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DNA & Molecular Biology

DNA: double helix molecule that is blueprint for almost every cell or organism on the planet. Deoxyribonucleic acid’s structure was discovered by Watson & Crick in 1953.

Chromosome: coiled length of DNA seen during cell division but not during interphase. Chromosome attaches to spindle fibres at the centromere in prophase.

Allele: Specific version of gene or function sequence of DNA. Version may be ‘normal’ (which is most common in gene pool), or any one of many visibly (measurably) different mutant versions

Homologous Chromosomes: 1 of 2 almost identical copies of DNA which normally contains at least a few genes that are heterozygous. Note: Homologous chromosomes pair when in diploid condition, but do not pair when in the haploid (gamete) state.

Ploidy Number: number of complete sets of chromosomes in a cell which contains all necessary genes for normal development, growth, and function. Note: Diploid (2n) has sets of chromosomes in almost every cell of the body except the gametes which are haploid (monoploid). Most organisms are diploid, but some species are polyploidy (Chinook salmon are decaploid).

Mitosis: Sexual cell division in almost every body cell type, resulting in genetically identical diploid cells (a clone).

Meiosis: Sexual cell division in the gonads (ovaries or testes) resulting in haploid cells that each are samples of the parental genotype.

Segregation of Genes: basis for meiosis which occurs in anaphase 1 when chromosomes are pulled apart without centromeres splitting. This process virtually guarantees gametes will be of many different genetic make-ups.

Phenotype: Physical appearance of a cell or organism as controlled by the genetic make-up (genotype). Most common appearance is usually termed ‘wild type’ (normal), while less common appearances are seen as mutant.

Genotype: Description of paired alleles for any gene. Can be heterozygous, homozygous or hemizygous.

Dominance: Condition in diploid organisms where one allele masks the phenotypic expression of its paired allele (which is recessive).

Recessiveness: Condition in diploid organisms where one allele’s phenotypic expression is masked by its paired allele (which is dominant).

Heterozygous: Genotype in a diploid organism where the alleles (copies of a gene) are different (one dominant, the other recessive).

Homozygous: Genotype in a diploid organism where both alleles are the same. Organism would ‘breed true’ for this characteristic

Hemizygous: Genotype in which a gene has no homologous pair with which to interact. Single allele always shows up in the phenotype and is nearly always found in males (Drosophila melanogaster or Homo sapiens)

Gamete: haploid sex cell resulting from meiosis; can be egg/ovum or sperm.

Zygote: diploid cell resulting from the fusion of an egg and sperm. Zygote immediately begins rapid mitosis.

Chromosome: Coiled length of DNA seen during cell division but not during interphase. Chromosome attaches to spindle fibres at the centromere in prophase.

Chromatid: Dpublicated ½ chromosome b4 separation in anaphase of mitosis or anaphase 2 of meiosis. Note: any DNA connected by a centromere is termed a chromosome; therefore, 2 sister chromatids = 1 chromosome.

Linkage: Condition where 2 or more genes are located on the same chromosome and therefore segregate in anaphase 1 of meiosis as if they were attached to each other. Note: after Mendel’s time, crossing over was discovered which showed that linkage is not permanent.