Genes

Everything you need to know about genes and DNA for dual award biology. Includes genetic modification, natural selection etc.

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  • Created by: CoolKid
  • Created on: 30-03-13 10:07

Useful beginner

The nucleus of the cell contains the chromosomes, genes are on the chromosomes.

A gene is a section of a DNA molecule that controls the development of certain characteristics.

A gene codes for a specific protein.

Genes exist as alleles which allow differences in inherited characteristics.

Diploid: 46 chromosomes, Haploid: 23 chromosomes.

Sex is determined by chromosmes, XX female, XY male.

Mutation is a rare, random change in genetic material. Mutations can be harmful (cancer) or useful (adapting to environment).

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Chromosomes, genes & DNA

Chromosomes are in the nucleus. They are made of DNA which carries all inherited information. Small sections of DNA control different characteristics. These are known as genes and control production of specific proteins.

Genes come in two alleles, each coding for a different form of the same character, eg. Brown eyes, blue eyes. Some alleles are dominant - a characteristic that shows up even if only one allele is inherited. Others are recessive - a characteristic which only shows when both of the alleles are inherited. Many genes code for several different characteristics but some code for single characteristics such as free or attached earlobes.

Normal body cells are diploid, with 46 chromosomes arranged as 23 pairs. 22 pairs are similar, the 23rd pair are the sex chromosomes. These are XX or XY.

Gametes only have one of each pair so are haploid with only 23 chromosomes. (They become diploid when fertilisation occurs.)

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Mutations

A mutation is a change in the DNA of a cell. It can occur in single genes or whole chromosomes.

They take place when the DNA is replicating as the cell divides, it causes the sequence of DNA to change. They occur faster when exposed to ionising radiation and some other chemicals (some included in cigarette smoke).

Mutations can be harmful, eg cancer, or can make no difference if it doesn't chnge the protein coded for. They can however also be positive. If a mutation occurs in a sex cell it can be passed on to the next generation, if it is harmful it is known as a genetic disorder. 

Whole chromosome mutations often result in the death of the cell. If chromosomes do not divide properly in a sex cell, too many or few chromosomes might get passed on at fertilisation, eg. if a fertilised egg has 3 copies of chromosome 21 the individual will have down's syndrome.

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Genes and Inheritance

Genes exist in alternative forms called alleles that give rise to differences in inherited characteristics, there are dominant alleles and recessive alleles (Card 2). If an organism is homozygous for a characteristic, both alleles are the same, if an organism is heterozygous for a characteristic, it has two different alleles. Codominance occurs when neither allele is dominant and both contribute to the appearance of the offspring.

The phenotype describes the physical characteristics of an organism (eg tall/short). The genotype desribes the alleles that a cell or organism has for a particular feature (eg TT, tt).

The sex of a person is controlled by sex chromosomes, you can use genetic diagrams to show how the sex is determined. The pattern of inheritance is shown using family pedigrees.

Test crosses are used to tell whether an organism with a dominant characteristic is homozygous or heterozygous for that trait. You cross it with a homozygous recessive individual. If the original organism is homozygous all the offspring will show the dominant characteristic. If the original is heterozygous the half the offspring will show the dominant and half the recessive.

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Example Genetic diagram

You can show the patterns of single gene inheritance in a genetic diagram to predict the outcome of various genetic crosses involving single genes.

Phenotype of parents       tall       dwarf

Genotype of parents         TT         tt                    (both homozygous. t shows recessive allele)

Gametes                           T           t

Punnett square:         T       T

                          t      Tt       Tt

                          t       Tt      Tt

All offspring heterozygous with a dominant tall allele. Therefore they are all tall.

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Family Pedigrees

These are diagrams that show how a genetic characteristic or a genetic condition is passed on through generations of a family. Different symbols are used for male/ female/ affected/ unaffected. It is often used to work out the genotypes of the individuals.

Use textbooks to look up one of these.

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Natural selection and Evolution

Evolution takes place through natural selection. Natural selection leads to change and adaptation within a population, which can lead to the formation of a new species.

Mutations can be passed from one generation to the next. Some are harmful, some neutral and a few are beneficial. Eg mutations can cause an increase in the resistance of bacterial populations to antibiotics.

Evolution is the process by which the range of organisms on Earth change. New species evolve, while old ones become extinct, this is known as natural selection. The organisms best suited to their environment survive best and breed, passing on their well suited genetics. If the environment changes then different genes are needed and other breeds survive better.

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Selective Breeding & Cloning

Peole use the principles of natural selection in the selective breeding of animals and plants for desired characteristics. Plants may be cloned to keep desirable features. Animals may also be cloned or genetically engineered for different purposes.

Animals or plants with desirable characteristics are bred to pass on the desired alleles. Selective breeding in plants is used to give disease resistance, heavy crops and cold resistance. In animals for more milk, meat, eggs, offspring and disease resistance.

Plants can be cloned using cuttings (taking a cutting and replanting it) or micropropogation. This involves taking tiny amounts of tissue from a plant (explants), growing them in special nutrient media, supplying them with hormones and minerals. Then transferring to a hormone medium for root growth, they are then planted in a greenhouse.

Mammals can be cloned by taking an unfertilised egg cell, removing the nucleus then inserting a nucleus from a normal body cell of another animal of the same species. The cell is given an electric shock to start development before being inserted into the womb of another of the same species. There is a very high risk of failure and often other problems.

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Genetic Modification

DNA is the molecule of inheritance, it is made of two strands coiled into a double helix, these strands are linked by a series of paired bases, a gene is a section of DNA. The bases are A, T, G and C, and they are always in pairs A-T, C-G. Genetic modification involves cutting a gene out of one organism and inserting it into the DNA of another. A genetically modified material is called transgenic and has recombinant DNA.

Bacteria can be modified to produce large quantities of human proteins eg. insulin. Plants can be modified to improve food production eg. improving resistance to pests.

The gene is cut out of the DNA using a restriction enzyme, the same restriction enzyme if used to cut out a gene from the plasmid. The gene is then placed inside the plasmid and glued in using ligase enzymes. The plasmid has now been modified and is placed back inside its cell.

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