Lec 10: Protein Synthesis

Question 1

what is the purpose of transcription? why does it occur in the nucleus?

Answer 1

Transcription is the process by which DNA is copied (transcribed) to mRNA, which carries the information needed for protein synthesis

it occurs in the nucleus because this is where the DNA is located

Question 2

what is the template strand? state the other name given to this polymer

Answer 2

single strand of mRNA

antisense strand/non conding strand

Question 3

what is the cap for Pre-mRNa composed of?

Answer 3

phosphorylated 7 ‐methyl guanosine

Question 4

what adds the cap to pre-mRNA. where is the cap place on the mRNA molecule?

Answer 4

guanyltransferase adds the cap to the 5’ end of the mRNA

Question 5

what is the importance of pre-mRNA capping?

Answer 5

– Ensures mRNA is exported out of nucleus

– Blocks degradation of mRNA by 5’ exonucleases

– Promotes translation

Question 6

what is the polyadenylation of Pre-mRNA and why is this important in transcription

Answer 6

this is where appr.200 adenosine residues are added to pre-mRNA

So:

• pre-mRNA is cleaved by an endonuclease near a signal AAUAAA sequence at the 3’ end
• the adenosine residues are added at the cleavage site by poly‐A polymerase

importance:

• poly‐A‐tail protects the mRNA from degradation by 3’ exonucleases
• aids in termination of transcription
• ensures export from the nucleus

Question 7

what are the difference between exons and introns?

Answer 7

*EXtra special & *IN the way

exons code for proteins

introns do not code for proteins so need to be spliced out to produce the final mRNA

Question 8

what is alternative splicing?

Answer 8

this is where the pre-mRNA sequence is spliced to produce different proteins from the same gene

Question 9

what is mature mRNA

Answer 9

mRNA that has been spliced and processed

and is ready for translation

Question 10

what does mature mRNA consists of?

Answer 10

Mature mRNA consists of:

• 5’ cap
• 5’ UTR (untranslated region)
• Coding Region (to be translated into a protein)
• 3’ UTR (untranslated region)
• Poly(A) tail

Question 11

what is the name given to 3 bases e.g ACG

Answer 11

DNA base triplet

Question 12

what is the name given to base triplets in mRNA

Answer 12

codons

Question 13

how many mRNA codons are there?

Answer 13

64 (4³)

Question 14

what is meant by degenerate with respect to the genetic code?

Answer 14

More than one codon codes for each of the 20 amino acids

Question 15

why are the first two codons most crucial?

Answer 15

gives tolerance against mutation

Question 16

state the bases that code for the START and STOP codons

Answer 16

– START codon: AUG (methionine)

– STOP codons: UAG, UAA, UGA

Question 17

how does a mutation in the amino acid sequence affect haemoglobin?

Answer 17

change in amino acid sequence causes haemoglobin to crystallise when oxygen levels are low, causing the sickle shape which gets stuck in small blood vessels

Question 18

what is the consequence of sickle cell anemia in terms of red blood cell function?

Answer 18

compared to normal red blood cells, sickle red blood cells can not move as easily through the blood stream

resulting in;

– Downstream tissue ischaemia, causing pain and infarction

– In severe cases, organ damage and ischaemic stroke

Question 19

in sub-saharan africa what does sickle cell anemia provide protection against

Answer 19

malarial parasite

Question 20

what is meant by frameshift mutation?

Answer 20

small deletions or insertions into a DNA sequence can resulting in significant changes of protein sequence

Question 21

what is translation? where does it occur?

Answer 21

genetic code carried by mRNA is decoded to produce the specific sequence of amino acids in a polypeptide chain

in the ribosome

Question 22

what is a ribosome composed up of?

Answer 22

60S subunit and smaller 40S subunit

Question 23

what name is given to the triplet bases complementary to the codon on a mRNA molecule

Answer 23

anticodon

Question 24

what is a aminoacyl tRNA molecule?

Answer 24

a tRNA molecule bonded to an amino acid

Question 25

what is a deacylated or uncharged tRNA molecule?

Answer 25

tRNA molecule that has had its amino acid removed

Question 26

what is a peptidyl tRNA?

Answer 26

a tRNA molecule bound to a growing polypeptide chain

Question 27

what is initiation?

Answer 27

binding of ribosome to 5’ end of mRNA and hydrogen binding of the anticodon of an aminoacylated tRNA carrying methionine on the AUG start codon

Question 28

what is elongation?

Answer 28

the addition of further amino acids to the growing polypeptide chain brought by corresponding aminoacylated tRNAs.

Question 29

what is the function of peptidyl transferase?

Answer 29

creates covalent peptide bonds between amino acids

Question 30

what is termination?

Answer 30

when the stop codon (UAG, UAA, UGA) is reached and the peptide and ribosomal subunits are released

Question 31

where do proteins synthesised on free ribosomes end up?

Answer 31

cytoplasm

Question 32

where do proteins synthesised on ribosomes attached to RER end up?

Answer 32

secreted our of the cell

Question 33

how are proteins secreted?

Answer 33

proteins are incoporated into vesicles

for transport to the golgi apparatus

post translational modification of proteins occurs in the goldi cavaties e.g glycosylation

modified proteins traverses the GA and is packaged into secretory vesicles

these vesicles move to the cell membrane, fuse with it and expel their contents into the extracellular space (excytosis)