DNA

Question 1

Genes are carried on

Answer 1

Chromosomes

Question 2

What are chromosomes made of

Answer 2

DNA and Proteins

Question 3

Genes are made of

Answer 3

DNA

Question 4

What directions to strands run

Answer 4

In antiparallel
Run from 5’ to 3’
- This means that each DNA strand has a complementary sequence to the other strand
- This is important for copying DNA

Question 5

What is the structure of the double helix

Answer 5

It has two grooves

A major and a Minor

Question 6

How are nucleotides held together

Answer 6

Hydrogen bonds
double or triple
These form base pairs

Question 7

How does the genetic code work

Answer 7

Not fully understood- seemingly random when first discovered

Now seeing general patterns

Question 8

In genes you have coding and non coding regions

Answer 8

Coding regions code for a transcript of a protein

Non coding regions - depends on where they lie in relation to the genes

Question 9

DNA Replication

Answer 9

Goes from 5’ to 3’ ( only ever in this direction)

Dangerous–> incorrect base pairing can cause mutations (leads to diseases such as cancers)

Question 10

Templates

Answer 10

Complementary DNA sequences

Question 11

How does DNA replicate

Answer 11

DNA begins at the replication origin
Indicator proteins open up the helix at the replication origin.
Starts in lots of places along the strand and works in two directions outwards–> More efficient

Question 12

How many base pairs are there per second in humans

Answer 12

100 pb/ second

Question 13

DNA Replication steps

Answer 13

DNA polymerase makes a new DNA from the template
Catalyses addition to the 3’ end of the chain
This forms a phosphodiester bond from a phosphoanhydride bond
Pyrophosphate is released and further hydrolysed into 2 inorganic phosphates
DNA polymerase stays bound to the chain after each round of catalysis
DNA moves along the chain and is kept in place by another special protein

Question 14

How, if replication only proceeds from 5’ to 3’, does DNA replication work for both strands

Answer 14

DNA polymerase can only catalyse DNA synthesis in one direction
Therefor the replication form is asymmetrical and this problem is overcome with backstitching
The growing 5’ end is made discontinuously in short stretches and these short sequences are then stitched together.
Continuous strand -> Leading
Discontinuous strand -> Lagging

Question 15

What happens to errors during replication

Answer 15

DNA Polymerase is able to self correct mistakes
Errors occur at 1 in every 10^7 base pairs
–> This can form less stable base pairs which causes DNA mutation

DNA Polymerase can proofread - checks if the correct nucleotide is inserted and if so it continues –> If not it cleaves it out and tries again

The correcting system corrects 99% of mistakes

DNA repair mistakes are called mismatched nucleotides

A complec of mismatch repair proteins recognises the mismatch–> excises the new strand and replaces it

Still do not understand how the old and new pairs are recognised–> thought that maybe the new strands are preferentially nicked

Question 16

How many and what are the functions of DNA polymerase

Answer 16

2 Enzyme functions

5’ to 3’ polymerase

3’ to 5’ nuclease

Question 17

How does back stitching work

Answer 17

Starts with a primer - these are short lengths of RNA which bind and kick start the next stage of the process.
DNA polymerase adds to the RNA primer to start a new DNA fragment.
Once this is fully formed the old RNA primer is erased and replaced by DNA

Question 18

How long are back stitches

Answer 18

About 100 nucleotides

Question 19

Why does DNA grow from 5’ to 3’

Answer 19

Growth from 3’ to 5’ is not energetically favourable

Question 20

What is responsible for the synthesis of the RNA primers

Answer 20

Primase

Question 21

What enzymes work on the lagging strand

Answer 21

Nuclease
Repair polymerase
Ligase

Question 22

What is at the head of the replication machine

Answer 22

Helicase
Proteins at the replication fork form a protein machine
Replicative enzymes are thought to be present in a large multienzyme complex which enables both strands to be synthesised at the same time

What this replicative complex looks like is not entirely known.

Question 23

What do mutations cause and what causes them

Answer 23

Diversity of organisms is due to genetic change
Genetic change is frequently detrimental
For species survival organisms must be genetically stable
DNA changes are the cause of mutations –> e.g. sickle-cell anaemia

Question 24

Sickle cell anaemia

Answer 24

Mutation of Glutanic acid –> a valine in the beta globin protein
This was caused by a single nucleotide change
This is a single amino acid mutation

The protein folds in a different way so does not bind to oxygen as well

Question 25

Diseases

Answer 25

The same protein will be impacted

But the mutation for each individual will be slightly different –> the amino acids changed will be different

Question 26

What has happened to cancer rates in recent years

Answer 26

30% of deaths in Europe and the USA

Older you are the more likely it is in general.

Question 27

Mutations

Outside of synthesis

Answer 27

Energy has to go into the DNA
One type of mutation is a depurination –> you loose a base, you still have the sugar phosphate back bone but one of the bases vanishes. –> This means when you come to reproduce the DNA the logical sequence is lost–> This is a frame shift mutation

Ultraviolet light forms thymine dimers –> if left unrepaired these changes can be potentially fatal

Question 28

DNA Repair

Answer 28

Involves 3 steps
Damaged DNA is recognised and removed by a nuclease forming a gap.
A repair DNA polymerase synthesises new DNA using the old strand as a template
The remaining gap is closed using DNA ligase