DNA REPLICATION AND PROTEIN SYNTHESIS

The DNA double helix unwinds and unzips, using an enzyme, to make two individual strands of DNA

In the nucleus, there are nucleotides to which two extra phosphate groups have been added These extra phosphates “activate” the nucleotides, enabling them to take part in reactions

The bases of the activated nucleotides pair up with their complementary base on each of the old DNA strands. An enzyme, DNA polymerase links the sugar and innermost phosphate groups of next door nucleotides together. The two extra phosphate groups are broken off and recycled.

DNA polymerase will only link an incoming nucleotide to the growing new chain if it is complementary to the base on the old strand. Very few mistakes are made, perhaps one in every 10 8 base pairs (1:1,000,000,000)

How do we know the mechanism of DNA replication is as described? There are three possible ways that it could actually happen. Conservative Replication, where one completely new double helix is made from an old one. Semi Conservative Replication, where each new molecule would contain one new strand and one old strand.

Dispersive replication, in which each new molecule would be made of old bits and new bits scattered randomly through the molecules.

The Genetic Code A gene is a sequence of bases in DNA that codes for the sequence of amino acids in a polypeptide (protein) The ‘language’ of a gene has only 4 letters - these are? A T C and G

The Genetic Code The ‘language’ of a protein has 20 letters - these are? The 20 different amino acids that make up proteins

The Genetic Code  If 1 base coded for one amino acid in a protein then, only 4 amino acids could be coded for  If 2 bases coded for one amino acid in a protein then, only 16 amino acids could be coded for  If 3 bases coded for one amino acid in a protein then, 64 amino acids could be coded for – more than enough 4 1 = = = 64 The genetic code is a triplet code

The Genetic Code  There are 20 amino acids to be coded for and 64 base triplets to use to code them  Each amino acid has more than one code word – that is the genetic code is degenerate.

The Genetic Code The genetic code is non-overlapping ATTCGAGGCGGT is ‘read’ as ATT CGA GGC GGT Each base is a part of only one triplet.

The Genetic Code is:  A triplet code  Degenerate  Non-overlapping  Universal

Protein synthesis  2 major processes involved Transcription Translation

Transcription  The relevant gene in the DNA in the nucleus is ‘copied’ into a molecule of RNA called mRNA or messenger RNA

Transcription  DNA double helix unzips as  hydrogen bonds between complementary bases break  and the two polynucleotide strands separate A G C T A G C T

Transcription  One strand called the sense strand acts as a template, free RNA nucleotides complementary base pair to the exposed bases on this strand by forming hydrogen bonds  RNA polymerase forms sugar-phosphate bonds between nucleotides A G C T AGCU A G C T

Transcription  The mRNA detaches from the sense strand  The two DNA strands join together by complementary base pairing  The DNA molecules winds back up into a helix A G C T A G C U A G C T

Transcription  The sequences of 3 bases on the mRNA coding for amino acids are called CODONS.  Not all the bases in the DNA code for amino acids so the mRNA just transcribed contains non-coding regions known as INTRONS

Transcription exonintron exon enzymes These introns are removed by enzymes before the mRNA leaves the nucleus This leaves just EXONS or coding regions of mRNA

Transcription exon enzymes intron These introns are removed by enzymes before the mRNA leaves the nucleus This leaves just EXONS or coding regions of mRNA

nucleus Transcription to translation mRNA ribosome Following the removal of introns the mRNA moves out through a nuclear pore and attaches to a ribosome

tRNA GGG aa2 Translation AUG CCC GGG CGC ACA CGU UUC UGA tRNA UAC aa1 start codon anticodon stop codon

tRNA GGG aa2 AUG CCC GGG CGC ACA CGU UUC UGA tRNA UAC aa1 peptide bond formed

tRNA GGG aa2 AUG CCC GGG CGC ACA CGU UUC UGA tRNA UAC aa1 ‘empty’ tRNA leaves to pick up another specific amino acid

tRNA CCC aa3 tRNA GGG aa2 AUG CCC GGG CGC ACA CGU UUC UGA aa1 Ribosome moves along mRNA by one codon

tRNA CCC aa3 tRNA GGG aa2 AUG CCC GGG CGC ACA CGU UUC UGA aa1 peptide bond formed ‘empty’ tRNA leaves to pick up another specific amino acid

tRNA ACU AUG CCC GGG CGC ACA CGU UUC UGA aa2 aa1 This process is repeated until the ribosome reads a stop codon aa4 aa3 aa6 aa5 aa8 aa7