Genetics word list. the molecule which contains genes. This will be looked at in more detail. it is shaped in a double helix (spiral)

Transcription

1 Genetics word list DNA the molecule which contains genes. This will be looked at in more detail. it is shaped in a double helix (spiral) Chromosomes X-shaped objects found in the nucleus of a cell. The x is made from long coils of DNA (so as DNA contains genes, so do chromosomes) Gene Allele Dominant Recessive Homozygous Heterozygous Genotype Phenotype Mitosis is a section of DNA molecule and also part of a chromosome. also a gene. When you have two different versions of the same gene, you have to call them alleles instead of genes the dominant allele is the one that will determine the characteristic that will appear even if another allele (recessive) is present this allele does not affect the characteristic that will appear because of the dominant one. But, if there are only two recessive alleles, then it will. when two alleles are the same for the particular gene when the two alleles are different for a particular gene a description of the genes that you have a description of the physical appearance given by that gene cell division when one cell splits into two cells another process of cell division which creates gametes. only happens in the ovaries or testes. Diploid this is the name given to the cells which have all the chromosomes (i.e. 46) Haploid Gamete Zygote Blastocyst this is the name given to the cells, which have half the chromosomes (i.e. 23) either a sperm or egg cell. All gametes are haploid. this is the name given when the egg and sperm has fused together this is the name given after the zygote has begun to split via mitosis

2 Cell division Fertilisation: the fusion of male and female gametes to form a zygote The zygote eventually forms the new organism by means of cell division. This is called MITOSIS. The elements received by the parents are in the nuclei of the gametes. These are formed by MEIOSIS. Gametes are sex cells. A haploid has ½ the number of chromosomes. Gametes contain a nucleus and a cytoplasm. Both of these are produced by meiosis: Egg Larger: - food for initial stages of cell division - target for sperm non-motile (can t move by itself) few are produced Model of inheritance Male gamete n Sperm Smaller: - little food reserve motile mitochondria produced in large numbers Organism 2n Fertilisation Mitosis Zygote 2n Organism 2n Female gamete n Assumption: number of male and female gamete contribute equally to zygote. If the amount in the gamete = n, the amount in the zygote = 2n. The number of chromosomes in human being cells = 46. Therefore, the number of chromosomes in gamete = 23 There are 2 types of cell division: 1. Mitosis 2.

3 Mitosis - each daughter cell produced is genetically identical to each other AND parent cell - the chromosome number remains the same - this occurs in animals for growth and repair - this occurs for plants in the tip of the stem and the tip of the root - mitosis only has one division - mitosis produces 2 cells - the chromosome number is halved - each cell produced is genetically different (due to crossing over) - this takes place in gamete formation in humans: testis and ovary - this takes place in plants in the anther and the ovary - meiosis divides twice successively - meiosis produces 4 cells The chromosomes have replicated to produce homologous pairs of chromosomes (pairs of the same chromosomes) The nuclear membrane breaks down. The homologous pairs of chromosomes are lined up together at the equator by the spindle fibres The homologous pairs of chromosomes are pulled apart by the spindle fibres. Half the number of chromosomes are attached to each spindle fibre. The homologous pairs of chromosomes are now separated by new nuclear membranes. The cytoplasm pinches inwards at the equator. 5. Each cell has divided again. The result is the production of four haploid gametes. Each of these cells are genetically different to each other.

4 Mitosis This is the name given to the form of cell division which occurs in normal body cells. The result of this division is: - 2 genetically identical daughter cells - the chromosome number is maintained This type of cell division is used for growth, repair and replacement. Once the process of cell division has started, it is a continuous process and 4 distinct stages have been identified by humans: P M A T Interphase - this is often termed the resting phase (but it is not really lots does occur) - no chromosomes are visible - DNA replication occurs in this phase. Prophase - chromosomes appear; longitudinally split (i.e. double stranded) - each strand = a chromatid (i.e. a molecule of DNA) - a chromosome is made of two chromatids - the 2 chromatids making up a chromosome are genetically identical centromere - the centriole: DNA chromatid o a small body just outside the nucleus o this divides and each half migrates to the opposite side of the nucleus o as the centrioles move apart, they give rise to spindle fibres o the spindle fibres become attached to the centromere of the chromosomes when the chromosomes are lined up at the equator of the cell

5 Metaphase - the double membrane of the nucleus disappears - the chromosomes become aligned along the equator of the cell - the chromosomes are attached to the spindle fibres by their centromere centriole centromere spindle fibre Anaphase - a short, rapid phase - the centromere splits in half and each half repels each other - at the same time, the spindle fibres shorten Telophase - this covers the period in which the two groups of chromosomes arrive at the poles and become surrounded by a nuclear membrane - the spindle fibres disappear and the cell cleavage appears - the cytoplasm divides into two by means of formation of a cell membrane (or lamella in plants) between the two sets of chromosomes - the telophase may be either a transitory period short enough only for DNA replication to occur before the cells divide again or long : permanent interphase may follow.

6 Separation of the chromatids due to the centromeres split and each half repels each other half the spindle fibres shorten Role of spindle fibres attach the chromosomes / chromatids by the centromere shorten to separate chromatids (i.e. movement of chromosomes) Energy for process from ATP (i.e. respiration) Is interphase a resting phase? No DNA replication occurs (i.e. each chromatid manufactures a genetically identical partner see DNA stages later on) a build up of ATP for the process of DNA replication and cell division Outcome (c) 2 daughter cells same chromosome number as each other daughter / parent cell genetically identical to each other and parent cell (i.e. no variation) Consequence of mitosis (c) (d) from fertilised egg: - repeated cell divisions give rise to a population of cells which later become tissues and organs - all cells have the same genetic complement each individual of a species has a characteristic number of chromosomes e.g. humans: 46 (i.e. 23 pairs) Genetic information carried on chromosomes varies Some cells lose their genetic material once they are developed (e.g. red blood cells, xylem cells) mitosis is used for growth, repair and replacement