Nature, Structure and Organisation of the Genetic M aterial

1. Nature, Structure and Organisation of the Genetic Material • genome, gene expression; genes as units of inheritance • transmission of heritable characteristics; gene regulation, eukaryote chromosomes, alleles Chapter 10

2. DNA

3. Where is the DNA in a cell? • In human cells DNA is found in the nucleus (all of the chromosomal DNA) • The mitochondria of animals and plants contains circular mitochondrial DNA.

4. Organisation of the genetic code • DNA codes for RNA codes for Protein • Proteins have many functions • structural • contractile • enzymes • immune system • oxygen-carrying proteins • hormones • gene regulatory

5. Organisation of the genetic code • DNA consists of four different bases • Proteins consist of 20 different amino acid sub-units

6. Organisation of the genetic code • The genetic code is a triplet code (3 bases) . • Therefore 64 combinations but only 20 amino acids. • It is a redundant code: there is more than one codon for an amino acid.

7. Organisation of the genetic code e.g. TACAAACAAGCTCCTACT TAC starts translation and codes for met AAA adds the amino acid phe CAA adds the amino acid val GCT adds the amino acid arg CCT adds the amino acid gly ACT stops translation

8. Main features of genetic code • Pieces of information consist of triplets • The code is non-overlapping. A 12 base piece of DNA codes for 4 amino acids • The code is universal (some exceptions) • The code is redundant • The information encoded in DNA is the set of instructions to assemble amino acids into proteins • The information also includes a START instruction and STOP instructions. • TAC – start • ATT, ATC and ACT – stop

9. How much DNA in a chromosome? • On average, each chromosome has about 120 x 106 base pairs. • This also includes several thousand genes on each chromosome.

10. Human genes and the human genome • Human genes are commonly around 27,000 base pairs long and some can be up to 2 x 106 base pairs long. • The human genome contains approximately 24,000 genes.

11. Mitochondria contain DNA • Mitochondrial DNA (mtDNA) is found in the mitochondria. • Each mitochondrion is estimated to contain 2-10 mtDNA copies. • Mitochondrial DNA is inherited from the mother. • Each double-stranded circular mtDNA molecule consists of 15,000-17,000 base pairs. • mtDNA codes for 37 genes, 13 are for proteins (polypeptides), 22 are for transfer RNA (tRNA) and two are for the small and large subunits of ribosomal RNA(rRNA).

13. Gene structure • A DNA molecule contains a template strand and a complementary strand. • A new strand of DNA is made using the template strand.

14. Genes • The DNA strand coding for the gene is called the “template strand” • This mRNA is known as the Precursor mRNA (pre-mRNA) • DNA sequences which code for genes have two parts: • Introns • Exons • Introns are sequences of DNA which do not code for the final product. These are sometimes called ‘junk DNA’. • Exons are parts of the DNA sequence which code for the final mRNA product

15. mRNA Processing

16. Coding and Flanking Regions • The coding region is the segment of DNA on the template strand that is transcribed into mRNA • Upstream of the coding region are particular coding sequences rich in A and T called the ‘TATA’ box which occurs on the anti-sense strand. Another region further upstream is called the CAT or CAAT box. The purpose of these regions is regulation and control of transcription. This region is called the ‘promoter’. • Downstream about 20 base is the sequence AATAAA. This region includes an ‘end transcription signal’. Upstream Coding Downstream CAT …. ATATA …. TAC…….CTCCGGGGGAT……ACT ….AATAAAA…..

17. Gene Structure