1.2 RNA and protein synthesis

Genes and chromosomes

A gene consists of a unique sequence of DNA nucleotides that code for a polypeptide chain, protein or an RNA molecule.

Genome is the total number of genes in an organism.

RNA – ribonucleic acid

Made up of repeating nucleotides containing.

5 carbon sugar

Phosphate group

1 of 4 bases a, c g, u

When R molecules are synthesises, they temporarily bond with their complementary base pair on a DNA strand.

A bond to U

C bonds to G

Introns and exons

Exons are the coding sequence of DNA translated into polypeptides/proteins.

Introns are non-coding.

Introns can be transcribed into RNA molecules such as RRNA NAD TRNA

Genes are transcribed into Pre-mRNA strands during protein synthesis.

Introns are spliced out forming mature mRNA before being translated.

Called RNA splicing – ensures the synthesis of the correct sequence of amino acids.

Changes to the codes of exons could result in changes to the translated protein.

DNA of prokaryotes contain no introns.

mRNA

Synthesised using DNA as a template during transcription – occurs in the nucleus.

Codes for a specific set of amino acids as every 3 bases codes for one specific amino acid – codons

tRNA 

Contains an anti-codon consisting of 3 bases that are complementary to an mRNA codon.

Also carry a specific amino acid at the opposite end to the anti-codon

Function of tRNA is to place a specific amino acid in the correct sequence in the polypeptide being synthesised.

After it has transferred its specific amino acid, it can collect another from the cytosol.

rRNA

Ribosomes are made up of rRNA and protein molecules.

Ribosomes are the site of translation during protein synthesis.

Ribosomes move across the mRNA strand, translating the mRNA code into a sequence of amino acids.

Amino acids are the nuilding blocks of polypeptides and protein molecules.

Polypeptides consist of a specific sequence of amino acid acids linked together by peptide bond.

Transcription

Occurs in the nucleus.

MRNA is synthesised from a gene on DNA.

RNA polymerase separates the 2 strands of DNA and joins RNA nucleotides together in the 5’ to 3’ direction.

RNA nucleotides are added to the 3’ end of the DNA template.

Translation

Occurs in the ribosomes.

A polypeptide chain is built using a sequence of codons in a mature mRNA molecule.

Stage 1: initiation: use of start codon AUG

Stage 2. Chain elongation: building of amino acid sequence from codon.

Stage 3. Termination: completion of the mRNA sequence with a stop codon

Ribosomes read mature mRNA strands 5’ to 3’.

2 tRNA molecules move to the ribosome’s sites and bond with the mRNA strand determined by their codons.

Ribosomes facilitate the formation of a peptide bond between the 2 amino acids.

Ribosomes moves along the mRNA resulting in a growing polypeptide chain.

TRNA molecules continue to bring amino acids to the ribosome until a stop codon is reached. 

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