Transmission of genetic variation

Question 1

Define central dogma

Answer 1

Theory stating that genetic information flows only in 1 direction, from DNA to RNA, to protein, or RNA directly to protein

Question 2

When is transcription initiated?

Answer 2

When RNA polymerase binds to a DNA template at a promoter sequence

Question 3

What happens during the elongation process?

Answer 3

• DNA double helix unwinds
• RNA polymerase reads template DNA strand and adds nucleotides to 3’ end of growing RNA transcript

This terminates when RNA polymerase reaches a stop codon, and the mRNA transcript and RNA polymerase are released from the complex

Question 4

What does mRNA contain?

Answer 4

• 5’ UTR
• Exons
• 3’ UTR
• Cap
• Polyadenylated tail

Question 5

How is the sense strand related to mRNA?

Answer 5

Has same sequence as mRNA molecule, travels from 5’-3’

Question 6

How is the antisense strand related to mRNA?

Answer 6

Antisense strand used as template to generate this identical mRNA strand, travels from 3’-5’

Question 7

How is mRNA modified after transcription?

Answer 7

• 5’ capping
  
  • Protect 5’ end from degradation
  • Facilitate transport into cytoplasm
  • Enhance translation
• Introns spliced
• 3’ polyadenylation

Question 8

What is the triplet code?

Answer 8

• RNA read in 3s = Codons
• 64 possible combinations of 4 bases
• Codon → Specific amino acid
• 20 amino acids
• Degenrate - >1 codon can code for same amino acid

Question 9

Summarise the factors involved in translation

Answer 9

• Cytoplasm
• mRNA → Protein
• tRNA carries amino acids
• Specific to codon
• Needs initiation factors
• Needs elongation factors
• Important sites on UTR

Question 10

Describe translation initiation

Answer 10

• mRNA binds to small subunit
  
  • Ribosome binding site sequence binds to complementary sequence in RNA molecules in small subunit of ribosome, with help of protein initiation factors
• Initiator aminoacyl tRNA binds to start codon
• Large subunit of ribosome binds, completes ribosome assembly, translation begins

Question 11

Describe translation elongation

Answer 11

• Incoming aminoacyl tRNA - New tRNA moves into A site, where anticodon base pairs with mRNA codon
• Peptide bond forms - Amino acid attached to tRNA in P site transferred to tRNA in A site
• Translocation - mRNA ratcheted thorugh ribosome by elongation factors. tRNA attached to polypeptide chain moves into P site, A site empty
• Incoming aminoacyl tRNA - New tRNA moves into A site, anticodon base pairs with mRNA codon (again)
• Peptide bond formation - Polypeptide chain attached to tRNA in P site is transferred to aminoacyl tRNA in A site.
• Translocation - mRNA ratcheted through ribosome again, tRNA atached to polypeptide chain moves into P site. Empty tRNA from P site moves to E site, tRNA ejected, A site empty again

Question 12

Describe translation termination

Answer 12

• Release factor binds to stop codon - When translocation exposes stop codon, release factor fills A site. Release factor breaks bond linking tRNA in P site to polypeptide chain
• Polypeptide released - Hydrolysis reaction frees polypeptide, released from ribosome. Empty tRNAs released either along with polypeptide or when ribosome units separate
• Ribosome units separate - This is when tRNA can also be release, 2 ribosomal subunits dissociate. Subunits ready to attach to start codon of another message and restart translation

Question 13

Describe the stages of the cell cycle

Answer 13

• G1 - Cellular contents (bar chromosomes) duplicate, cell makes variety of proteins needed for DNA replication
• S - Each of 46 chromosomes duplicated by cell, each chromosome now consits of 2 sister chromatids (identical)
• G2 - Cell double checks duplicatied chromosomes for error, can repair if so. Synthesis of proteins, especially microtubules
• Mitosis
• Cytokinesis
• G0 - Cell cycle arrests

Question 14

At which phase of the cell cycle do transcription and translation occur?

Answer 14

Interphase