Genetics, variation and independence

Prokaryotic DNA

• single-stranded

  • single, circular loop of DNA = nucleoid (not a chromosome)

• 1 or more plasmids

  • small circular DNA molecules

  • can be exchanged between cells

  • usually only contain a few genes

  • plasmids > accessible for proteins required for gene expression and therefore contain genes required often, quickly/ in emergencies

  • contain genes for antibiotic resistance

Eukaryotic DNA

Eukaryotic chromosomes

• human cells have 23 pairs

• haploid (n) = cell/ nucleus containing single copy of each chromosome

• diploid (2n) = cell/ nucleus containing 2 sets of chromosomes/ 2 copies of each chromosome

  • no of chromosomes diff for all organisms

• loci = position of gene on chromosome

Gene

= base sequence of DNA that codes for the amino acid of a polypeptide/ a functional RNA (e.g. tRNA/ rRNA)

• allele = alternative form of a gene

• shape and behaviour of protein molecule depends on exact sequence of these amino acids (i.e. primary structure)

• genes in DNA control protein structure

The triplet code

• each triplet of DNA bases codes for one amino acid, or is start/ stop signal

• 64 possible triplets

  • but only 20 amino acids so multiple codons code for same amino acid

• degenerate: there’s >1 triplet codon for amino acid

• non-overlapping: each base only “read” once (as have stop signals)

• universal: same code for all species

Non-coding DNA

• genome contains many non coding sections of DNA (doesn’t code for amino acids)

• found between genes, as non-coding multiple repeats

  • means contain same base sequence repeated multiple times

• can be found within genes: introns

  • coding exons separated by 1 or more introns

• during transcription, eukaryotic cells transcribe whole gene (all introns and exons) to produce pre-mRNA molecules

• before pre-mRNA exits nucleus, introns removed and exons (coding sections) joined in process = splicing

Proteome

full range of proteins produced by genome

• usually large amount of post-translational modification of proteins (often in Golgi)

• each gene also capable of producing multiple different proteins via alternative splicing

genome = complete set of genes (e.g. in a cell)

• but not every gene expressed in every cell, depends on cell type