2. Gene Expression

Question 1

Importance of bonds that hold DNA strands together?

Answer 1

Hydrogen bonds

1. Easy to break for helicase/RNA polymerase to separate and unwind DNA strands
2. Allow for semi-conservative DNA replication/transcription via complementary base pairing
3. Stabilise the DNA double helix

Question 2

Difference between transcription and translation ?

Answer 2

Transcription in nucleus but translation in cytoplasm
Transcription template is DNA, translation template is mRNA
Transcription product is RNA, translation product is polypeptide
Transcription: phosphodiester bonds formed between nucleotides, translation peptide bonds formed between amino acids
Transcription involves enzyme RNA polymerase while translation involve peptidyl-transferase

Question 3

Why does a codon have three bases?

Answer 3

20 common amino acids present thus need minimum of 20 codes
if 1 base, only 4 codes
if 2 bases, only 16 codes
thus need 3 bases to give 64 codes;;
allow for stop codes to terminate translation;;

Question 4

DNA vs RNA

Answer 4

DNA: double stranded, pentose sugar is deoxyribose, nitrogenous bases can be A thymine C G
RNA: single stranded, pentose sugar is ribose, nitrogenous bases can be A Uracil C G

Question 5

How does tRNA bind to mRNA?

Answer 5

Complementary base pairing via Hydrogen bonds between anticodon on tRNA and codon on mRNA
A-T C-G

Question 6

How is translation terminated ?

Answer 6

1. stop codon (UAA,UGA,UAG) on mRNA reaches in A site
2. protein release factor binds
3. addition of water molecule, hydrolysis of completed polypeptide from tRNA in P site
4. freeing polypeptide from ribosome, translational complex disassembled

Question 7

The flow of genetic information from DNA to polypeptide is an accurate process.
Explain why.
jiayous jiayous 宝贝

Answer 7

1. DNA is transcribed to mRNA by complementary base pairing
2. mRNA codons are non-overlapping triplets that have complementary tRNA anticodons during translation
3. tRNA carries specific amino acid
4. amino-acyl tRNA synthase has active site complementary to a specific amino acid and also recognises the specific anticodon of the tRNA

Question 8

Difference in gene expression between eukaryotes and prokaryotes

Answer 8

In prokaryotes, both transcription and translation can occur simultaneously while in eukaryotes transcription in nucleus and translation in cytoplasm
Post-transcriptional modification does not occur in prokaryotes (no introns)

Question 9

Features that make tRNA adapted to its role in translation?

Answer 9

1. anticodon that is complementary to codon on mRNA
2. 3D structure complementary to active site of aminoacyl tRNA synthetase
3. binding site at 3’ OH end / CCA stem attachment site for specific amino acid corresponding to anticodon

Question 10

How is messenger RNA is synthesized in a eukaryotic cell?

Answer 10

• general transcription factors bind to TATA box
• RNA polymerase recognises and binds to promoter
• the DNA unwinds and separates
• one strand act as template, RNA polymerase assembles dNTPs through complementary base pairing and catalyses formation of phosphodiester bond between 5’ phosphate end of incoming dNTP and 3’ OH end of growing polypeptide
• RNA polymerase moves in 3’ to 5’ direction, RNA strand formed in 5’ to 3’ direction
• termination occur when AAUAAA polyadenylation sequence is transcribed

Question 11

Role of RNA polymerase in synthesis of mRNA

Answer 11

• unwinds double stranded DNA to expose bases for synthesis of mRNA in 5’ to 3’ direction
• catalyses assembly of ribonucleotides which form complementary base pairs with template to form mRNA
• catalyses formation of phosphodiester bonds between 5’ phosphate group of incoming nucleotide and 3’ OH end of growing RNA polynucleotide chain
• carries out proofreading to remove incorrectly inserted ribonucleotide
• reanneals unwound DNA, and dissociate growing RNA chain from the template