question 6

How may hydrogen bonds between C and g?

Answer 6

3

Question 7

How many hydrogen bonds between A and T?

Answer 7

2

Question 8

What is a locus?

Answer 8

The position of a gene on a chromosome is called a locus

Question 9

Where are chromosomes found?

Answer 9

Cytosol in prokaryotes

Nucleus in eukaryotes

Question 10

What is DNA and protein known as?

Answer 10

Chromatin

Question 11

What is a nucleosome?

Answer 11

The structure formed when the DNA double helix coils around a core of eight histone molecules

Question 12

When does DNA replication occur?

Answer 12

During the S (synthesis) stage of the cell cycle

Question 13

What is the replication origin for eukaryotes?

Answer 13

• In yeasts specific sequence of DNA at replication origin (ARS)
• For mammals, a chromatin structure rather than DNA sequence at replication origin
• A complex of proteins that forms the origin recognition complex (ORC)

Question 14

What is the sliding clamp?

Answer 14

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

Question 15

For dna replication in E.coli what does helicase do?

Answer 15

It separates the strands of the dna

Question 16

For DNA replication in E.coli what does DNA topoisomerase (gyrase) do?

Answer 16

It relieves supercoiling

- Single stranded binging proteins keep the strands apart

Question 17

What does DNA polymerase III do in e.coli DNA replication?

Answer 17

It is responsible for DNA replication itself but needs a short double sanded region to start (primer RNA)
- Reads DNA 3’ to 5’ and can only add based to 3’ end of the growing strand so the strand grows 5’ to 3’

Question 18

How does DNA replication occur on the lagging strand?

Answer 18

• Must open the strand a little and then prime it
• Short sequences replicated called Okazaki fragments
• Primers are removed and gaps filled to complementary bases
• Fragments joined with ligase

Question 19

What are the differences between prokaryotic and eukaryotic DNA replication for Okazaki fragments?

Answer 19

Prokaryotes have 1000 bases while eukaryotes 100-200 bases

Question 20

What is the difference between prokaryotic and eukaryotic DNA replication in terms of speed?

Answer 20

Humans are 3000 bases per minute while bacteria 30,000

Question 21

What is the difference between prokaryotic and eukaryotic DNA replication in terms of polymerase?

Answer 21

Prokaryotes use DNA polymerase III while eukaryotes use lots of different DNA polymerases

Question 22

What three things do chromosomes require to function?

Answer 22

Telomeres, centromeres, origins of replication

Question 23

What do telomeres do?

Answer 23

They are repeated sequences that protect the ends of the chromosome by looping or recruiting protective proteins

Question 24

Why do we need telomere sequences?

Answer 24

• Problem with replication of lagging strand as there is not enough DNA template for primer
• The overhang is not replicated

Question 25

What is the function of telomerase?

Answer 25

• Present in germ cells, stem cells and cancer cells

- It extends the lagging strand so the primer attaches and DNA polymerase completes the replication

Question 26

What does telomerase consists of?

Answer 26

RNA part which is the template for extension, complementary to the telomere repeat

Question 27

What is werner syndrome?

Answer 27

Mutation in WRN gene which produces a protein which has a role in preserving the ends of chromosomes