DNA, Genes and Chromosomes

How do eukaryotic cells store DNA?

• Eukaryotic cells contain linear DNA molecules that exist as chromosomes- thread-like structures, each made up of one long molecule of DNA. Chromosomes are found in the nucleus

• The DNA is really long, so it needs to be wound up so it fits in the nucleus

• It is wound up around proteins called histones which also help to support the DNA

• This is then coiled up very tightly to make a compact chromosome

How do prokaryotic cells store DNA?

• Prokaryotes also carry DNA as chromosomes- but the DNA molecules are shorter and circular.

• The DNA isn’t wound around histones- it condenses to fit in the cell by supercoiling

• Mitochondria and chloroplasts in eukaryotic cells have DNA similar to prokaryotic DNA

What is a gene?

A gene is a sequence of DNA bases that codes for either a polypeptide or functional RNA.

How does a gene code for a polypeptide?

• Different polypeptides have different numbers and order of amino acids. It is the order of the bases in a gene that determines the order of amino acids in a particular polypeptide

• Each amino acid is coded for by a sequence of three bases in a gene called a triplet

What is a genome and a proteome?

• A cell’s genome is the complete set of genes in a cell

• A cell’s proteome is the full range of proteins that the cell is able to produce.

What are introns and exons?

• Some genes don’t code for polypeptides at all- they code for functional RNA

• In eukaryotic DNA, genes that do code for polypeptides contain sections that don’t code for amino acids

• These sections are introns

• The part of the gene that do code for amino acids are exons

• Introns are removed during protein synthesis

• Eukaryotic DNA also contains regions of multiple repeats which also don’t code for amino acids- they are called non-coding repeats

What are alleles?

• Alleles are different types of the same gene

• The order of bases in each allele is slightly different, so they code for slightly different versions of the same polypeptide