The role of DNA in protein The role of DNA in protein synthesissynthesis

ALBIO9700/2006JK

DNA controls protein DNA controls protein synthesissynthesis

• The shape and behaviour of a protein The shape and behaviour of a protein molecule depends on the exact sequence of molecule depends on the exact sequence of the amino acids (primary structure)the amino acids (primary structure)

• DNA controls protein structure by DNA controls protein structure by determining the exact order in which the determining the exact order in which the amino acids join together when proteins are amino acids join together when proteins are made in a cellmade in a cell

• The triplet code:The triplet code:– The sequence of bases or nucleotides in a DNA The sequence of bases or nucleotides in a DNA

molecule is a code for the sequence of amino molecule is a code for the sequence of amino acids in a polypeptideacids in a polypeptide

– The code is a three-letter or The code is a three-letter or triplettriplet code code– Each sequence of 3 bases stands for 1 amino acidEach sequence of 3 bases stands for 1 amino acid– Sequence read in 1 direction and on 1 strand of Sequence read in 1 direction and on 1 strand of

DNA moleculeDNA molecule

ALBIO9700/2006JK

ALBIO9700/2006JK

• DNA molecule unwinds and unzipsDNA molecule unwinds and unzips• Free activated RNA nucleotides pair up Free activated RNA nucleotides pair up

with exposed bases from one strandwith exposed bases from one strand• Forms sugar-phosphate backboneForms sugar-phosphate backbone• Single stranded molecule – Single stranded molecule – mRNAmRNA

((messenger RNAmessenger RNA) leaves nucleus) leaves nucleus• Free amino acids and Free amino acids and transfer RNAtransfer RNA

((tRNAtRNA) in cytoplasm. Each tRNA ) in cytoplasm. Each tRNA bonds with a particular amino acidbonds with a particular amino acid

• mRNA attaches to ribosome small mRNA attaches to ribosome small subunit and subunit and codoncodon exposed to the exposed to the large subunit (first 3 bases always large subunit (first 3 bases always AUG)AUG)

ALBIO9700/2006JK

Protein synthesisProtein synthesis

• tRNA molecule with complementary tRNA molecule with complementary anticodonanticodon forms hydrogen bonds. This tRNA forms hydrogen bonds. This tRNA has the amino acid attached to ithas the amino acid attached to it

• A second tRNA (bringing a different amino A second tRNA (bringing a different amino acid) bonds with the next 3 exposed bases. acid) bonds with the next 3 exposed bases. The 2 amino acids are held closely together The 2 amino acids are held closely together and peptide bond is formed between them and peptide bond is formed between them (reaction catalysed by peptidyl transferase – (reaction catalysed by peptidyl transferase – found in small subunit of ribosome)found in small subunit of ribosome)

• Ribosome moves along mRNA ‘reading’ next 3 Ribosome moves along mRNA ‘reading’ next 3 bases. Several ribosomes work on the same bases. Several ribosomes work on the same mRNA strand at the same time mRNA strand at the same time ((polyribosomespolyribosomes). First tRNA leaves as the ). First tRNA leaves as the third one joins to the second onethird one joins to the second one

• Polypeptide chain continues to grow until a Polypeptide chain continues to grow until a ‘stop’ codon (UAA, UAG or UGA) is exposed on ‘stop’ codon (UAA, UAG or UGA) is exposed on the ribosomethe ribosome

ALBIO9700/2006JK

ALBIO9700/2006JK

ALBIO9700/2006JK

Genes and genomeGenes and genome

• GeneGene – a part of a DNA molecule which – a part of a DNA molecule which codes for just one polypeptide. One codes for just one polypeptide. One DNA molecule contains many genes.DNA molecule contains many genes.

• GenomeGenome – total set of genes in a – total set of genes in a cell/total information in one cell/total information in one cell/genetic code of organismcell/genetic code of organism

ALBIO9700/2006JK

ALBIO9700/2006JK

ALBIO9700/2006JK

ALBIO9700/2006JK