Biology, Paper 2- Inheritance, Variation and Evolution


Asexual reproduction

this is reproduction with only one parent. this means there is no fusion of gametes, and so there is no mixing of genetic information- hence off spring are clones. this reproduction is done through mitosis and the replication of DNA. 

Examples include:

- Straberry plants send out runners which are shoots which form a new plant when they touch the ground. 

- Daffodils also produce smaller bulbs which grow into new plants.

- Fungi can reproduce using spores

- Malarial parasites can reproduce asexually when in the human host body, 

Gardners can use asexual reproduction to produce identical plants. 

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Sexual reproduction

sexual reproduction is the fusinf of male and female gametes: this is with the sperm and egg cells in animals, and the pollen and egg cells in plants. Because genes of the offspring are taken from both parents there is variation in the offspring. 

this formation of gametes is called Meiosis. In meiosis the cell containing the genetic information splits the first time the same as in mitosis (chromosones replicate, move to opposite poles and the cell divides) however these copies of the DNA seperate and the cell divides a second time. This forms four haploid cells or gametes with 23 chromosones- half the number of chromosones as the parent cell so 46 chromosones are gained from the mother and father in total. 

In total one cell divididing by meiosis forms 4 gametes. because the genetic material has only actually copied once, for the first split, the gametes are all genetically different. 

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Asexual VS Sexual Reproduction

Advantages of sexual: variation is prouced in the offspring- meaning if the environmment changes variation enables at least some organisms to be suited to survive, it allows humans to use selective breeding to get the best plants and animals. 

Advantages of asexual: only one parent is needed, time and energy efficient as a mate does not need to be found, it is faster than sexual reproduction, identical offspring make the most of good conditions suited to the organisim. 

Advantages of both: some plants can produce seeds sexually and copie themselves asexually. eg fungi can make spores sexually or asexually, the malarial paracite reproduces sexually in the mosquito and asexually in the human. 

Generally organisms reproduce asexually when conditions suit them and are good, and sexually when conditions are changing. 

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Genetic material in the neucleus of the cell is stored in the chemical DNA, this is stored in small lenghts on genes and the genes are contained in structures of chromosones. Each of the genes code a secuence of amino acids which make up a protien, these proteins are what determine the way our bodies form and our characteristics. 

the genome is the entire genetic material of an organism, this has been throurghly reasearch and knowledge of the human genome can lead to:

- doctors being able to see wich genes are linked to certian disorders with the body,

- Understand the cause of inherited disorders and how to treat them.

- See how humans have changed over time (looking at inherited gene from ancestors). 

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DNA (2)

the DNA is made up of a polymer, which is a repeating strand of nucleotodies, these all have the same properties: 

- a sugar

- a phosphate 

- One of the bases A, T, G, C.

These are joined to make long strands, joined by the Phosphate linking to the dioxiribose sugar on the next polymer. These are twisted to make the double helix structure. 

The DNA includes two strands linked together by a force of attraction between strands. This comes from the G (gyuanine) always linking to the C (cytosine) on the opposite strand, and the T (thyomine) always linking to the A (adenine) on the oposite strand. 

Think of: Gemma Collins says ThAnks. 

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Protein Synthesis

A protien is a string of amino acids, which are coded for by DNA, this is indicated by the precense of C,G,A,T  pairs bonded with hydrogen bonds. Because each of these pairs are complementary (A an T, G and C) when making protiens the DNA is unzipped, meaning RNA bases can attatch to the single strand of DNA to make m-RNA. the m-RNA then leaves the neucleus into the cytoplasm- via the nuclear pore. 

RNA- is only one strand (DNA is two), Thymine is replaced with Uracil. 

When the m-RNA has left the nueclues, translation occurs in the cytoplasm. this is where the mRNA attatches to the ribosome, which moves along the mRNA strand decoding it into base triplets (or codons). Each of these code for a different amino acid. 

Amino acids are formed out of polypeptides, these are created by tRNA anti codons. tRNA is found in the cytoplasm and bring the amino acids to the ribosomes. the tRNA bonds complimentary to the mRNA. this means once the amino acids have linked up the tRNA leaves to colllect another amino acid. 

When the chain of amino acids is complete it folds and twists or joins other chains to form a specific protien coded by the DNA. 

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a mutation is a chang in the structure of DNA. 

If bases in the DNA are changed it could change the order of amino acids in the protien coded by the gene. These occur all of the time but most do not alter the protien- or if they do so only slightly so it still works. 

Mutations causing protiens to change shape could: mean the substrate can no longer fit into the active sight if it is a enzyme, if it is a structual protien (like collagen) it may become weaker. 

Not all DNA codes for protien, some turn genes on and off, so they can/cannot make specific protiens. This means mutations to these parts of DNA may change the way genes are expressed 

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Gregor Mendel

Gregor mendel investigated the characteristics we inherit by experimenting with pea plants. He found that characteristics are determined by 'units' inherited and passed on instead of a blend of the parents characteristics. 

After Mendels work late in the 19th centuary, the behaviour of gchromosones in cell division was observed, and then even later 'units' were named genes and were shown to be located on chromosones. 

Mendels discovery was not recognised during his life time as:

- he was a monk in a monastry, not a scientist in a university 

- his work was not published in a well known book or journal. 

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Gamete - sex cell formed by miosis 

chromosone- molecules found in the nucleus of all cells containing lengths of genes. 

genes - parts of chromosones which are made of DNA coding for a protein. 

alleles - different forms of a particular gene. 

Dominant - an allele which only needs to be present once and is expressed. shown by a capital. 

recessive - an allele only expressed if there are two present (no dominant), lower case. 

Genotype- the combination of alleles a person has for a gene. 

Phenotype- the physical expression of a gene. 

Homozygous - two copies of the same allele for a gene, like aa or AA. 

Hetrozygous - copies of two different alleles like Aa. 

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Genetic diagrams

Used to show the likelyhood of expressing a certian gene. 

Crosses, genetic 

using TT instead of HH the punnet square shows how you could determine the likelyhood of inheriting characteristics instead of crossing over the lines. 

in both of these the likelyhood of inheriting a dominant to recessive allele is 3:1. 

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Genetic Disorders

Polydactyly- Extra fingers or toes, is caused by a dominant allele (so you can get the gene with just one faulty allele. 

Cystic fibrosis - disorder of cell membranes in areas linng the airways and pancreas. this means you have thick mucas in these areas. this is caused by the resecive allele. 

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Sex Determination

Only one of the pairs out of all 23 pairs in the human body posseses the gene to determine sex. 

In female these chromosones are identical (**). 

Males posses the X chromosones, and a shorter chromosone- Y. 

This means all people inherit one sex chromosone from the mother (X) and one from the father (X or Y) so there is theoretically a 50% chance of being born male or female. 

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The differences between the characteristics of individuals in the same population. This can be due to:

- The genes they have inherited (genetics)

- The conditions in which the organisms have grown up in (environmental)
- A combination of both

Reproducion creates variation by creating different combinations of allelles, but only mutations create completely new alleles. 

Continous variation is a variation in any characteristic which can have any value between an upper and lower limit eg. Shoe Size. 

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Selective Breeding

Humans selectivly breed animals inorder to produce offspring with desireable genetics. This invloves:

1) Breeding parents that show the most of the desired characteristics

2) Choosing the offspring with the most promising characteristics and breed them 

3) Repete untill the characteristic is expressed suficiently. 

There are many reasons for doing this, including to increase crop yeild, to increase milk production or just to produce attractive offspring. 

However this can be dangourous as it reduced the gene pool of the animals by inbreeding them. this means their is a higher chance of them inheriting a disadvantagous gene- which may not be obvious at first. For example all crops may end up with susecptablility to a certian desiese 

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Genetic Engineering

Genetic engineering is the modification of a genome by moving genes from one organism to another, usually across species. 

this is done by:

1) the chromosone for the characteristic is found, and the lengh of gene needed for this is cut out by enzymes. 

2) this is placed into a plasmid cut with the same enzyme so there is space for the gene. the plasmid with the gene in it is then returned to the bacterium it was taken from, this then grows and divides to make many copies of the same gene. 

if the gene is placed in an embryo the cell will grow and divide so the whole organism has the gene. 

Many plants are genetically modified to increase crop yeild. however some people are afraid this will reduce variation in the species and could have impacts on health if eaten. 

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Cloning- Adult Cell

You can clone an organisim by:

1) removing the body cell from an organism and taking the neuclues. 

2) put the neuclues into a sex cell from any animal of the same species, with its neucleuse removed. 

3) Inserting this emryo into a surrogate mother, to be born as a clone of the organism you took the body cell from. 

this works because the offspring has all 46 chromosones from its mother, instead of a mix of 23 and 23 from both parents. the egg cell is used to trick the embryo into thinking it was fertilised. 

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Cloning- Embryo Transplants

This can be used to create indentical children. this is done by:

1) Using IVF to fertalise sperm and eggs from prize animals. 

2) Let the embryo grow and divide into 8 cells- just before they begin to speciallise. 

3) Implant these embryos into 8 different mothers and let them grow and divide as if they were the origional embryos. 

This means 8 animals are born with identical genes. 

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Cloning- in Plants

There are two methods of cloning plants, these are cuttings; where a cutting of the plant is taken and planted and so grows into a copy of the origional plant. 

You can also use the tissue culture technique. this is done by scraping a sample of the plant and placing it in agar jelly to grow. this sample will then grow into tiny plantlets, which will be planted and then grow to be copies of the same plant. this is most commonly used commercially or to preserve rare plants. However it is expensive, and could all inherite a suseptiblity to a diesise. 

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Natural Selection

Evolution is the gradual change in the inherited characteristics of a population over time, sometimes resulting in new species.

One of the most celebrated theories of this is Darwins Natural Selection this is where an environment changes and therefore only the most adapted to the new environmnet- through genetic mutation - survive. This means they then pass on their andvantageous genes through reproduction, and the less advantageous genes die out with their owners who dont have the chance to reproduce. Meaning in time the whole population has the new gene. 

He discoverd this when visiting the Galapogas islands, when looking at different types of bird. 

the ideas took some time to catch on because:

- it challenged the idea that God created all the organisms which live on earth. 

- there was not enough evidence to convince scientists. 

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Alfred Russel Wallace also had the same idea as Darwin, this made Darwin realise he could publish his ideas. in the Origin of Species book. 

Lamarck also had a theory before Darwin, this was that all organisms have an 'inner need' to express a characteristic, and so try to change themselves to express this. this means they eventualy pass it on to their children. However this is not true. 

6 Mark summary-  

A population of organisms shows genetic variation. this genetic variation is caused by genetic mutation. there may be a change in the environment that means certian individuals are better adapted to survive than others. these individuals are more likely to survive and reproduce, passing their advantageous genes to their offspring. as this process continues over time species change- this is the process of evolution. 

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Warning Colouration, Mimicry and Speciation

Warning colouration - an organism who will be dangourous to predators will often blatantly express this by developing a skin colouration. this is effective as they are left alone as it is obvious they are harmful. 

Mimicry- an organism mimics the shape and colour of a poisonous animal. this means predators believe they are of the same species and so do not eat them. however this only works if the percentage of real population is higher. 

Speciation - where a population is divided for some reason (eg a river) individuals within these seperate populations have natural variations, and some are best suited to the new environment. these favourable characteristics are passed on through natural selection. when the species come back together they are now so different they can no longer breed to produce fertile offspring and so are not the same species. 

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Types of fossils:

- the hard parts of animals that dont decay easily

- organisms which have not decays as one of the conditions for decay was absent.

- preserved traces of organisims 

- parts of organisms replaced by minerals as they decay. 

Scientist can look at fossils over time to see how organisms have adapted, and eveloved into what they are now.

however fossil records are often sparse as evidence has been destroyed, or lie unfound. also most early life was soft bodied and was not traced. 

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- makes things easier to study, allows us to make sense of the living world, helps to uderstand how living things are related, helps recognise biodiversity, gives scientists common means of communication.

species can be classified by: Kingdom, Phylum, Class, Order, Family, Genus, Species. this is based on groups in groups with no overlap. this means it goes from large to small. this can be remembered by: King Prawn Curry Or Fat Greasy Sausages. 

this system was divised by Carl Linnaeus. hwever now due to more reseach and biochemistry there are three domains above kingdoms. they are: Archea, Bacteria, Euyrota. 

this can be shown in evolutionary trees showing how species are linked. by showing dates and closest ancestors on brances. 

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Causes of Extinction:

- Natural disasters 

- new predators

- new competitiors

- environmental changes

- humans (hunting, developments, destroying ecosystems faster than they can save them)

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