topic 3-Genetics

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CB3-Genetics

CB3a-Meiosis

Gametes (sex cells) are produced by meiosis. There are two sets of chromosomes in the nucleus. The DNA replicates to form 4 sets of chromosomes in the nucleus. Then cell division splits the cell so that there are two daughter cells. Then, without replicating the chromosomes again, the cells split into four, making four haploid cells.

Haploid gametes are needed for sexual reproduction because they only have half the genetic information needed to fuse in order for a zygote to have a full set of genes.

An organism's genome is its complete set of DNA. Each genome contains all the information needed to build and maintain that organism.

Genes are found in the nucleus of a cell. They contain all the information needed.

Genes code for proteins. It is a small section of DNA that determines a particular feature.

CB3b-DNA

In a Eukaryotic cell, DNA is found in the nucleus as it is stored as chromosomes.

In DNA, there are four base pairs: Adenine and Thymine, and Guanine and Cytosine.

DNA strands are made up of polymers called nucleotides. These are sugar and phosphate that join together to make the sugar phosphate backbone. They are held together by the base pairs on either side that are held together by hydrogen bonds which are weak forces of attraction.

There are two helices. This is called a double helix. To make up one helix, there is an alternating pattern of deoxyribose sugar molecules and phosphate groups. This is called the sugar phosphate backbone. They are then held to gether by two of four nitrogenous bases: Adenine, Cytosine, Guanine and Thymine. These split into complementary base pairs. The helix is held together by weak forces of attraction called hydrogen bonds. There are two bonds between A and T and 3 between C and G.

To extract DNA from a fruit it must first be mashed up. This mechanically breaks the fruit apart and increases the surface area of cells to expose to the ingredients used further on. This is then mixed with detergent and salt to break down the membranes and for the DNA to stick together. This mixture is then filtered so the insoluble parts are left as residue. Then it is mixed with ethanol as DNA is not soluble in ethanol. The DNA will start to come out of the solution and appear as stringy white precipitate.

CB3c-Alleles

A gene is a section of DNA that codes for a protein whereas an allele is a different form of a gene that codes for a different version of a characteristic.

A genotype is a description of the pair of alleles present for a characteristic whereas a phenotype is the physical expression of the alleles.

A homozygous allele would be to letters that are the same e.g. HH or hh. A heterozygous allele would have one dominant and one recessive e.g. Hh.

If an organism has a heterozygous allele for a certain feature, the dominant one would be displays and the recessive one would not. Therefore the genotype displays both alleles from the parents but the phenotype only displays what is shown.

CB3d-Inheritance

In humans, the female gamete is always known as X. In a male, a sperm can either have the letter X or Y. If an X sperm fuses with the egg, the baby will be a girl. If a Y sperm fuses with the egg, the baby will be a boy.

CB3e-Gene mutation

Polygenic inheritance is characteristics controlled by multiple genes. These can include height, skin colour, eye colour and weight.

Mutation is a change in a gene caused by a mistake in copying the DNA base pairs during cell division, or by radiation or certain chemicals.

Potential applications of mapping human genomes include: Improved genetic testing, Locations of genes linked to increased chances of inheriting a disease, new gene therapy treatments, new knowledge of evolution and personalised medicines.

When a mutation occurs, it will change the pattern of the DNA which leads to a different programming of the gene and therefore results in variation.

many mutations have no effect on the phenotype because they are not in the DNA sequence that is being copied and therefore cannot affect the protein produced.

CB3f-Variation

Genetic variation is variation due to the inheritance of certain alleles that make someone have similar characteristics to their parents. Environmental variations are causes from the environment that affect a human e.g. language and religion.

Continuous variation is when there is no final number or solution to the data. for example height, mass and time. Discontinuous variation is when there is an finite amount of data, e.g. eye colour, hair colour.

Genetic variation is caused by both mitosis and sexual reproduction. It can be caused by the mutation of a gene as this changes the DNA sequence. Also, sexual reproduction takes the parents alleles into account and therefore they do not look identical to another but have similar characteristics.

Causes of environmental variation include the influences around a person, e.g. what country they are brought up in and who grows up with them. His can also be affected by the sun or other types of radiation and chemicals.

Certain characteristics are inherited and that makes a person look the way they do. However, over time, looks can change de to the environment therefore both genetic and environmental variation occurs n humans.