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

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Mitosis
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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
Image: Mitosis
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
Image: Meiosis
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
Image: Homologous chromosomes
Synapsis
The pairing up of homologous chromosomes at the start of prophase 1 of meiosis
Image: Synapsis
Crossing over
An exchange of genetic material between non-sister chromatids of homologous chromosomes, resulting in recombinant chromosomes
Image: Crossing over
Chromosome
Threadlike structures made of DNA which carry genes determining the characteristics of an individual, found in the nucleus of eukaryotic cells
Image: Chromosome
Gene
A small segment of DNA in a chromosome where specific hereditary information is stored, and codes for the synthesis of a single polypeptide
Image: Gene
Alleles
Different forms of a gene which occupy the same relative positions on a pair of homologous chromosomes
Image: Alleles
Dominant allele
A form of a gene that expresses itself and give the same phenotype in both the homozygous and heterozygous conditions
Image: Dominant allele
Recessive allele
A form of a gene that expresses itself only in the homozygous condition
Image: Recessive allele
PhenotypeThe observable characteristic of an organismGenotypeGenetic composition of an organism, i.e. the combination of alleles it possessesHomozygousAn organism who has identical alleles for a particular trait. Eg. TT or TtHeterozygousAn organism who has unlike alleles for a particular trait. Eg. TtCodominanceCondition which arises when both alleles in a heterozygous organism are dominant and are fully expressed in the phenotypeMutationThe sudden or spontaneous change in gene structure or a chromosome, or chromosome number, and may be inheritableMutagenA physical, chemical or biological agent that causes mutation by altering the genetic materialContinuous variationRange 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 variationClearly 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 selectionProcess which results in the best adapted organisms in a population surviving to reproduceCodonTriplet 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 cellTranscriptionProcess where message stored in template DNA is copied to an mRNA moleculeTranslationProcess which uses the message stored in the mRNA to make a protein moleculeGenetic engineeringA technique used to transfer genes from one organism to anotherVectorAnother DNA molecule that is used to carry the genes of one organism into anotherPlasmidCircular DNA found in bacteriaRecombinant plasmidCircular DNA from bacteria that contains a foreign geneTransgenic organismAny organism which acquires a foreign gene, and a result of genetic engineering