THE GENETIC CODE A2 Biology

DNA Deoxyribonucleic acid Carries the instructions for the behaviour and construction of cells Can make a perfect copy of itself

Structure Nucleotide Sugar (deoxyribose) Phosphate Nitrogenous base Complementary strands base pairs A=T C=G

Functions of DNA 1. Replication – in dividing cells 2. Expression - Carrying information for protein synthesis – in all cells (and future generations)

Replication Each daughter cell needs to receive a complete exact copy of the DNA so it will have all the genes required to carry our it’s functions and so will future generations Occurs during interphase before cell division Genome = sum total of all the genes in an organism (60,000-80,000 genes in the human genome)

Semi Conservative Replication The DNA double helix unwinds and each strand acts as a template to make a new strand The result is 2 helices of DNA each made up of half the original DNA and half new DNA (semi conservative)

3’ vs 5’ Anti-parallel nature 3’ to 5’ strands

Steps 1. Replication starts at a specific sequence on the DNA molecule called the replication origin. 2. The enzyme Helicase unwinds and unzips DNA using energy, breaking the hydrogen bonds that join the base pairs, and forming two separate strands (replication fork forms). McGraw Hill animation 1

Replication Fork

3. The new DNA is built up from the four nucleotides (A, C, G and T) that are free floating in the nucleoplasm. The 2 halves of the DNA act as templates. 4. These nucleotides attach themselves to the bases on the old strands by complementary base pairing. 5. The enzyme DNA polymerase joins the new nucleotides to each other by strong covalent bonds, forming the sugar-phosphate backbone. Polymerase can only add nucleotides to the 3’ side (free OH end).

This produces one complete strand (leading strand) and one strand of fragments (lagging strand)

6. RNA Primase creates a short primer to start copying for the lagging strand. 7. DNA Ligase joins the fragments together. McGraw Hill Animation

8. A winding enzyme winds the new strands up to form double helices. 9. The two new molecules are identical to the old molecule. Harvard animation

FYI DNA replication can take a few hours, and in fact this limits the speed of cell division. One reason bacteria can reproduce so fast is that they have a relatively small amount of DNA /student/animations/dna_replication/ind ex.html

Meselson-Stahl Experiment (1958) Originally Watson and Crick proposed the semi- conservative hypothesis. The evidence for the semi-conservative hypothesis came from an experiment performed in 1958 by Meselson and Stahl. They used the bacterium E. coli together with the technique of density gradient centrifugation/(ultra- centrifuge) which separates molecules on the basis of their density. Sumanasinc animation

Possibilities:

1. Initial test to see if it will work: Grow bacteria on medium with normal 14 NH 4 then centrifuge Grow bacteria for many generations on medium with 15 NH 4 then centrifuge 2. Culture bacteria in 15 NH 4. Return heavy DNA bacteria to 14 NH 4 medium for 20 minutes (one generation) The DNA has replicated just once in 14 N medium. The resulting DNA is not heavy or light, but exactly half way between the two. (Thus rules out conservative replication) 3. Grow on 14 NH 4 medium for 40 mins (two generations) After two generations the DNA is either light or half-and-half. (This rules out dispersive replication) The results are all explained by semi- conservative replication. McGraw Hill M-S expt

The Genetic Code The sequence of bases on DNA codes for the sequence of amino acids in proteins. There are 20 different amino acids and only 4 different bases, so the bases are read in groups of 3. This gives 4 3 or 64 combinations, more than enough to code for 20 amino acids. A group of three bases coding for an amino acid is called a codon, and the meaning of each of the 64 codons is called the genetic code.

i.e. If the code consisted of: 1 nucleotide – it could code for 1 amino acid each 2 nucleotides - ? 3 nucleotides - ?