Gene Expression U1

Transcription RNA splicing Translation

DNA - Deoxyribose sugar - Bases A, T, G and C - Double stranded RNA - Ribose sugar - Bases A, U, G and C - Single stranded

mRNA rRNA tRNA

Made up from DNA in the nucleus in the first stage of gene expression. Carries a copy of the DNA code from the nucleus to the ribosome in the cytoplasm where it is translated into a protein. Each triplet of bases on the mRNA molecule is called a codon and codes for a specific amino acid.

rRNA molecules combine with proteins to form the ribosome - the organelle responsible for assembling proteins.

tRNA molecules are single but fold due to complementary base pairings. tRNA carries amino acids to the ribosomes to form proteins. tRNA has a sequence of 3 bases which is called the anti-codon. These are complementary to the codons on mRNA.

In the nucleus, RNA polymerase unwinds the double helix and breaks the hydrogen bonds between the complementary base pairs. RNA polymerase adds complementary RNA nucleotides to the 3’ end (A-U & C-G) This synthesises the primary transcript of mRNA.

The primary mRNA transcript is composed of introns and exons and undergoes RNA splicing in the nucleus. Introns are non-coding regions whilst exons are coding regions. Introns are removed and exons are joined together to create the mature mRNA transcript. The order of the exons remains unchanged.

mRNA attaches to the ribosome. Each tRNA molecule has an amino acid attachment site and carries a specific amino acid to the ribosome. tRNA has an anti-codon which is complementary to the codon of mRNA. Translation begins at a start codon and ends at a stop codon. The order of bases determines the order of amino acids. Peptide bonds form between the amino acids creating a polypeptide chain.

Different proteins can be expressed from one gene through alternative RNA splicing. Different mature mRNA transcripts are produced from the same primary transcript depending on which exons are retained. The order of exons does not change.

Polypeptide chains fold to form the 3D shape of a protein. This is held together through hydrogen bonding and other interactions between amino acids. Proteins have a large variety of shapes which determines their function e.g, SHARE. Phenotype (physical appearance) is determined by proteins produced as the result of gene expression. Environmental factors (diet, UV radiation) also influence phenotype.