DNA, genes and chromosomes

In a nutshell

DNA is stored differently in eukaryotic and prokaryotic cells. Eukaryotic cells contain lots of non-coding DNA that doesn't code for polypeptides or functional RNA.

Definitions

Key word	Definition
Gene	A DNA base sequence that codes for a polypeptide or functional RNA.
Locus	The position of a gene on a DNA molecule.
Triplet/codon	The sequence of three bases in a gene that code for an amino acid.
Functional RNA	Functional RNA includes RNA molecules other than messenger RNA. These RNA molecules perform specific tasks during protein synthesis.
Genome	The complete set of genes in a cell.
Proteome	The full range of proteins a cell is able to produce.
Genetic code	The rules that allow one triplet to code for one amino acid, which leads to the formation of a polypeptide.
Allele	The number of different forms a gene can exist in.
Intron	A non-coding sequence of RNA.
Exon	A coding sequence of RNA.
Non-coding repeats	Regions where DNA sequences are repeated over and over.
Homologous pairs	Pairs of matching chromosomes.

Eukaryotic DNA

In eukaryotic cells, DNA is linear and is associated with proteins called histones. The linear DNA molecules are found as tightly coiled chromosomes in the nucleus. This is because DNA is incredibly long so it must be wound up to allow storage in the nucleus. Histone proteins help this coiling and also support the DNA.

Biology; Biological molecules; KS5 Year 12; DNA, genes and chromosomes — 1. DNA molecule, 2.Histone, 3. DNA wrapped around histone, 4. Coiling and condensing, 5. A pair of chromosomes.

Mitochondria and chloroplasts

In eukaryotic cells, mitochondria and chloroplasts also contain their own DNA. However, it is circular and much shorter than nuclear DNA; it is also not associated with histones.

Prokaryotic DNA

In prokaryotic cells, the DNA is circular and shorter but is still found as chromosomes. Prokaryotic cells do not have histones, instead the DNA condenses and fits into the cell by supercoiling.

Biology; Biological molecules; KS5 Year 12; DNA, genes and chromosomes — 1. DNA double helix, 2. Circular chromosome, 3. Coiled chromosome, 4. Supercoiling.

The genetic code

A gene codes for a polypeptide or functional RNA, it is able to do this because the specific sequence of DNA bases code for a specific sequence of amino acids. Three bases, or a triplet, code for one amino acid and these amino acids are joined together by peptide bonds to form polypeptides. This specific sequence of amino acids determines the primary structure of the protein.

Proteins are produced in the three stage process of protein synthesis: transcription, splicing and translation.

Non-coding DNA

Not all genes code for polypeptides, some code for functional RNA. Equally, genes have sections that do code for polypeptides (exons) and sections of genes that don't code for polypeptides. These sections are called introns and they are removed from mRNA in a process called splicing.

Biology; Biological molecules; KS5 Year 12; DNA, genes and chromosomes — 1. Gene, 2. Exon, 3. Intron, 4. Exon.

Eukaryotic DNA also contains regions where DNA sequences are repeated over and over. These are known as non-coding repeats.

Example

CCGGCCGGCCGG

Alleles

Humans have $23$ homologous pairs of chromosomes, making the total number of chromosomes $46$ . Homologous pairs have chromosomes that are the same size and contain the same genes. They may have different alleles. This is because one chromosome in each pair is inherited from each parent.

Alleles that code for the same characteristic are found on the same locus of each chromosome in a homologous pair.

Biology; Biological molecules; KS5 Year 12; DNA, genes and chromosomes — 1. Allele for one characteristic, 2. Position of allele on the gene, 3. Allele for the same characteristic.