Genetic Information, Variation and Relationships Between Organisms

AS BIOLOGY NOTES UNIT 4

Genetic Information, Variation and Relationships Between Organisms

DNA, Genes and Chromosomes

DNA

In eukaryotic cells, DNA molecules are very long, linear and are associated with proteins called histones.  The DNA coils and wraps around histone proteins, which are then coiled again, very tightly, to make a compact chromosome.  This allows the chromosomes to reside in the nucleus of cells.

DNA can also be found in chloroplasts and mitochondria, however it is shorter than DNA in the nucleus (as it doesn't code for as many proteins), it is also circular and not associated with histone proteins.  It is found in these locations so that enzymes involved in photosynthesis and respiration can be made directly where they are needed.

In prokaryotes, the DNA is organised differently.  It is shorter, circular and not wound around histones.  It condenses to fit in cells by supercoiling.

Genes

Chromosomes contain genes.  A gene is a sequence of bases that code for a polypeptide chain or functional RNA.  If a gene doesn't code for a polypeptide, it will code for functional RNA.  These are RNA molecules, other than mRNA, that perform specific tasks during protein synthesis, e.g. tRNA, which carries amino acids to the mRNA to form the polypeptide chain, and rRNA, which forms part of the ribosome.

Genome:The complete set of genes in the cell.

Proteome:  The full range of proteins it can provide

Non-coding DNA

Much of the DNA on chromosomes does not code for polypeptides or functional RNA.  Even genes that do code for polypeptides contain sections that don't code for any amino acids, they are called introns.  There can be several introns within a gene and their purpose is not known.  They are removed during protein synthesis so they don't affect the amino acid order in a process called splicing.

DNA sequence

I_____EXON_______INTRON_______EXON_______INTRON_______EXON____I

Pre mRNA sequence

I_____EXON_______INTRON_______EXON_______INTRON_______EXON____I

SPLICING

mRNA sequence

I_____EXON_________EXON_________EXON____I

The part of the genes that do code for amino acids are called exons.  Prokaryotes have no introns.

Eukaryotic DNA also has regions of multiple repeats outside of genes.  These are DNA sequences that repeat over and over, e.g. CCTTCCTTCCTT.  They don't code for amino acids and are called non-coding multiple repeats.

Chromosomes

In human cells there are 46 chromosomes that are arranged in 23 pairs.  One half of each pair comes from the egg cell and the other half comes from the sperm cell.  Pairs of matching chromosomes are known as homologous.  In a homologous pair, both chromosomes are the same size and have the same genes located in the same positions, although they could have different alleles.

Alleles are different versions of the same gene, e.g. eye colour, where the alleles are brown and blue, so the sequence of bases will be slightly different in each gene.

Alleles coding…