Inheritance, Variation and Evolution

  • Created by: India.02
  • Created on: 07-06-19 06:41

DNA

- DNA - strands of deoxyribonucleic acid - chemical that all genetic material is made from - contains coded information, which helps with structure anf unctions of an organism - determines inherited characteristics - found in the nucleus of animal and plant cells, in chromosomes - these normally come in pairs - DNA is a polymer made of two strands coiled into a double helix

- Gene - small section of DNA that is found on a chromosome - each gene codes for a particular sequence of amino acids which are then put together to forma a specific protein - only 20 amino acids are used but they make thousands of proteins - gene tell cells the order of amino acids - DNA also determines what proteins the cell produces, which determines what cell it is

- Genome - the entire set of genetic material in an organism - scientists have worked out the complete human genome - allows them to identify genes that are linked to specific diseases - knowing which genes are linked to genetic diseases could help us understand them better and help with developing effective treatments

- They can look at genomes to trace the migration of certain populations of people around the world - all modern humans are descended from a common ancestor who lived in africa - as populations migrated away from africa, they gradually developed small differences in their genomess - investigating these differences means that scientists can work out when populations split off, and what route they took

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DNA Structure and Protein Synthesis

- DNA strands are polymers made up of repeating units called nucleotides - each nucleotide has one sugar molecule, one phosphate molecule and one 'base' - sugar and phosphate form a backbone as they alternate - one of the four bases joins to each sugar - each bases links to the opposite strand - complementary base pairing (A and T, C and G) - order of bases decides order of amino acids in a protein - each amino acid is coded for by 3 bases in a gene - there are parts of a gene that don't code for anything - come non-coding parts switch genes on and off, so they control whether a gene is expressed (used to make a protein)

- Proteins made in cytoplasm on ribosomes - DNA is found in the nucleus and is too big of a molecule to move out of the nucleus - the mRNA molecule copies the code from the DNA and acts as a messenger between the two organelles - correct amino acids are brought in the correct order by carrier molecules

- The chain of amino acids coils into a unique shape to form a protein that has a specific function - enzymes that act as biological catalysts to speed up chemical reactions in the body - hormones that are used to carry messages around the body - structural proteins are physically strong (collagen strengthens connective tissues)

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Mutations

- A random change in an organism's DNA - can sometimes be inherited

- Occur continuously and spontaneously (when a chromosome isn't replicated properly) - chance of mutation is increased by exposure to certain substances and certain types of radiation 

- They change the sequence of the DNA bases, which produces a genetic variant (different form of gene) - as DNA codes for sequence of amino acids that make up a protein, mutations to a gene sometimes lead to changes to the protein that it codes for

- Most mutations have a very little or no effect on the protein - the extent can be so small that the function and appearance are unaffected

 - However, some can have a serious effect - mutation can code for an altered protein with a change in shape, which could affect its ability to perform its function - if shape of enzymes active site is changed, the substrate may no longer be able to bind to it - structural proteins could lose their strength if their shape is changed

- If there is a mutation in non-coding DNA, it could change how genes are expressed

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Sexual Reproduction

- Where the genetic information from two organisms is combined to produce offspring which are genetically different to either parent - mother and father produce gametes by meiosis - in humans each gamete contains 23 chromosomes, which is half the number of chromosomes in a normal cell - egg from the mother and sperm cell from father fuse together to form a cell with a full number of chromosomes

- Offspring inherits features from both parents becuase it recevied a mixture of chromosomes from its mum and dad - produces variation in offspring - flowering plants can reproduce in the same way (they have egg cells and sperm is known as pollen)

- Advantages - variation, which increases the chance of survival in the environment - chracteristics that make them better adapted to the environment means they have a better chance of survival because they are more likely to breed successfully and pass the genes on (natural selection) - selective breeding can be used to speed up natural selection, which allows us to produce animals with desried chracteristics

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Asexual Reproduction

- Only one parent and offspring are gentically identical to the parent

- Happens by mitosis - an ordinary cell makes a new cell by dividing into two - the new cell has the exact same genetic inforamtion as the parent cell - it is a clone

- There is no fusion of gametes, no mixing of chromosomes and no genetic variation between parent and offspring

- Bacteria, some plants and some animals can reproduce in thsi way

- Advantages - only one parent - uses less energy because ther eis no need to find a mate - faster than sexual reproduction - identical offspring can be produced in favourable conditions

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Reproduction Via Both Methods

- Malarial parasites - malaria is caused by a parasite that is spread by mosquoitoes - when it bites the human, the parasite can be transferred to the human - parasite reproduces sexually in the mosquito and asexually in the human host

- Fungi - many species of fungus release spores, which can become new fungi when they land in a suitable place - spores can reproduce sexually and asexually - asexually produced spores form genetically identical fungi - sexually produced spores introduce variation and are often produced in repsonse to an unfavourable change in the environment, which increases the chance that the population will survive the change

 - Plants - asexual reproduction can happen in different ways - strawberry plants produce runner, which are stems that grow horizontally on the surface of soil away from a plant - at various points along the runner, a new straawberry plnt forms, which is identical to the parent - when plants grow from bulbs, new bulbs can form from the main bulb and divide off - this can then grow into an identical new plant

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Meiosis

- The cell duplicates the gentic information, which forms two armed chromosomes - they arrange themselves into pairs - in the first division, the chromosome pairs line up in the centre of the cell - pairs are then pulled apart so each new cell only has one copy of each chromosome - in the second division, the chromosomes line up again and the arms are pulled apart - you get four gametes, each with a single set of chromosomes - each gamete is genetically different from the others because the chromosomes get shuffled during the process - each gamete only gets half

- After two gametes have fused during fertilisation, the resulting new cells divides by mitosis to make a copy of itself - repeats many times to produce lots of new cells in an embryo - as the embryo starts to develop, the cells being to differentiate 

- Mitosis - chromosomes line up in the centre of the cell and cell fibres pull them apart - two arms of each chromosome go to opposite ends of the cell - membranes form around each set, and they become nuclei - nucleus has divided - cytoplasm and cell membrane divide - there are now two daughter cells that contain the same DNA - genetically identical - DNA is identical to parent

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Characteristics

- The genes you inherit control your characteristics - some characteristics are controlled by a single gene (red-green clour blindness) - most are controlled by several genes interacting

- All genes exist in different versions called alleles - you have two versions of every gene in your body - one on each chromosome in a pair

- If an organism hass two alleles for a particular gene that are the same, it is homozygous - if the two alleles are different, it is heterozygous

- If two alleles are different, only one can determine what characteristic is present - the allele for the characteristic shown is called the dominant allele and the other is the recessive allele

- For an organism to disply the recessive characteristic, both alleles must be recessive - to display the dominant characteristic, both have to be dominant or there needs to be a dominant allele present because the dominant alleles overrule the recessive ones

- Genotype - combination of alleles that you have

- Alleles work at molecular level to determine what charcteristics you have - phenotype

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