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Genetics is the study of how information is passed on from parents to their offspring and how characteristics and diseases are inherited. The genetic material in humans is deoxyribonucleic acid (DNA). DNA has a double helix structure and is comprised of nucleotides which in turn are comprised of deoxyribose (a 5-carbon sugar), a phosphate group and a base. There are four bases: adenine (A), cytosine (C), guanine (G) and thymine (T), which can bind together reversibly to form base pairs – A binds with T and C binds with G. Combinations of these bases then specify the proteins to be encoded.
DNA is a double helix formed by base pairs attached to a sugar-phosphate backbone.
The majority of DNA is found in the nucleus of the cell (nuclear DNA) and a small amount is found in the mitochondria (mitochondrial DNA):
Nuclear DNA is wound up and packaged with proteins to form structures called chromosomes. Humans have 23 pairs of chromosomes in each cell (46 chromosomes in total). Twenty-two of these pairs are very similar to each other in size and shape, termed the autosomes. One member of each pair is inherited from the mother while the other is inherited from the father. Additionally, humans have a pair of sex chromosomes. Females have two X chromosomes while males have one X and one Y. A karyotype is a species-specific characteristic set of chromosomes.
Mitochondria DNA has a specific role in cell's energy metabolism. It is found outside of the nucleus in mitochondria and is all inherited from an individual’s mother.
The key functions of DNA are to:
a) Be reproducible to enable maintenance and growth of cells and reproduction
b) Encode proteins or RNA
A gene is a sequence of nucleotides that codes for a specific protein or RNA. Genomics is the term for the study of all the genes in an individual. Within the human genome, there are 3 million base pairs and ~20-25,000 genes. The sequence of a set of three bases is called a codon; each of these codes for a specific amino acid which then form the building blocks for proteins. Genes contain protein coding sequences (exons) and non-coding sequences (introns), with only 2% of DNA made up of exons.
Genes are responsible for encoding proteins in an organism, with each gene coding for one polypeptide, a chain of amino acids. These polypeptides then become proteins after folding to become a functional three-dimensional form. The stages in protein production are as follows:
The DNA sequences of the gene acts as a template to produce ribonucleic acid (RNA). RNA is similar to DNA but has a slightly different sugar making up its backbone (ribose vs. deoxyribose), is single stranded and has the base uracil (U) instead of thymine (T).
Introns, the non-coding sequences, are removed from the RNA.
Unlike DNA, RNA is able to leave the nucleus of a cell and is exported. This type of RNA is termed messenger RNA.
Messenger RNA is used to direct the assembly of a specific protein chain through production of amino acids. These are then joined together to produce a peptide in the ribosome (a molecule in every cell where protein synthesis takes place which is comprised of ribosomal RNA and ribosomal proteins). RNA involved in transferring amino acids to the polypeptide chain is termed transfer RNA.
e) Protein modification
Peptides will assemble together to form a protein which will involve folding as it is functional only in three-dimensional form.
The final stage involves moving the protein to where it is needed in the cell.
First, both coding and noncoding regions of DNA are transcribed into mRNA. Some regions are removed (introns) during initial mRNA processing. The remaining exons are then spliced together, and the spliced mRNA molecule (red) is prepared for export out of the nucleus through addition of an endcap (sphere) and a polyA tail. Once in the cytoplasm, the mRNA can be used to construct a protein.
Genotype and phenotype
Corresponding chromosomes from different individuals of the same species generally carry the same sets of genes in the same order. However, variation between humans results from these genes. Apart from the sex chromosomes which contain different sets of genes, there are two copies of every gene in each cell. These are termed alleles, where there are two different versions of the same gene at the same location (locus) on corresponding chromosomes. These versions can be the same (homozygous alleles), or they can differ (heterozygous alleles). The set of alleles a person has is known as their genotype. This genotype then codes a set of observable characteristics which are then expressed, known as the phenotype.
While variation can result between humans from alleles, most of the variation in DNA is in the non-coding regions, with regulatory sequences controlling whether the gene is expressed or not and if the subsequent protein is produced. Therefore, despite cells containing the same DNA, control of gene expression allows for cells with different specialised functions. These specialised cell types maintain their gene expression patterns through modification to DNA such as methylation and binding of regulatory proteins that are retained when the cells divide. This complex interplay of regulatory processes continues throughout the life of the organism.
There are two types of cell division. Mitosis results in the production of two cells each with the same number and type of chromosomes as the parent cell which is typical of ordinary tissue growth. Meiosis is the process of cell division resulting in the production of gametes (sex cells). Each gamete contains one set of chromosomes (23 single chromosomes instead of 23 paired sets of chromosomes). Gamete production can result in variation. During the process of sexual reproduction, genes are shuffled, resulting in different genetic combinations and variation in offspring.
Mutations are rare changes in the sequence of DNA that can result in variation. Most mutations are generally harmful, but some can improve the organism’s ability to survive and reproduce, forming the basis of evolution by natural selection. Mutations most commonly result from mistakes made when DNA is being replicated during cell division. During mitosis, mutations are normally due to natural events such as replication errors or cosmic radiation, though they can result from environmental hazards such as man-made radiation or exposure to mutagenic chemicals. Most of these alterations are repaired but if not, they can lead to changes in a protein that can result in genetic disease. When mutations occur in the gametes, they are passed on to the next generation.
Such mutations can be small scale or large scale.
While small scale mutations usually only affect a single protein, it can have significant consequences:
- A point mutation changes one codon. For example, a mutation altering a normal codon to a stop codon. This can result in truncation of the protein which will then likely result in the protein not being functional.
- A frameshift mutation involves a change such as deleting a base. This can alter the sequence and subsequent codons after this change, affecting the functionality of the resulting protein.
- If a mutation occurs in the regulatory part of the gene, this could cause the protein to be expressed in the incorrect location.
Large scale mutations can often cause severe problems. Examples include:
- Large deletions completely removing a gene;
- Large duplications which may affect cell growth though overproduction of protein;
- Modification of arrangement of genes which can affect their relationship to their control sequences (e.g. an inversion, a portion of the chromosome breaking off and reattaching after turning upside down);
- Transfer of DNA segments from one chromosome to another which can affect gene expression (translocation).
When an individual is missing a chromosome, it is known as monosomy. An example is Turner syndrome where a female is born with only one X chromosome. When an individual has extra chromosome copies it is known as trisomy. An example is Down syndrome, also known as trisomy 21 where there is an extra copy of chromosome 21. The majority of known types of chromosomal abnormalities involve sex chromosomes. Female abnormalities tend to be due to variations in the number of X chromosomes (e.g. Triple-X syndrome in women who inherit three X chromosomes), while male abnormalities are the result of irregular numbers of either the X or Y chromosome or both (e.g. Klinefelter syndrome in men who inherit one or more extra X chromosomes).
While the term mutation is used to refer to rare and deleterious changes in the genome, the term polymorphism is used to refer to the variation in a population which has resulted from the accumulation of even rarer, advantageous genomic changes over time. Polymorphisms are defined as the existence within a population of several subtly different normal variants of a DNA sequence.
Finally, inheritance of characteristics does not solely result from the genetic information; the environment in which the organism lives and develops interacts with the genetic makeup and expression. Alterations in gene expression can then result in genetic diseases. These changes are known as epigenetic and are persistent and heritable. They may arise during foetal development and be influenced by the wider environment.
© Public Health Genetics Unit 2006, H Green 2017