Gene Expression and Protein Synthesis Higher Human Biology Subtopic 3 (b)

Learning Intentions Describe the structure of tRNA Describe the process of Translation Discuss alternative splicing and modification

Translation Translation is the synthesis of a protein in the form of a polypeptide chain. The strand of mRNA is divided into triplet bases called codons. Each codon contains the code for one amino acid – the building block of a protein.

Transfer RNA - tRNA tRNA is found in the cells cytoplasm Its function is to pick up a SPECIFIC AMINO ACID and bring it to the mRNA on the ribosome.

The Structure of tRNA Singled stranded, folded structure There is an ANTI-CODON at one end There is an AMINO ACID BINDING SITE the other

Anti-codons The anticodon is composed of a triplet of bases which are complementary to a codon on the mRNA .

THE CODE….. There are 64 combinations of 3 bases. The mRNA codon AUG is always the start codon in a sequence There are 3 mRNA codons that don’t code for an amino acid – these are the stop codons.

Stage 3 : Tranlsation

The Ribosoms Small structures that contain rRNA and enzymes needed for protein synthesis A ribosome has 3 sites within it. P site : holds the tRNA carrying the growing chain A site : holds the tRNA carrying the next amino acid to join the chain E site : discharges a tRNA once the amino acid is part of the chain.

The Stages………… 1 – mRNA moves to ribosome and binds at 5’ end 2 – AUG codon gets into place at the P site 3 – the tRNA with complimentary anticodon is held in place by hydrogen bonds 4 – the next complimentary tRNA joins at the A site 5 – strong peptide bonds form between the amino acids

The Stages………… 6 – the ribosome moves along opening up the A site so another complimentary tRNA can join 7 – the polypeptide chain continues to grow and the tRNA discharge at the E site 8 – a stop codon on the mRNA is reached which signals the end of the protein chain as there is no matching tRNA anti-codon 9 – a release factor occupies the A site and frees the polypeptide chain from the ribosome translation animation

Overview of gene expression Nucleus A G A G G T T G A C G A T C T C C A A C T G C T DNA Transcription Overview of gene expression mRNA U C U C C A A C U G C codon ser pro thr ala Ribosome Translation Protein

Polyribosomes This is where several ribosomes attach to an mRNA strand.

Definitions Term Definition Transcription Copying of the DNA code onto the mRNA Translation Converting the information given on the mRNA into a sequence of amino acids Triplet Sequence of bases specifying one amino acid Codon Triplet of bases on mRNA Anticodon Triplet of bases on tRNA

One Gene Can Code for Many Proteins ALTERNATIVE SPLICING + POST TRANSLATIONAL MODIFICATION OHow can one gene How can this be? Hmmm..

CRACK THE CODE CODON: UUU = CODON: CCA = CODON: CAU = CODON: AAU =

CRACK THE CODE What is the CODON for: Leucine Tyrosine Lysine Stop

Alternative Splicing One gene can code for different proteins When the primary transcript is spliced after transcription, alternative segments of RNS may be treated as introns and exons. This means that one primary transcript can lead to different mature mRNA strands……………….. ……………..This leads to different polypeptide chains. One gene can code for different proteins

Primary transcript Different proteins are formed

Post Translational Modification After translation the polypeptide chain can be modified to produce many different proteins by: Cleavage Molecular addition

1. Cleavage The polypeptide chain can be cut and combined with other poly peptides.

2. Molecular addition A PHOSPHATE or CARBOHYDRATE group can be added to the polypeptide chain.

Gene Expression Your PHENOTYPE are determined by the proteins you produce. The proteins you produce are controlled by the gene being expressed (switched on) Only a fraction of genes in a cell are expressed. GENE EXPRESSED PROTEIN PHENOTYPE

Gene activity in different specialised cells Cell type Activity of genes coding for production of:   antibodies lysosome enzymes enzymes for synthesis of urea pepsin phagocyte off on liver stomach lining lymphocyte

Gene Expression Gene expression can be influenced by intra cellular and extra cellular environmental factors. It is controlled by the regulation of transcription and translation

Learning Intentions Describe the structure of tRNA Describe the process of Translation Discuss alternative splicing and modification