DNA, genes and chromosomes
Select Lesson
Exam Board
Select an option
Practical skills
Control systems
Origins of genetic variation
Modern genetics
Microbiology and pathogens
Energy for biological processes
Exchange and transport
Surface area to volume ratio and gas exchange
Cell membrane structure and permeability
Simple and facilitated diffusion
Osmosis and investigating water potential
Active transport and co-transport
Gas exchange in fish, insects, plants and humans
Mammalian circulatory system and blood vessels
Haemoglobin and the Bohr effect
Heart structure and the cardiac cycle
Heart rate regulation and electrocardiograms
Transport in plants - xylem
Transport in plants - phloem
Classification and biodiversity
Cells, viruses and reproduction of living things
Biological molecules
Carbohydrates: types, structure and function
The structure and function of lipids
The structure and function of proteins
Nucleic acids: DNA and RNA
DNA replication
DNA, genes and chromosomes
RNA, protein synthesis and the genetic code
Enzymes: activation energy, properties and structure
Factors affecting enzyme activity
Inorganic ions: iron, hydrogen, sodium and phosphate
The properties and functions of water
Explainer Video
Summary
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.
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.
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.
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 homologous pairs of chromosomes, making the total number of chromosomes . 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.
FAQs - Frequently Asked Questions
What is an intron?
An intron is a non-coding sequence of DNA.
How many pairs of homologous chromosomes do humans have?
Humans have 23 homologous pairs of chromosomes.
What is a gene?
A gene is a DNA base sequence that codes for a polypeptide or functional RNA.