Biology Additional Science

In miosis, a diploid cell divides twice to produce four haploid cells with genetically different sets of chromosomes. This happens in sexually reproducing organisms to produce gametes.

In mitosis, a cell divides to produce two cells with identical sets of chromosomes. This happens in order to produce new cells for growth and repair or replacement of tissues.

Chromosomes

In normal human cells, there are only 23 pairs of chromosomes. They consist of long coiled molecules of DNA. Genes are sections of DNA which code for a particular inherited characteristic e.g. blue eyes. 

The DNA molecule itself consists of two strands which are coiled to form a double helix. The strands are linked by a series of paired bases, adenine, cytosine, guanine, and thymine.  

Adenine is only ever linked to Thymine. (A-T)

Cytosine is only ever linked to Guanine. (C-G)

The instructions are in the form of a code, made up of 4 bases which hold the strands of molecule together. These bases are the important bit, as they represent the order in which amino acids should be assembled to make proteins by living cells. Each group of 3 bases represent 1 amino acid in a protein chain. Since there are only about 20 amino acids, the code contained in the 4 bases is quite sufficient. 

 During cell division, chromosomes make copies of themselves. In order to do this, they must replicate their DNA and this involves uncoiling it into separate strands.

The 2 new DNA molecules produced are genetically identical since their bases are complimentary. This means Strand A makes a copy of Strand B, and Strand B makes a copy of Strand A.

•  Allele - alternative form of gene that for instance, there were genes for eye colour you would say that there were two alleles. 
• Dominant - This refers to an allele which controls the development of a characteristic when it is present on only one of the chromosomes in a pair.
• Recessive - This refers to an allele which controls the development of a characteristic only if it is present on both of the chromosomes in a pair.

Dominant alleles express themselves if present only once, so an individual can be homozygous dominant (BB) or heterozygous (Bb) for brown eyes. Recessive alleles express themselves only if present twice, so an individual can be homozygous (bb) for blue eyes.

• Homozygous - If both chromosomes in a pair contain the same allele of a gene, then the individual is homozygous for that gene or condition.
• Heterozygous - If the chromosomes in a pair contain different alleles of a gene, then the individual is heterozygous for that gene or condition.
• Genotype - This refers to the particular pair of alleles representing a certain characteristic e.g. we can refer to a homozygous dominant (BB) as a genotype for eye colour.
• Phenotype - To the outward expression of a genotype. e.g. BB and Bb are two different genotypes but each produce a brown eyed phenotype - only bb produces blue eyed phenotype.

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