AS Biol2

Definitions and Important things to know.



Variation exists between all species.

Interspecific - the variationthat exists between different species.

Intraspecific - the differences that occur within a species.

Intraspecific variation is caused by genetic and environmental factors.

Genetic - different alleles.

Environmental - minerals available, temperature of climate for camoflauge.

Investigating Variation -

You have to sample a population.

The sample has to be random.

S.D. tells you about the variation within a sample.

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DNA is made of Nucleotides that contain a Sugar, a phosphate and a base.

- Pentose Sugar

- Phosphate Group

- Nitrogenous Base

The sugar is a Deoxyribose sugar.

Each nucleotide contains the same sugar and phosphate but the base can vary.

There are 4 possible bases. - Adenine

                                              - Thymine

                                              - Cytosine

                                              - Guanine

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DNA Cont.

Two polynucleotide chains join together to form a double-helix.

DNA nucleotides join together to form polynucleotide strands.

The neuclotides join up between the phosphate group of one nucleotide and the sugar of another, creating a sugar-phosphate backbone.

Two DNA polynucleotide strands join together by hydrogen bonds between the bases. There is specific bare pairing. A + T    C + G

The two strands wind up to form the DNA Doule-helix.

DNA contains your genetic information. These molecules are very long and are coiled up tightly. It has a paired structure which makes it easier to copy itself.

The double-helix structure means the DNA is very stable in the cell.

Eukaryotic DNA is linear and associated with proteins.

DNA molecules are shorter and circular in Prokaryotes.

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DNA contains genes which are instructions for proteins.

Genes are sections of DNA found on chromosomes. Genes code for proteins. Proteins are made from amino acids. Different proteins have a different order and number of amino acids. The order of the nucleotide bases in a gene that determines the order of amino acids in a particular protein. Each amino acid is coded for by a sequence of three bases called a triplet in a gene. Different sequences of bases code for different amino acids.

Not all the DNA in Eukaryotic cells codes for proteins.

Bits which code for proteins - Exons

Bits which do not code for proteins - Introns

Inrons are removed during protein synthesis.

There are also multiple repeats of DNA sequences over and over.

E.g. CCTTCCTTCCTT This does not code for an amino acid either.

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Genes Cont.

The Nature and Development of organisms is determined by Genes.

1. DNA sequence determines amino acid sequence

2. Proteins and enzymes are formed

3. Enzymes enable metabolic pathways.

4. Metabolic pathways help determine nature and development

Genes can exist in different forms called alleles - slightly different versions of the same characteristic. e.g. blood type

As humans we have 23 pairs of chromosomes. 46 in total. Alleles are found in the same position (locus) on each chromosome in a homologous pair.

Gene mutations can result in non-functioning proteins.

Mutations are changes in the base sequence. Mutations can produce new alleles of genes. e.g. incorrect active site shape for binding to a protein, so a non-functional enzyme.

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Meiosis and Genetic Variation

DNA from one generation is passed on to the next by gametes.

Gametes are the sperm and egg cells in males and females.

Gametes have a haploid number (n) of chromosomes.

At fertilisation a haploid sperm fuses with a haploid egg, making a cell with the normal diploid (2n) number of chromosomes.

Gametes are formed by Meiosis - a type of cell division

1. DNA unravels and replicates so there are two copies of each chromosome, called chromatids.

2. DNA condenses to form double-armed chromosomes, made from two sister chromatids.

3. The chromosomes arrange themselves into pairs. These are then separated halving the chromosome number.

4. The pairs of sister chromatids are then separated, four haploid cells (gametes) that are genetically different are produced.

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Meiosis and Genetic Variation Cont.

Chromatids cross over during the first division.

Chromatids then end up with a new combination of alleles.

Meiosis produces cells that are genetically different.

1. Crossing over of chromatids

2. Independant segregation of chromosomes

During crossing over gour daughter cells are formed each with different alleles.

During independant segregation four daughter cells are formed with completely diferent combinations of chromosomes. (Separation of the chromosomes)

These two events then lead to genetic variation.

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Genetic Diversity

Variation in DNA can lead to genetic diversity.

The more variety in a population's DNA, the more genetically diverse it is. The more alleles also means that it is more genetically diverse. Genetic Diversity is increased by mutations and migration of individuals.

Genetic Bottlenecks reduce genetic diversity.

- An event which causes a big reduction in a population. This reduces the number of different alleles in the gene pool and so reduces genetic diversity. The survivors reproduce and a larger population is created from a few individuals.

The Founder Effect is a type of genetic bottleneck.

- Describes what happens when just a few organisms from a population start a new colony. Only a small number of organisms have contributed their alleles to the gene pool. There's more inbreeding in the new population, which can lead to a higher incidence of genetic disease.

- The founder effect can occur as a result of migration leading to geographical separation or if a new colony is separated from the original population for some reason.

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Genetic Diversity Cont.

Selective Breeding involves choosing which organisms reproduce.

Selective Breeding of plants and animals by humans has reduced genetic diveristy in some populations.

Humans select which domesticated animals or strains of plants to reproduce together in order to produce high-yielding breeds.

Selective breeding leads to a reduction in genetic diversity.

- Once an organism with the desired characteristics has been produced, only that type of organism will continue being bred.

- So only similar organisms with similar traits and therefore alleles are bred together.

- It results in a type of genetic bottleneck as it reduces the number of alleles in a gene pool.

Selective breeding can cause problems for the organisms involved e.g. shorter life expectancy and increased susceptibility to new diseases.

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Variation In Haemoglobin

Oxygen is carried around the body by Haemoglobin.

- Red blood cells carry Haemoglobin.

- Hb is a large protein with a quaternary structure. It also has a very high affinity for oxygen.

- In the lungs O2 joins with Hb in the red blood cells to form Oxyhaemoglobin.

- This is a reversible reaction. When O2 leaves the Oxyhaemoglobin near the body cells it turns back into Haemoglobin. Hb + 4O2 <--> HbO8

Hb saturation depends on the partial pressure of O2. This is the measure of O2 concentration. The higher the concentration the higher the partial pressure. Hb affinity for oxygen varies dependng on the partial pressure of O2. O2 loads onto Hb - High pp of O2. Oxyhaemogloin unloads O2 when low pp of O2.

O2 enters blood capllaries at the alveoli in the lungs. Alveoli have a high pp of O2 so loads onto Hb to form Oxyhaemoglobin. When cells respire they use up O2 so lowers the pp of O2. Hb then returns back to the lungs to pick up more O2.

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Variation In Haemoglobin Cont.

Dissociation curves show how affinity for oxygen varies - They show how saturated the Hb is with oxygen at any given partial pressure.

The graph is 's' shaped and shows a positive correlation between the % saturation of Hb and the partial pressure of O2.

Carbon Dioxide concentration affects oxygen unloading.

- When cells respire there is a raise in pCO2. This increases the rate of oxygen unloading. The curve shifts down. This is called the Bohr effect.

Haemoglobin is different in different organisms.

- Most vertical and left - depleted oxygen environment - lugworm

- Next one to the right - high altitude - llama

- Next one the right - human

- Next one to the right - active animal - hawk

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Variation in Carbohydrates and Cell Structure

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