Introduction to the GENOMES and gene expression

Considering chromosomes, alleles, genotypes, phenotypes, variations

Doc Brown's Biology Revision Notes

Suitable for GCSE/IGCSE/O level Biology/Science courses or equivalent

 This page will help you answer questions such as ...

 What is a gene?  What is a chromosome?

 What is the genome?  What are alleles?

 What is the difference between genotype and phenotype?

 



The connection between DNA, genes, alleles and the genome

The genome is the whole of the genetic material of an organism - all of the DNA!

In animal and plant cells the genetic material (DNA) is contained in the cell nucleus and arranged in 'packages' called chromosomes.

Chromosomes often occur in pairs e.g. human cells have 23 pairs of chromosomes, 46 chromosomes in all.

Every chromosome is a very long strand of DNA that is coiled up to give it a characteristic shape.

Reminders: DNA is a very long natural polymer in which the monomer is a nucleotide that makes up the repeating unit in the molecular chain. The DNA molecule consists of two strands wound and bound together to form the double helix molecule.

For more on structure of DNA see DNA and Protein Synthesis  gcse biology revision notes

A gene is a relatively short strand of DNA that forms a section of a chromosome.

Each gene has the coded instructions to tell a cell to combine a particular sequence of amino acids to form a specific protein. In this case the monomer unit is an amino acid and the resulting polymer is called a protein.

Proteins control the development of an organism's characteristics and all its functions.

As if this wasn't complicated enough, there is an extra layer of complexity due to the existence of alleles!

Genes can exist in different versions called alleles - subtle differences in the genetic DNA code.

Each allele produces a different form of the same characteristic of an organism.

e.g. brown or blue eyes is a good example.

Each chromosome in a pair carries the same genes, BUT, they may carry different alleles.

The diagram below sums up the relationship between all the terms described and explained above.

Note: Genetic variants are caused by alterations in the common nucleotide sequences in the DNA of genes. The term variant can be used to describe an alteration that may be benign (harmless), pathogenic (harmful), or of unknown significance. The term variant is increasingly being used in place of the term mutation. Variants are key to successful evolution because genotype changes (usually of the smaller type) can lead to changes in phenotype. 

Human genetic variation is the genetic differences both within and among populations. There may be multiple variants of any given gene in the human population. A mutation may defined as any change in a DNA compared to normal that results in a rare and abnormal variant.

For more on structure of DNA see DNA and Protein Synthesis  gcse biology revision notes

For more on mutations and variants see Genetic variation and mutations


Genetic instructions and the characteristics of an organism

The combination of all alleles for each gene of an organism is called its genotype.

It is the genotype of each organism that makes it unique.

However, the characteristics shown by an organism are called its phenotype.

The phenotype of an organism is primarily determined by the genotype, but the phenotype can be influenced by the environment the organism is interacting with.

e.g. under what conditions does an organism grow and develop?

The diet of an animal can affect how well it grows and how healthy it is. A well nourished child grows strong and healthy. A malnourished child short of protein, vitamins etc. may have stunted growth, be too thin, physically weak and the immune system weakened so the individual is more susceptible to infectious diseases.

A flower exposed to lots of sunlight, rich soil and adequate water may grow a healthily rich green and have attractively coloured flowers. If a plant is deprived of enough sunlight, nutrients or water, it grows somewhat thinly and becomes limp, it tends to be yellowish rather than green and petal colours may fade.

In both of the above cases, the genotype determines the maximum healthy growth of organism, but this may be reduced by environmental factors.

Therefore the variation of the phenotype is determined by a combination of the genotype (genetic factors) and the conditions of growth and development (environmental factors).

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