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GCE O Level Biology: Definitions of Keywords from Cell division, Heredity and Molecular Genetics

Terms in this set (28)

Mitosis
A form of nuclear division in which the daughter nuclei produced contain the same number of chromosomes as the parent nucleus
Meiosis
A form of nuclear division in a single cell divides twice to produce haploid daughter nuclei, each containing half the number of chromosomes as the parent nucleus
Homologous chromosomes
A pair of chromosomes, one from each parent, which have the same chromosomal length, centromere position, gene sequence and loci, but may contain different alleles
Synapsis
The pairing up of homologous chromosomes at the start of prophase 1 of meiosis
Crossing over
An exchange of genetic material between non-sister chromatids of homologous chromosomes, resulting in recombinant chromosomes
Chromosome
Threadlike structures made of DNA which carry genes determining the characteristics of an individual, found in the nucleus of eukaryotic cells
Gene
A small segment of DNA in a chromosome where specific hereditary information is stored, and codes for the synthesis of a single polypeptide
Alleles
Different forms of a gene which occupy the same relative positions on a pair of homologous chromosomes
Dominant allele
A form of a gene that expresses itself and give the same phenotype in both the homozygous and heterozygous conditions
Recessive allele
A form of a gene that expresses itself only in the homozygous condition
Phenotype
The observable characteristic of an organism
Genotype
Genetic composition of an organism, i.e. the combination of alleles it possesses
Homozygous
An organism who has identical alleles for a particular trait. Eg. TT or Tt
Heterozygous
An organism who has unlike alleles for a particular trait. Eg. Tt
Codominance
Condition which arises when both alleles in a heterozygous organism are dominant and are fully expressed in the phenotype
Mutation
The sudden or spontaneous change in gene structure or a chromosome, or chromosome number, and may be inheritable
Mutagen
A physical, chemical or biological agent that causes mutation by altering the genetic material
Continuous variation
Range of differences that can be observed in many characteristics in a population. It is brought about by combined effects of many gene and can be affected by environment.
Discontinuous variation
Clearly defined differences in a characteristic that can be observed in a population. It is usually controlled by only one or a few genes and is not affected by the environment.
Natural selection
Process which results in the best adapted organisms in a population surviving to reproduce
Codon
Triplet of nucleotides within a molecule of messenger RNA that functions as a unit of genetic coding, usually by specifying a particular amino acid during the synthesis of proteins in a cell
Transcription
Process where message stored in template DNA is copied to an mRNA molecule
Translation
Process which uses the message stored in the mRNA to make a protein molecule
Genetic engineering
A technique used to transfer genes from one organism to another
Vector
Another DNA molecule that is used to carry the genes of one organism into another
Plasmid
Circular DNA found in bacteria
Recombinant plasmid
Circular DNA from bacteria that contains a foreign gene
Transgenic organism
Any organism which acquires a foreign gene, and a result of genetic engineering
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