1. DNA Structure and replicationreplication

2. Nucleotides 3 components Sugar Phosphate Organic base

3. A bit more about nucleotides Nitrogen-containing base Pentose sugar Base has H sticking out

4. Bases 4 different bases: –Guanine –Adenine –Thymine –Cytosine Purines = A & G (bigger, 2 rings) Pyrimidines = C, T (smaller, 1 ring)

5. Joining the nucleotides The nucleotides join together Condensation reaction ‘Sugar phosphate backbone’ Polynucleotide strand

6. Joining the strands 2 polynucleotide strands running in opposite directions complimentary base pairing hydrogen bonds A with T C with G

7. The Double Helix A-T 2 hydrogen bonds G-C 3 hydrogen bonds ‘twisted ladder’ 10 base pairs for every complete turn of the helix

8. DNA replication

9. DNA Replication

10. DNA unzips Nuceotides in the cytoplasm attach to the two strands by base-pairing DNA polymerase catalyses the process Each strand acts as a template

11. DNA Replication Meselson and Stahl Grew microbes in 15 N growth medium Then repeatedly on 14 N growth medium DNA was extracted and separated by centrifugation

12. DNA Replication

13. Making a Protein

14. Genetic Code The code is a 3-letter triplet code. Each sequence of 3 bases = 1 amino acid. e.g. ATG=Met, TTT=Lys (called a codon in mRNA) 20 different amino-acids used to make proteins. Triplet codes for 4 3 =64 (spares are repeats, stops, start is always Met)

15. Protein Synthesis The DNA sequence encodes for the primary protein sequence. Cell functions are determined by proteins (enzymes) so DNA determines cell activities by determining protein synthesis. Click to watch an animation

16. Stage 1: Transcription DNA base sequence determines the amino acid sequence. Takes place in nucleus DNA unwinds Complementary copy (by base pairing) of the coding sequence is made from RNA (mRNA) using one strand of DNA as template. Click to watch an animation

17. More about Transcription Synthesis is always 5'->3' (extends from 3'OH). Carried out by RNA polymerase mRNA leaves nucleus. [Splicing out of introns occurs in nucleus] Click to watch an animation

18. Stage 2: Translation mRNA leaves nucleus and attaches to ribosome in CYTOPLASM. Ribosome made of rRNA & protein mRNA binds to small subunit 1 st amino acid is always AUG (start codon) = Met In cytoplasm there are 64 different molecules of tRNA each with a specific triplet anticodon. Click to watch an animation

19. More about Translation Each tRNA has a specific amino acid attached by a specific amino-acyl tRNA synthetase enzyme. Process uses ATP & forms activated molecule to provide energy for peptide bond The anticodon of the correct tRNA then pairs with the codon of the mRNA. This brings two tRNAs together in the ribosome and allows a peptide bond to be formed between the two amino acids by peptidyl transferase. Continues until reach one of the three stop codons (UAA, UAC, UGA). Click to watch an animation

20. Genes & Genomes Human genome 3 x 10 9 bp 3% protein coding 97% other functions e.g. telomeres or unknown function (‘junk DNA’) Section of DNA that codes for a polypeptide is a GENE. In humans  100,000 genes (not all expressed in each cell) Genome=total set of information in one cell. Click to read more