Life Sciences Term 1

Autosomes:

Chromosomes that are not sex chromosome.  There are 22 pairs of autosomes in a diploid cell.

Bivalent

A pair of homologous chromosomes physically held together by at least one DNA crossover.

Centriole:

An organelle in the cytoplasm of the cell, which gives rise to spindle fibres during meiosis and mitosis.

Centromere:

Structure that holds two chromatids together to form a chromosome.

Chiasma:

Point where crossing over takes place between chromatids of the                   homologous chromosome during prophase 1.

Chromatid:

It is a single thread of a double stranded chromosome. Two chromatids are joined by a centromere to form a chromosome.

Chromosome:

A structure made up of two chromatids joined by a centromere that                          carries the hereditary characteristics within the DNA.

Diploid number (2n):

Complete chromosomal number represented in pairs, which is                                     characteristic of an organism.

Gametes:

Haploid cells (n) which contain half the chromosome number of the diploid generation.  Egg cells and sperm cells are the gametes necessary in sexual reproduction where the fusion of the two gametes results in a new individual.

Gene:

The unit of heredity transmitted in the chromosome, which controls the development of the characteristics.

Gonosomes:

Sex chromosomes.  There is one pair of sex chromosomes in a diploid cell:  the XX chromosomes in females and XY chromosomes in males.

 Haploid number (n): 

Half the number of chromosomes present in gametes after  meiosis has occurred.

Homologous chromosomes:

Maternal and paternal chromosomes having the same shape and size which are paired but differs in genetic material.

Maternal: 

From the mother / female parent.

Meiosis:

A process of cell division whereby the chromosomal number is halved for the production of haploid gametes (sperm cells and egg cells).

Mitosis:

A process of cell division where the resulting daughter cells have the same diploid chromosomal number as the original parent cell.

Non-disjunction:  

The homologous chromosomes do not separate due to failure of the centromere to divide during meiosis I & II. The resulting gametes will have either an extra chromosome/copy or another gamete will have one less chromosome.

Paternal:

From the father / male parent.

Somatic cells:

Normal diploid body cells.

Spindle fibres:

Micro-tubules that form during cell division which radiate out from the centrosomes and draw the chromosomes to the poles.

Variation:

The morphological and physiological differences that can be seen between members of the same species.

Zygote:

The resulting diploid  cell after fertilization has occurred