Nature, Structure and Organisation of the Genetic Material

Chapter 10genome, gene expression; genes as units of inheritance transmission of heritable characteristics; gene regulation, eukaryote chromosomes, alleles

DNA

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.

Organisation of the genetic codeDNA codes for RNA codes for ProteinProteins have many functions

structuralcontractileenzymesimmune systemoxygen-carrying proteinshormonesgene regulatory

Organisation of the genetic codeDNA consists of four different basesProteins consist of 20 different amino acid sub-

unitsNumber of nucleotides in a code

Total number of different codes possible

1 (e.g. T, G, C, A) 4

2 (e.g. AA, AT, GC) 16

3 (e.g. TTA, GCC, AAA) 64

4 (e.g. GGGA, TGCA) 256

Organisation of the genetic codeThe 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.

Organisation of the genetic codee.g.TACAAACAAGCTCCTACTTAC starts translation and codes for metAAA adds the amino acid pheCAA adds the amino acid valGCT adds the amino acid argCCT adds the amino acid glyACT stops translation

Main features of genetic codePieces of information consist of tripletsThe code is non-overlapping. A 12 base piece of

DNA codes for 4 amino acidsThe code is universal (some exceptions)The code is redundantThe information encoded in DNA is the set of

instructions to assemble amino acids into proteinsThe information also includes a START instruction

and STOP instructions.TAC – startATT, ATC and ACT – stop

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.

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.

Mitochondria contain DNAMitochondrial 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).

Gene structureA DNA molecule contains a template strand

and a complementary strand.A new strand of DNA is made using the

template strand.

GenesThe 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:

IntronsExons

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

mRNA Processing

Coding and Flanking RegionsThe coding region is the segment of DNA on the

template strand that is transcribed into mRNAUpstream 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’.

CAT …. ATATA …. TAC…….CTCCGGGGGAT……ACT ….AATAAAA…..Upstream Coding Downstream

Gene Structure