Gene Expression Part 1

Question 1

What is Crick’s Central Dogma

Answer 1

DNA contains instructions for making a protein, which are copied by RNA. RNA then uses the instructions to make a protein

Question 2

What is a gene? (3)

Answer 2

A stretch of DNA or locus on a chromosome
Unit of inheritance unit of transcription

Question 3

Where does transcription take place

Answer 3

The nucleus

Question 4

What happens in transcription (3)

Answer 4

1. DNA is copied to make small single stranded mRNA molecule
2. mRNA modified rand then exported through nuclear pore to be translated in ribosome
3. Ribosome complex found on RER

Question 5

5 types of RNA

Answer 5

mRNA - transition molecule between gene and protein
tRNA - read triplet code on mRNA
rRNA
miRNA
non-coding mRNA

Question 6

What makes up the highest percentage of RNA species?

Answer 6

Ribosomal RNA (rRNA) - 80% of total RNA species

Question 7

Which type of RNA has the most kinds?

Answer 7

MRNA - 100,000 of kinds as each corresponds to a specific gene - make up around 2% of total RNA species

Question 8

3 stage process of making a polynucleotide or peptide

Answer 8

Initiation, elongation, termination

Question 9

What 3 things are needed to make a polynucleotide or peptide?

Answer 9

Enzyme, activated substrates, a template

Question 10

3 things needed for DNA replication (enzyme, activated substrate, template)

Answer 10

Enzyme : ribosome
Activated substrate : amino acid (combined to make polypeptide chain)
Template : mRNA

Question 11

Describe how mRNA is synthesised in the first step of transcription

Answer 11

1. RNA polymerase binds to template strand and selects NTP with correct base attached
2. RNA polymerase links bases in a chain using phosphodiester bonds

Question 12

What is gene transcription initiated by?

Answer 12

By promoter sequences upstream

Question 13

Why are promoter sequences important in transcription? (2)

Answer 13

Important for RNA polymerase to bind and copy gene in correct 5’ to 3’ direction
Controls the timing of transcription

Question 14

What is the initiation codon

Answer 14

ATG (AUG) - amino acid methionine

Question 15

What is an enhancer in transcription?

Answer 15

Another upstream sequence that controls the location and level of gene expression - act in concert with promoter

Question 16

Describe elongation in transcription

Answer 16

1) Polymerase unwinds small bubble of DNA (3’-5’)
2) promoter binds upstream and mRNA transcript is copied- RNA nucleotides match up (5’-3’).
3) once mRNA copied, it leaves DNA template and leaves nucleus for processing

Question 17

3 processes that happen to turn pre-mRNA to mature mRNA

Answer 17

Capping at 5’ end
Tailing or Polyadenylation at 3’ end
Splicing in middle (removes introns)

Question 18

What is capping?

Answer 18

A 5’ -5’ linkage is created by triphosphate bride to add a cap to 5’

Question 19

What is tailing or polyadenylation at 3’ end and why is it important?

Answer 19

Addition of polyA tail (repeat sequences of base bait A) and protects against degradation and allows mRNA to exit nucleus

Question 20

What is the mRNA made up of?

Answer 20

Exons - coding sequences which are translated

Question 21

Where does translation happen?

Answer 21

Ribosome (Rough ER)

Question 22

What happens to 4-letter DNA code in translation?

Answer 22

GATC (or GAUC) converted to 20-letter protein sequence

Question 23

What molecule reads the genetic code?

Answer 23

Ribosome

Question 24

What are the stop codons of the genetic code? What do they do?

Answer 24

TAA, TAG, TGA
Terminates translation of protein

Question 25

What does the genetic code being degenerate mean?

Answer 25

Some amino acids are coded for by more than one 3bp combination

Question 26

Explain why there are only a few kinds of rRNA and 100,000 kinds of mRNA

Answer 26

Ribosomes have the same function so dont have a lot of variation therefore don’t require many different rRNA
mRNA comes from DNA template so there will be lots of variation - DNA has lots of functions so mRNA has to code for many different proteins

Question 27

Why is the capacity for proof reading in RNA polymerase quite limited compared to DNA polymerase?

Answer 27

If there is an error in DNA it will be passed down to offspring on the germ line and all proteins will be affected
Whereas rRNA only codes for one protein so a mutation will only affect one protein and not be passed down

Question 28

Why does cordycepin (3’deoxyadenosine) inhibit the synthesis of mRNA?

Answer 28

3’ carbon is used to extend nucleotides but if there is no oxygen, the phosphate of another nucleotide cannot attach so dna cant be replicated