Translation , Transcription , RNA Processing

Question 1

What are the three steps in transcription

Answer 1

1) initiation
2) elongation
3) termination

Question 2

In what direction are new RNA strands made

Answer 2

5’ to 3’ direction

Question 3

What is the +1 site also referred to !

Answer 3

Initiation site

Question 4

Nucleotides that come before the initiation step (+1) are given a ….. number and are said to be ….

Answer 4

• negative

- upstream

Question 5

Nucleotides that come after the initiation site are marked with ….. numbers and are said to be ….

Answer 5

Positive

• downstream

Question 6

Outline the initiation step of transcription in eukaryotes

Answer 6

1) basal transcription factors bind to promoter region , helping RNA polymerase in your cells get foothold of DNA.
2) eukaryote promoters have a special sequence called a ‘ TATA box’. It is recognised by the general transcription factor which eventually allows RNA polymerase to bind. Also this TATA box contains many A’s and T’s which are easier to break.
3) RNA polymerase separates the DNA strand.

Question 7

Outline the elongation step of transcription in eukaryotes

Answer 7

1) RNA polymerase moves along the 3’ to 5’ end of the template strand and adds complementary nucleotides to the 3’ end of the RNA strand.
2) pre mRNA is made - it does not contain thymine . Instead it contains uracil.

Question 8

Outline the termination step of transcription in eukaryotes

Answer 8

• when RNA polymerase reaches the terminator signal , the RNA polymerase stops synthesising mRNA and releases itself from DNA,
• once separated , the two DNA strands come back together and reform the double helix.

Question 9

How is the 5’ end of the pre-mRNA modified ?

Answer 9

A 5’ cap is added onto the 5’ end to protect against degradation.
- it is a modified guanine nucleotide ( methylated guanine ) that is added to the 5’ end.

Question 10

What occurs at the 3’ end of the pre-mRNA transcript?

Answer 10

• at the 3’ end when a sequence called a polyadenylation signal shows up in a RNA molecule during transcription , specific endonucleases cuts RNA into two.
• polyApolymerase adds about 100-200 adenine nucleotides to the cut end forming a polyA tail.

Question 11

What is RNA splicing ?

Answer 11

• introns are recognised and removed.
• the remaining Exons are pasted together.
• there is always one more exon that intron.

Question 12

Describe the structure of prokaryotes ribosomes

Answer 12

• they have 70s ribosomes
• they are made up of 30S and 50s subunits.
• 3rRNAs and 56 proteins

Question 13

Describe the structure of ribosomes in eukaryotes

Answer 13

• 80s ribosomes
• made up of 60s and 40s subunits
• 4rRNAs and 82 proteins

Question 14

What type of RNA is most abundant in a cell?

Answer 14

Ribosomal RNA ( more than 80%)

Question 15

Which rna is more abundant - tRNA or mRNA?

Answer 15

TRNA (15% ) than 2% of mRNA

Question 16

What codon acts as a start codon in translation ?

Answer 16

AUG

Question 17

What codons act as stop codons during termination step of translation!

Answer 17

UAA , UAG, UGA

Question 18

Outline the initiation step of translation in eukaryotes

Answer 18

• the first tRNA molecules carrying methionine amino acid attaches to a small ribosomal unit,
• together they bind to the 5’ of the mRNA by recognising the 5’ GTP cap.

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Question 19

Outline the elongation step in translation in eurkaryotes

Answer 19

• • methionine carrying tRNA starts out in the middle slot of the ribosome called the P site,
• next to it , a fresh codon is exposed in another slot called the A site which will be the landing site for the next tRNA.
• peptide bonds form between the two amino acids in P and A site by ribosomal RNA.
• methionine forms the N terminus of the polypeptide whereas the other amino acid forms the C terminus
• once the peptide bond is formed , the mRNA is pulled onward through the ribosome by exactly one codon.
• this shift allows the first , empty tRNA to drift out via the exit site and exposes a new codon in the A site - so the whole cycle can repeat.

Question 20

Outline the termination step of translation in eukaryotes

Answer 20

Termination occurs when a stop codon ( UAA , UAG,UGA) enters the A site.