Fundamentals of DNA, RNA and proteins

Question 1

DNA structure

Answer 1

Double stranded
2 antiparallel strands (Both strands of double helix grow in 5’ to 3’ direction, but they grow in opposite directions due to opposite orientation of the sugar molecule in them)
Backbone of DNA= alternating phosphate repeats and deoxyribose sugar (pentose)
nucleic acid base= connected to each sugar
Phosphodiester bonds in sugar phosphate backbone connect 5’ carbon of one nucleotide to 3’carbon of another nucleotide

Question 2

Complementary base pairing

Answer 2

Purine and pyrimidine
G+C=3 Hydrogen bonds
A+T= 2 Hydrogen bonds
So GC pairs are stronger than AT pairs

Question 3

What is denaturation of DNA?

Answer 3

Hydrogen bonds break between bases
But covalent bonds (phosphodiester bonds) holding chain don’t
So DNA 2 strands will break

Question 4

What is annealing of DNA?

Answer 4

When temperature has cooled down

DNA can reform

Question 5

Purines

Answer 5

Double C ring
Bigger
A+G

Question 6

Pyrimidine

Answer 6

Single C ring
Smaller
T+C

Question 7

What is a gene?

Answer 7

A small section of DNA that carries coded information for a sequence of amino acids to build a protein
A gene carries introns (non coding) and exons (coding) parts

Question 8

What is a regulatory sequence?

Answer 8

Segment of a nucleic acid molecule which is capable of increasing or decreasing the expression of specific genes within an organism

Question 9

What are DNA promoters?

Answer 9

Sites on DNA
Where transcription is initiated
Located near transcription site

Question 10

What are enhancers?

Answer 10

Short region of DNA- increase the likelihood that transcription of a particular gene will occur

Question 11

What is regulatory RNA?

Answer 11

Non-coding RNA molecules that play a role in cellular processes such as activation or inhibition processes

Question 12

Chromosomes in human somatic cell

Answer 12

46 chromosomes
23 pairs (22 autosomal and 1 pair of sex chromosomes)

Question 13

Why is it called semi-conservative replication?

Answer 13

New double helix DNA contains one newly made strand and one parental strand
DNA strand copied= template strand

Question 14

2 steps of DNA replication

Answer 14

1. Initiation

2. Elongation

Question 15

Semi-conservative replication- Initiation

Answer 15

1. Topoisomerases (DNA gyrase in bacteria) unwind supercoiling of DNA
2. DNA helicase break hydrogen bonds between bases
3. Where replication beings= Replication fork- open up DNA
4. Single strand contains single-strand binding protein= stabilises single stranded DNA- hold and keep replication forks open. Prevent reversion of double helix forming (H bonds)

Question 16

Semi-conservative replication- Elongation

Answer 16

1. One strand is synthesised continuously (5’ to 3’)= LEADING STRAND
2. One strand is synthesised discontinuously= Okazaki fragments= LAGGING STRAND as there is no 3’ OH at rep fork that a nucleotide can attach to and DNA is always synthesised in 5’ to 3’ direction
3. RNA primers made by primase (to provide free 3’ -OH groups)
4. Rnase H removes primer (Pol I)
5. DNA polymerase continues to synthesise DNA

Question 17

How are the okazaki fragments joined together in lagging strand?

Answer 17

DNA ligase
Synthesises a phopshodieseter bond between 3’ OH of deoxyribose of one okazaki fragment and 5’ phosphate of next of lagging strand

Question 18

What are RNA primers?

Answer 18

Complementary to DNA

Allows DNA rep to occur

Question 19

What is a consequence of removing RNA primers?

Answer 19

Chromosome ends are unreplicated

Therefore chromosome ends become shorter with each cell division

Question 20

Solution of unreplicated chromosome ends

Answer 20

Telomeres added = stretches of DNA with no informational role
Human telomeres lose about 100bp each mitotic division- telomere shortening correlates with ageing
Hence why we have a Hayflick limit- cells can divide up to 50 times as telomere shortens with every cell division
Some immortal cells e.g. cancer cells maintain telomere length by Telomerase
In tumours= telomerase always activated= uncontrollable cell divison= growth of tumour

Question 21

What is the fidelity of DNA replication like?

Answer 21

Low mutation rate

Question 22

Why does DNA replication have a low mutation rate?

Answer 22

Three Mechanisms:
1. Structural differences of purines / pyrimidines
2. Proof reading activity of DNA Polymerase - 3’→5’
DNA Pol I and III have exonuclease activity= enzyme that can remove nucleotides of a strand
3. Mismatch repair

Question 23

What is exonucleolytic proof reading of DNA polymerase?

Answer 23

DNA polymerase has 3’→ 5’ exonuclease activity (backward
Polymerase checks the last nucleotide
	added
	• If correct adds new nucleotide
	• If incorrect chops it off
	- Increases fidelity by 100-1000 times

Question 24

Mismatch repair

Answer 24

Recognise mismatched nucleotide- is removed

Correct nucleotide is inserted into daughter strand

Question 25

Chromosomal mutations

Answer 25

1) Deletions- If too much information is lost, it may be fatal to the organism and may result in early death
2) Duplications- Effect of base duplications depend on location within the chromosome – whether or not duplication resides in coding or noncoding region of DNA
3) Inversions
4) Translocation- 2 parts of 2 chromosomes swapped-Can be caused due to abnormal synapsis event at Meiosis I by incorrect chromosomes coming together