MGD 9-10: transcription and translation

Question 1

where does transcription and translation take place within the cell?

Answer 1

transcription in the nucleus, translation in the cytoplasm

DNA –(transcription)–> mRNA –(translation)–> protein

Question 2

what is a gene?

Answer 2

a stretch of DNA with a chromosomal locus

‘unit of inheritance’, ‘unit of transcription’

Question 3

what is the structure of heterochromatin and euchromatin?

Answer 3

heterochromatin: solenoid fibre - genes not expressed
euchromatin: beads on a string - genes expressed

Question 4

what are the different types of RNA?

Answer 4

mRNA
tRNA
rRNA
miRNA
noncodingRNA

Question 5

mRNA and protein structure

Answer 5

mRNA: 5’–>3’
protein: N–>C terminus

Question 6

what is needed in making a DNA (DNA replication)? what is the process?

Answer 6

enzyme: DNA polymerase (attach free bases)
activated substrate: dNTPs (free DNA bases)
template (DNA)
3 stage process: initiation, elongation, termination

Question 7

what is needed in making mRNA?

Answer 7

enzyme: RNA polymerase (attach free bases)
activated substrate: NTPs (free RNA)
template: DNA
3 stage process: initiation, elongation, termination

Question 8

what is required in translation (making a polypeptide)?

Answer 8

enzyme: ribosome (40s & 60s)
activated substrate: AA (to attach to codon)
template: mRNA
3 stage process: initiation, elongation, termination

Question 9

briefly describe what happens in each step of DNA replication

Answer 9

1. initiation: recognition origin of replication
   initiation proteins
   DNA polymerase
2. elongation: 5’ –> 3’ chain growth
3. termination: when replication forks meet

Question 10

briefly describe what happens in each step of transcription

Answer 10

1. initiation: promoter recognition
transcription initiation factors
RNA polymerase
2. elongation: 5'-->3' chain growth
3. termination: sequence dependent

Question 11

where is the transcription initiation promotor sequence for eukaryotes?

Answer 11

TATA box at -30
variety of upstream sequences
complex regulation

Question 12

where is the transcription initiator promotor sequence for prokaryotes

Answer 12

pribnow box at -10
upstream sequences at -35
‘simple’ regulation

Question 13

what is RNA processing and when does it occur / what does it produce?

Answer 13

from pre-mRNA to mature mRNA:

1. capping: at 5’end: protection against degradation (5’-5’ phosphate linkage)
2. tailing or polyadenylation: at 3’ end, polyA tail protection against degradation (AAUAA)
3. splicing: in the middle, removes introns (non-coding DNA) - sequence dependent

Question 14

what are the nucleases used to degrade nucleic acids?

Answer 14

1. endonucleases: breaks WITHIN polynucleotide, non-specific or specific
2. exonuclease: degrades polynucleotide from END, 5’ or 3’ specific

Question 15

how much of each type of RNA do we have?

Answer 15

rRNA - the most (80%) - few kinds but many of each
tRNA - mid (15%) - 100 kinds, many copies of each
mRNA - least (2%) - 100,000 kinds, few copies of each

Question 16

what types of rRNA and ribosomes are present in prokaryotes?

Answer 16

3rRNAs + 56 proteins
30s + 50s subunits
70S ribosomes

Question 17

what types of rRNA and ribosomes are present in eukaryotes?

Answer 17

4rRNAs + 82 proteins
40S + 60S subunits
80S ribosomes

Question 18

what are features of the genetic code?

Answer 18

4 letter DNA TO 20 proteins
read triplet
non-overlapping and no gaps
5' to 3' template produces N to C polypeptide chain extension
adaptor molecule required = tRNA

Question 19

what is the initiation and termination codes of the genetic code?

Answer 19

initiation: AUG
termination: UAA, UAG, UGA

Question 20

structure of tRNA, uncharged and charged?

Answer 20

tRNA - clover
uncharged - anticodon 3 bases
charged: amino acyl tRNA (has AA attached to tRNA)

Question 21

5’ base of anticodon attaches to what of codon?

Answer 21

3’ base

Question 22

where is the wobble position and what is it?

Answer 22

5’ base of anticodon & 3’ base of codon (complementary) (anticodon top, codon bottom)
allowing a single tRNA species to recognise more than 1 codon

Question 23

briefly explain what happens in translaiton

Answer 23

1. initiation: AUG codon recognised by methionyl tRNA ribosome
2. elongation: N to C chain growth, aminoacyl tRNAs
3. termination: stop codons

Question 24

in detail, explain what happens in initiation of translation

Answer 24

1. 5’ cap binds to the 5’ end (protect against degradation)
   initiation factors, 40S subunit ribosome, Met-tRNA (start codon) all bind to the mRNA using GTP
2. the met-tRNA and 40s ribosome will move along the mRNA until AUG is found
3. using ATP, Met-tRNA binds to AUG
4. 60S subunit then bind to the whole complex through attaching to the 40S subunit
5. GDP dissociates

Question 25

In detail, explain what happens in elongation of translation

Answer 25

1. the Met-tRNA sits in the p site (peptide)
2. the next triplet is recognised by the tRNA holding the complimentary AA
3. binding of the next aminoacyl-tRNA t the A site uses GTP
4. both sites are now occupied and a peptide bond forms between the 2 amino acids (peptidyl transferase)
5. the AA stay with the aminoacyl in the A site, the uncharged tRNA now moves out of the P site
6. using a GTP, the ribosome complex moves along the mRNA sequence so that the A site now becomes the P site and so the process continues

Question 26

what is used to form the peptide bond?

Answer 26

peptidyl transferase

Question 27

In detail, explain what happens in termination of translation

Answer 27

keep moving along the mRNA until stop codon reached (UAA, UAG, UGA)
no tRNA are complementary to stop codon
the stop codon is recognised and the peptide and tRNA are hydrolysed (H2O) to release the protein into the cytoplasm