3.4.2 DNA AND PROTEIN SYNTHESIS Flashcards
Why does transcription occur?
To make mRNA
Stage one of TC:
DNA helicase breaks the hydrogen bonds between complementary base pairings in DNA, causing the two strands to separate, exposing the nucleotide bases in that region
Stage two of TC:
The nucleotide bases on one of the two DNA strands known as the template strand pair with their complementary bases- RNA nucleotides attracted to exposed bases according to the base pairings rule, e.g. C-G
Stage three of TC:
Enzyme RNA polymerase then moves along the strand and joins the nucleotides together to form a pre-mRNA molecule
Stage four of TC:
- in prokaryotic cells, mRNA is produced directly from DNA
- in eukaryotic cells, pre- mRNA is formed from DNA- this needs to be spliced to remove introns, then leaves the nucleus via nuclear pores and mRNA then becomes attracted to ribosomes
Why does translation take place?
Forms polypeptides
Stage one of TL:
mRNA attaches to ribosomes on the rough endoplasmic reticulum
Stage two of TL:
tRNA anticodons bind to complementary mRNA codons
Stage three of TL:
tRNA brings a specific amino acid
Stage four of TL:
Amino acids join by a peptide bond
Stage five of TL:
Amino acids form peptides bond by use of ATP and an enzyme
stage 6 of TL:
tRNA is released after amino acids joins to polypeptide
Stage 7 of TL:
The ribosome moves along mRNA to form an polypeptide
What determines the sequence of codons on mRNA in translation?
DNA sequence of the triplets which make up a gene determines the sequence of codons on mRNA
When does translation end?
Synthesis occurs until a ribosome reachers a stop-codon, at this point, the ribosome mRNA and the last tRNA molecule all separate and the polypeptide chain is complete
