Biology AQA unit 2

interspecific variation: one species differs from another
intraspecific variation: members of the same species differ from each other
Causes of variation: Genetic differences (mutations, meiosis, fusion of gametes), Environmental influences (environment, conditions)
normal distribution curve- mean (max height of the curve), standard deviation (measure of the width of the curve)
1. 1. 1) calculate the mean value (add all divide by how many there are)
    1. 2) Subtract the mean value from each of the measured values
      1. 3) some may be negative so square all numbers
        4) add all squared numbers together
        5) Divide this number by original number of measurements minus 1 (n-1)*
        6) square root this number
Structure of DNA
1. Deoxyribonucleicacid
  1. Determines inherited characteristics and contains vast amounts of information in the form of genetic code
  2. Made up of 3 components to form a nucleotide
    1. Deoxyribose = sugar
    2. A phosphate group
    3. An organic base: single ring= Cytosine (C) / Thymine (T) double ring= Adenine (A)/ Guanine (G)
  3. All combined by condensation reactions
    1. = single nucleotide (mononucleotide)
2. Pairing of bases: A and T (have two hydrogen bonds joining them) C and G (have three hydrogen bonds joining them)
  1. Pairings are complementary- ladders need to be same length
3. Double helix- ladder like arrangement twisted (two polynucleotide chains twisted) structural backbone of DNA. for each complete turn of the helix there are 10 base pairs
4. Function of DNA: hereditary material- passes genetic information from cell to cell and generation to generation
  1. 3.2 billion base pairs in DNA of mammalian cell
    1. almost infinite variety of sequences of bases along the length of a DNA molecule
      1. This variety= immense genetic variety between organisms
  2. How is DNA adapted to do this?:
    1. very stable, can pass from generation to generation without change
    2. Two separate stands held together by hydrogen bonds, allow them to separate during DNA replication and protein synthesis
    3. Extremely large molecule- carries vast amounts of information
    4. base pairs inside helical cylinder of deoxyribose-phosphate backbone- genetic info is protected from being corrupted from outside chemical and physical forces
5. Proving DNA is a hereditary material: experiment using mice and bacterium that can cause pneumonia
  1. A safe form of pneumonia (R-strain) A harmful form of pneumonia (S-strain)
  2. Mice separately injected with living safe form and dead bacteria of harmful form
  3. Living safe form- mice: remained healthy, (as did the group with dead harmful form)
  4. So when mice injected with both types- would have expected nothing to happen
    1. BUT.... these mice developed pneumonia
      1. Possible explanation: experimental error(harmful forms not killed)
      2. Possible explanation: living safe form had mutated into harmful form form (possible but extremely unlikely as same results got each time)
      3. Possible explanation: Pneumonia is cause by a toxin- the harmful form had info on how to make toxin but being dead it can not- safe form could make the toxin but doesn't know how. information may have been transferred from harmful to safe form
        Considered worthy of further investigation:
        Living harmful bacteria found in mice with pneumonia
        various substances were isolated from bacteria and purified
        Each substance was added to living safe bacteria- see if they transform into harmful form
        Only substance that made harmful form was purified DNA
        when an enzyme that breaks down DNA added, ability to carry out information ceased
Triplet code
1. What is a gene?
  1. sections of DNA that contain coded information for making polypeptides
  2. In the form of specific sequence of bases along DNA molecule
  3. Polypeptides combine to make proteins, genes determine the proteins of an organism
  4. gene = sequence of DNA bases that determines a polypeptide, and a polypeptide is a sequence of amino acids
2. How bases code for amino acids
  1. Minimum of 3 bases that code for each amino acid
    1. Only 20 amino acids regularly occur in proteins
      1. Each amino acid must have its own code of bases on the DNA
        only 4 different bases- A, T, C, G
        if each base coded for a different amino acid, only 4 amino acids coded for
        using a pair of bases 16 (4^2) different codes are possible
        three bases produce 64 (4^3) different codes, more than enough to satisfy the requirements of 20 amino acids
        As the code has three bases= triplet code
        As there are 64 possible codes- only 20 amino acids= some amino acids have more than one code
        In eukaryotes much of the nuclear DNA does not code for amino acids- sections called introns (can occur within genes and as multiple repeats between genes
3. Features of triplet code
  1. A few amino acids have only a single triplet code
  2. The remaining amino acids have between two and six triplet codes each
    1. The code is known as 'degenerate code' because most amino acids have more than one triplet code
  3. The triplet code is read in one particular direction along DNA strand
  4. Start of sequence is the same triplet code
    1. codes for amino acid- methanionine
      1. if the first methanionine molecule does not form part of the final polypeptide- it is later removed
  5. Three triplet codes do not code for any amino acid. these are called 'stop codes', and mark the end of the polypeptide chain
DNA and chromosomes
1. prokaryotic cells (eg bacteria) have small circular loops of DNA and not associated with protein molecules. Prokaryotic cells do not have chromosomes
2. Eukaryotic cells: DNA molecules larger and form helices (linear) rather than circle and associates with proteins to form structures called chromosomes
3. Chromosomes only visible as distinct structures when cell is dividing
  1. rest of the time, they are widely dispersed throughout the nucleus
4. chromosome- chromatids joined by a centromere
5. DNA molecule
  1. DNA combined with proteins
    1. DNA-protein complex is coiled
      1. Coils fold to form loops
        Loops coil and pack together to form the chromosome
6. humans have 46 chromosomes
  1. Homologous chromosomes/ pairs
    1. each pair of chromosomes is derived from chromosomes by mother (in egg-maternal chromosomes) and by the father (in sperm-paternal chromosomes) Known as homologous pairs- total number is diploid number (46)
    2. Two chromosomes that determine the same genetic characteristics
      1. BUT determining the same genetic characteristics is not the same as being identical
        for example: homologous pair of chromosomes may each possess information of eye colour and blood group, but only one chromosome carry the codes (alleles) for blue eyes and blood group A, while the other carries alleles for brown eyes and blood group B.
        During meiosis: halving of the chromosomes is done which ensure that each daughter cell receives one chromosome from each homologous pair. each cell receives one set of information for each characteristic of the organism. when the haploid cells combine, the diploid state, with paired homologous chromosomes, is restored.
7. What is an Allele?
  1. genes=section of DNA, contain coded information in the form of specific sequences of bases. Each gene exists in two+ forms. Each form is called an allele
  2. Each person inherits one allele from each of its parents, could be the same or different
    1. when they are different each allele will code for a different polypeptide
      1. Any differences in the base sequence of an allele of a single gene may result in a different sequence of amino acids being coded for
        Different amino acid sequence= production of different polypeptide, hence different protein
        may not function properly/ or work at all
        when protein produced is an enzyme, may have a different shape
        new shape may not fit the enzyme's substrate and enzyme may not be able to function= serious consequences for the organism
Meiosis
1. why is it necessary?
  1. In sexual reproduction two gametes fuse to give new offspring. Each gamete has a full set of chromosomes (diploid number)
    1. Then the cell produced has double this number. diploid no. =46 X2 =96 . this would continue for each generation. To maintain constant number of chromosomes they must be halved at some stage in the life cycle. this halving occurs as a result of meiosis.
2. Every diploid cell of an organism has two sets of chromosomes, one provided by each parent
  1. During meiosis, the chromosome pairs separate, so that only one chromosome from each pair enters each gamete. Known as haploid number of chromosomes (which is 23)
    1. When 2 haploid gametes fuse at fertilisation the diploid number is restored
3. Process of meiosis
  1. meiosis involves two nuclear divisions that occur one after each other
  2. 1) the homologous chromosomes pair up and their chromatids wrap around each other. equivalent portions of these chromatids may be exchanged in process called CROSSING OVER. At the end of this stage, homologous pairs have separated , with one chromosome from each pair going into one of the two daughter cells
    1. 2) second meiotic division, the chromatids move apart. end of meiosis 2, four cells have been formed. In humans each of these cells contains 23 chromatids
4. Genetic variation- meiosis also produces genetic variation among offspring by : independent segregation of homologous chromosomes and recombination of homologous chromosomes by crossing over
  1. Independent segregation:
    1. each chromosome lines up alongside its homologous partner
      1. 23 homologous pairs of chromosomes lying side by side
        Do this randdomly
        One of each pair will pass to daughter cell
        Enter text here

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Biology AQA unit 2

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