Replication of DNA

1. Replication of DNA To be able to explain: • the semi-conservative mechanism of DNA replication, including the role of DNA polymerase. • How DNA acts as a genetic code by controlling the sequence of amino acids in a polypeptide. • That codons for amino acids are triplets of nucleotide bases. • Role of nucleic acids in protein and enzyme synthesis • The structure of RNA.

2. Function of DNA • DNA is the genetic material, and genes are made of DNA. • DNA therefore has two essential functions: replication and expression. • Replication means that the DNA, with all its genes, must be copied every time a cell divides. • Expression means that the genes on DNA must control characteristics. • Characteristics are controlled by genes through the proteins they code for.

3. Replication - DNA Synthesis • DNA is copied, or replicated, before every cell division. • Two new strands are built up by complementary base-pairing onto the two old strands.

4. Replication

5. The enzyme helicase unwinds and unzips DNA The enzyme DNA polymerase joins the new nucleotides to each other DNA replication can takes a few hours. Bacteria can reproduce so fast is that they have a relatively small amount of DNA. In eukaryotes replication is speeded up by taking place at thousands of sites along the DNA simultaneously

7. Original polynucleotide strand New polynucleotide strands SEMICONSERVATIVE REPLICATION

8. Structure in relation to function • sequence of bases so code for information storage • long molecule so large amount of information stored • complementary pairing so information can be replicated • double helix makes molecule stable so prevents code being corrupted • chains held together by weak hydrogen bonds chains so chains can unzip for replication / transcription

9. RNA • RNA is a nucleic acid like DNA, but with 4 differences: • RNA has the sugar ribose instead of deoxyribose • RNA has the base uracil instead of thymine • RNA is usually single stranded, but can fold into 3-dimentional structures, like proteins. • RNA is usually shorter than DNA

10. Messenger RNA (mRNA) • mRNA carries the "message" that codes for a particular protein from the nucleus (where the DNA master copy is) to the cytoplasm (where proteins are synthesised). • It is single stranded and just long enough to contain one gene only. • It has a short lifetime and is degraded soon after it is used.

11. Ribosomal RNA (rRNA) • rRNA, together with proteins, form ribosomes • rRNA is coded for by numerous genes in many different chromosomes. • Ribosomes free in the cytoplasm make proteins for use in the cell. • Ribosomes attached to the RER make proteins for export.

12. Transfer RNA (tRNA) • tRNA is an “adapter” that matches amino acids to their codon. • tRNA is only about 80 nucleotides long, and it folds up by complementary base pairing to form a looped clover-leaf structure.

13. At one end of the molecule there is always the base sequence ACC, where the amino acid binds. On the middle loop there is a triplet nucleotide sequence called the anticodon. There are 64 different tRNA molecules, each with a different anticodon sequence complementary to the 64 different codons. The amino acids are attached to their tRNA molecule by specific aminoacyl tRNA synthase enzymes.

14. Comparison of DNA and RNA Deoxyribose Ribose A T C G A U C G Double helix Single strand Nucleus and cytoplasm In nucleus Very stable Less stable Permanent Temporary One Three – m t and r Constant Variable V large smaller

15. How structure is related to function DNA • Function 1. To pass on hereditary information 2. To control production of proteins 1 Maintains information store 2 Order acts as instruction 2 Allow for a triplet code Allows unzipping for copying 1&2 1 Stability 1&2 Holds a lot of information together 1 Compact shape

16. m RNA Function 1. Translation Can pass through nuclear pore Act as code for amino acids

17. tRNA – pick up their specific amino acids from the cytoplasm

18. Transcription - RNA Synthesis • DNA never leaves the nucleus, but proteins are synthesised in the cytoplasm • A copy of each gene is made to carry the “message” from the nucleus to the cytoplasm. • This copy is mRNA, and the process of copying is called transcription.

20. t RNA Function 1. Amino acid activation For 20 amino acids To pair with codon on m RNA

21. Only one strand of RNA is made. • The DNA stand that is copied is called the template or sense strand because it contains the sequence of bases that codes for a protein. • The other strand is just a complementary copy, and is called the non-template or antisense strand.

22. The new nucleotides are joined to each other by strong covalent phosphodiester bonds by the enzyme RNA polymerase.

23. The introns are cut out and the exons are spliced together by enzymes. • The result is a shorter mature RNA containing only exons. • The introns are broken down. • The mRNA diffuses out of the nucleus through a nuclear pore into the cytoplasm.

24. Translation - Protein Synthesis • A ribosome attaches to the mRNA at an initiation codon (AUG). • A codon is a sequence of three nucleotides • The ribosome encloses two codons.

25. 1. • First the mRNA attaches itself to a ribosome (to the small subunit). • Six bases of the mRNA are exposed. • A complementary tRNA molecule with its attached amino acid (methionine) base pairs via its anticodon UAC with the AUG on the mRNA in the first position P. • Another tRNA base pairs with the other three mRNA bases in the ribosome at position A. • The enzyme peptidyl transferase forms a peptide bond between the two amino acids. • The first tRNA (without its amino acid) leaves the ribosome.

26. Translation 2 The ribosome moves along the mRNA to the next codon (three bases). The second tRNA molecule moves into position P. Another tRNA molecule pairs with the mRNA in position A bringing its amino acid. A growing polypeptide is formed in this way until a stop codon is reached.

27. End of Translation A stop codon on the mRNA is reached and this signals the ribosome to leave the mRNA. A newly synthesised protein is now complete!

29. Translation - animation