2.7 : DNA Replication, Transcription, Translation

Question 1

what is replication? In what phase does this take place?

Answer 1

• how DNA copies itself, which enter each daughter cell formed in cell division
• interphase

Question 2

what is semi-conservative replication? how does it work?

Answer 2

• first step : unwinding of the two strands in the DNA double helix –> helicase unwinds both of the strands at one region –> breaks hydrogen bonds in that region –> strands are separated –> unpaired nucleotides are exposed and surrounded by a pool of free floating nucleotides (A,T,G,C)
• -> the complementary nucleotides of the exposed bases line up opposite to each base –> hydrogen bonds form between the complementary bases to hold them in place –> condensation reaction links sugar & phosphate groups of adjacent nucelotides (phosphodiester bond)
• -> two strands are formed identical to the original strand (as DNA double helix)

Question 3

what enzyme is used in the process of semi-conservative replication? what role does it have?

Answer 3

• DNA polymerase

- ‘proof reading’ role : any mistakes between complementary bases is corrected.

Question 4

when a new pair of strands form a DNA double helix (end of semi-conservative replication) where do both strands come from?

Answer 4

• -> one strand : comes from original chromosome (template strand)
• ->other strand : newly synthesised strand

Question 5

why is the process referred to as ‘semi-conservative’ replication?

Answer 5

as one strand, that acts as the template strand, comes from the parent (original) chromosome, so both strands act as templates to form two different synthesised strands (both daughter cells have one original strand)

Question 6

what is the evidence for semi conservative replication?

Answer 6

experiment conducted –> grew e-coli & added radioactive isotopes (in a medium where a food store was available = nitrogen) –> synchronised bacterial divisions occurred
–> at the end of the experiments the DNA with heavy’ isotope had 50% of DNA with ‘intermediate’ isotope & 50% with ‘light’ isotope –> showing that semi conservative replication had occurred as only half of the cells were labelled with the DNA originally present –> showing newly synthesised ones were formed & formed new DNA double helices.

Question 7

what are the steps in protein synthesis? (of forming a protein)

Answer 7

• transcription

- translation

Question 8

what occurs in transcription? what is the enzyme involved in this process & what does it do?

Answer 8

(first step in protein synthesis)

• enzyme : RNA polymerase : catalyses the formation of complementary copy of the genetic code of a gene (takes form of mRNA molecule)
• DNA double helix of a particular gene unwinds (helicase) hydrogen bonds of the complementary bases are broken –> free nucleotides in the surrounding pool of nucleotides are added to the 3’ of growing RNA molecule –> coding strand of DNA acts as a template –> single stranded RNA molecule formed by complementary base pairing (A,U,G,C) –> mRNA strand leaves nucleus through pores in nuclear membrane –> goes to cytoplasm –> then ribosome –> info is ‘read’ & translation begins.

Question 9

what is the genetic code? what does this dictate?

Answer 9

(genetic information in DNA lies in the sequence of nitrogenous bases)

• dictates the order in which specific amino acids are assembled & combined
• genetic information = code = lies in coding strand –> codon

Question 10

what occurs in protein synthesis? what does tRNA do? what role does enzyme specificity play in this?

Answer 10

• after transcription –> the mRNA strand enters the ribosome, where tRNA is present.
• tRNA translates codons in the mRNA strand into amino acids attached to specific tRNA-amino acid complexes (specific to codon)
• tRNA contains anticodons which are complementary to the codons on the mRNA strands
• amino acids are attached to tRNA (forming tRNA-amino acid complex) via enzyme reaction & ATP.
• enzyme specificity ensures that the correct amino acids are used in the right sequence (connected to correct codons)

Question 11

what occurs in translation?

Answer 11

(last stage of protein synthesis)

• the protein chain is assembled, 1 amino acid residue at a time.
• -> a ribosomal subunit moves along the mRNA strand (forming a polysome = mRNA, ribosome, growing) –> ‘reads’ the strand, begins at ‘start’ codon
• -> for each mRNA codon –> a tRNA-amino acid complex with the complementary anticodon –> anticodon temporarily binds with its codon (held by hydrogen bonds) –> occurs with many codons and specific tRNA-amino acid complexes –> adjacent complexes form peptide linkages –> frees the first tRNA, goes into cytoplasm (available for reuse) –> this process continues until the ‘stop’ codon on the mRNA strand is reached –> a polypeptide chain is formed (one or more polypeptides = protein - two polypeptide chains held by hydrogen bonds)