Molecular basis of DNA transcription

Question 1

What did the 1944 Avery: DNA as genetic material show

Answer 1

Proved that isolating DNA- pure DNA held the information for converting one bacteria to another

Question 2

What did the Crick Watson 1953 experiment show

Answer 2

Hypothesis of DNA being double helix

Question 3

What is “the central dogma of molecular biology”

Answer 3

How information flows between:

gene –> mRNA—> protein

Question 4

What are the 3 sets of experiments which defined the relationships between nuclear DNA and the protein expression?

Answer 4

A messenger exists that transmits information from the nucleus to the cytoplasm

The genetic code is read in triplets

The amino acid sequence is heavily related to the DNA sequence

Question 5

Summarise the process of transcription

Answer 5

Using the 3’-5’ strand as a template a complementary mRNA strand is produced

Question 6

Describe the differences between RNA and DNA

Answer 6

Ribose vs deoxyribose
(At the 2’ for ribose there is a OH whistle for deoxyribose there is a H at the 2’)- THEREFORE THE PHOS BACKBONE IS DIFFERENT

Uracil replaces thymine in RNA

Question 7

How is RNA synthesised

Answer 7

the template strand = 3’-5’ using complimentary base sequence- the genetic code WILL be the same

RNA polymerase synthesises the RNA strand in the 5’-3’ direction

REQUIRES DNA STRANDS TO SEPARATE

Question 8

Describe the role of RNA polymerase

Answer 8

The double helix is being unzipped- H bonds between complimentary bases are broken.

Temporary H bonds between one of the strands and the enzyme

Question 9

What are the 3 main steps of transcription

Answer 9

Initiation
Elongation
Termination

Question 10

Comment on non coding regions for transcription of Ecoli- helping initiate transcription

Answer 10

Promotor region indicates the region to start code for transcription-

A sigma factor associates with a sequence known as the pribow box “TATAAT”
RNA polymerase binds to the sigma factor
The sigma factor dissociates shortly after the transcription initiation starts

Question 11

Describe the elongation part of the the prokaryotic transcription

Answer 11

Due to the helicase function of DNA polymerase a transcription bubble is formed due to H bonds temporarily forming between the RNA and DNA nucleotides
The reading of the DNA sequence on the template, allowing for the synthesis of mRNA in the 5’-3’

Question 12

Describe the termination part of the prokaryotic transmission

Answer 12

Hair pain loop forms- due to the many G-C bonding mRNA folds within itself

This causes the detachment

Question 13

How is eukaryotic transcription any different?

Answer 13

More complex- more proteins

polymerases
initation factors
elongation factor
helper proteins

ADDITIONAL mRNA PROCESSING

Question 14

What are exons?

Answer 14

Expressed regions

regions of DNA which are found in mature mRNA

Question 15

What are introns?

Answer 15

Intervening regions

regions of DNA within a gene unit that are not found in mature mRNA
They do not come for any amino acids in mature protein- as they are not in the RNA sequence

Question 16

What is splicing

Answer 16

Splicing is the removal. of introns and in doing so converts pre mRNA to mRNA

This is necessary because in eukaryotes, only processed mRNA can be exported form he nucleus and so is available for translation
Some hn RNAs can be spliced in different ways thereby resulting in deferent product from the same gene

Question 17

Conment on mRNA processing

Answer 17

Capping- for50-2mation of 7-methylguanosine cap at 5’ end
GTP reacts with 5’ end resulting in addition of 5’ phosphate group

Polyadneylation- adding 50-250 adenosine residues at the end of the RNA chain

Protection at both end

Question 18

Transcriptional Output?

Answer 18

long rRNA (Pol I) 
mRNA- proteins (Pol II)

miRNA- control amount of mRNA that is translated (Pol Ii)

rRNAs short (Pol III) 
tRNA (Pol III )

Question 19

Outline how protein output is regualted

Answer 19

mRNA is degraded- leading to inactive version- this is due to mRNA

Question 20

Outline how transcription is regulated

Answer 20

TFs - enhancers and repressors
Enhancers

Both of the above make contact with the RNA polymerase

Chromatin structure- where the gene expression depends on the cell type

Methylation

Question 21

Explain methylation in context of structure

Answer 21

Gene is switched on—> active chromatin—> unmethylated cytosines—-> acetylated histones

Gene is switched off—> inactive chromatin—> methylated cytosines—> deactylated histones PACKED TOGETHER

eg un methylated cell promoters- active