3.5: Transcription and translation
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Compare DNA and RNA.
Feature -- DNA -- RNA
- Shape -- Double helix (two strands) -- Usually single strand
- Sugar -- Deoxyribose sugar -- Ribose sugar
- Bases -- Adenine, thymine, cytosine, guanine -- Adenine, uracil, cytosine, guanine.
Outline what is happening at each stage of transcription.
- In the nucleus, the section of DNA for the gene to be expressed unwinds and unzips.
- The template strand of DNA is transcribed with complementary ribose nucleotides and uracil instead of thymine. RNA polymerase separates the two DNA strands and hooks the RNA nucleotides together as they base-pair along the DNA template.
- A single strand of mRNA forms.
- mRNA leaves the nucleus.
What is a codon?
A codon is the basic unit of the genetic code. IT is a 3-nucleotide sequence of DNA or RNA that specifies a particular amino acid or termination signal, e.g. codon AGC refers to the three bases adenine-guanine-cytosine which form the amino acid serine.
Outline what is happening at each stage of translation.
- Initiation stage: tRNA for starter codon AUG brings amino acid methionine to mRNA chain. They are held together by hydrogen bonds. The ribosome is then constructed from its parts around the point.
- Elongation stage: amino acids are added one after the other after methionine. Enzymes form peptide bonds between the amino acids. mRNA moves along the ribosome.
- Termination: a stop codon reaches the attachment site of the ribosome and a release factor causes translation to stop. The release factor causes hydrolysis and the addition of water instead of another amino acid to the polypeptide chain. The polypeptide chain is freed and the ribosome dissociates from the mRNA.
Discuss the relationship between genes and the products of gene expression.
The relationship between a gene and the produce produced by gene expression is the 'one gene-one polypeptide' hypothesis. This means that each gene codes for one polypeptide. A protein that consists of two or more polypeptide chains, e.g. haemoglobin will have a gene for each polypeptide cahin (alpha haemoglobin and beta haemoglobin). However, some genes code for RNA molecules and are not translated into proteins and some sections of DNA called introns (used to be called 'junk' DNA) are non-coding and have no corresponding polypeptide.
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