Biology Edexcel Topic 3 definitions

Genetics definitions

Allele

A version of a gene

Amino acids

Small molecules from which proteins are assembled

Anticodon

A triplet sequence of bases on a tRNA molecule that is complementary to a specific codon in mRNA

Asexual reproduction

A form of reproduction involving a single parent that creates genetically identical offspring (uses mitosis)

Blood group

Classification of blood type as A, B or O based on surface antigens on red blood cells and the presence of certain antibodies in the plasma

Chromosome

A long coiled molecule of DNA that carries generic information in the form of genes

Coding DNA

A sequence of DNA that codes for the production of a protein

Codominance

When both alleles for a gene in a heterozygous organism equally contribute to the phenotype

Codon

A sequence of three bases in a gene that code for a particular amino acid (also known as a base triplet)

Complementary base pairing

Describes how weak hydrogen bonds form between complementary base pairs - A pairs with T and C pairs with G

Detergent

A chemical that is used to disrupt cell membranes in the extraction of DNA

Diploid cell

A cell that contains two copies of each chromosome (i.e. a full set of chromosomes)

DNA

A double-stranded polymer wound to form a double helix, that the genetic code

Dominant

Describes an allele that is always expressed - represented by a capital letter

Environmental variation

Differences in phenotype acquired during the lifespan of an organism due to environmental factors such as diet, lifestyle, climate, exposure to light

Family pedigree

A chart used to show the inheritance of genetic disorders in a family

Fertilisation

The fusion of the nucleus of male and female gametes which restores the full chromosome number

Gametes

Sex cells (sperm and egg cells) with half the usual number of chromosomes

Gene

A length of DNA on a chromosome that codes for the production of a specific protein

Genetic variation

Differences in the genotypes of organisms of the same species due to the presence of different alleles, arising through mutations and sexual reproduction which creates variation in phenotypes

Genome

The complete genetic material of an organism

Genotype

An organism's genetic composition which describes all alleles

Haploid cell

A cell that contains a single copy of each chromosome (i.e. half the number of chromosomes)

Heterozygous

When someone has two different alleles of a gene (e.g. Ff)

Homozygous

When someone has two identical alleles of a gene (e.g. ff)

Human genome project

An international research project involving thousands of scientists which successfully mapped the entire human genome

Meiosis

A form of cell division that involves two divisions which produces four genetically different daughter cells (gametes) with a haploid number of chromosomes

Monohybrid inheritance

The inheritance of a single gene

mRNA

A polymer of nucleotides that carries genetic information from the nucleus to the ribosomes during protein synthesis - it is a single helix and uses U instead of T

Multiple alleles

The existence of more than two alleles of a gene (e.g. ABO blood groups are controlled by three alleles, IA, IB, and IO)

Mutation

A random change in the base sequence of DNA which may result in genetic variants which may be beneficial, damaging or neutral

Non-coding DNA

DNA which doesn't code for a protein but instead controls gene expression by influencing the binding of RNA polymerase to DNA (tells the RNA polymerase how often to bind to the DNA and how much protein will be made)

Nucleotides

The monomers of DNA consisting of a common sugar, a phosphate group and one of four chemical bases (A, T, C, G) attached to the sugar

Phenotype

An organism's observable characteristics due to interactions of the genotype and the environment

Protein

A large molecule that is synthesised from amino acids

Protein synthesis

The formation of a protein from a gene

Punnett square

A grid used to predict the potential outcomes of a genetic cross

Recessive

Describes an allele that is only expressed in the absence of a dominant allele - represented by a small (lower case) letter

Reproductive cycle

The time required to produce independent offspring

Ribosomes

Sub-cellular structures that are the site of protein synthesis

RNA polymerase

An enzyme involved in transcription that binds to a region of non-coding DNA, unzips the DNA strands and joins free RNA nucleotides to complementary bases on the coding DNA strand

Sex chromosomes

A pair of chromosomes responsible for the determination of gender - XY in males, XX in females

Sex-linked characteristics

A characteristic that is coded for by an allele founded on a sex chromosome

Sex-linked genetic disorder

A disorder caused by a faulty allele located on a sex chromosome

Sexual reproduction

A form of reproduction that creates genetic variation, involving the fusion of male and female gametes

Transcription

The first stage of protein synthesis in which mRNA is formed from a DNA template

Translation

The second stage of protein synthesis that takes place in the ribosomes - amino acids are joined in a specific order dictated by mRNA to form a protein

tRNA

An RNA molecule that carries specific amino acids to the ribosomes during protein synthesis

Variation

The differences between individuals due to genes, the environment or a combination of both

Zygote

A diploid cell formed by the fusion of the nucleus of a male gamete with the nucleus of a female gamete