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iGCSE Biology VARIATION & SELECTION - Coggle Diagram

- - - - each chromosome contains
        1x double stranded DNA molecule
        
        folded & coiled around
        histones (proteins)
      - human cells = 46 chromosomes
        
        22 homologous pairs
        plus sex chromosomes
        X-Y (male) or X-X (female)
        
        homologous pairs carry
        genes for the same feature
        at same position & sequence
        
        cells with chromosomes
        in pairs are diploid cells
        
        except red blood cells
        no nucleus =
        no chromosomes
        
        except sex cells
        which have half (23)
        and are haploid
      - genes can have >1 form
        different forms = alleles
        
        homologous chromosomes
        carry genes for same features
        in same sequence
        
        BUT alleles of
        genes can be different
        
        one allele can be
        dominant
        
        one can be recessive
        only shown if no dominant
        
        DNA in the two chromosomes
        is not quite identical
  - - - one nucleotide:
        
        phosphate group
        
        holds the nucleotides
        in each strand together
        
        deoxyribose
        (sugar molecule)
        
        nitrogen-containing base
        always in pairs
        
        adenine (A)
        
        thymine (T)
        
        cytosine (C)
        
        guanine (G)
        
        complementary bases
        always bind with each other
        A - T and C - G
        the "base pairing rule"
        
        hydrogen bonds
        hold the pairs together
  - - - some DNA doesn't code
        but instead switches genes
        on and off
        
        operon
        
        group of structural genes
        
        headed by non-coding
        sequence of DNA an operator
        
        plus another sequence
        of DNA the promoter
        
        starts transcription by
        binding to RNA polymerase
        
        close to promoter is
        regulatory gene which
        codes for a repressor protein
        
        repressor binds with the operator
        preventing it from binding to
        the RNA polymerase
        = STOPS transcription
- - - - all sequences in a protein
        = gene
        
        proteins contribute
        to features
        
        myosin (muscles)
        
        enzymes -> processes
        in cells
        
        haemoglobin
        
        hormones
        
        made of chains of amino acids
        
        keratin (skin)
        structural protein
        
        sections that
        determine features
      - are the same in all organisms
        = UNIVERSAL CODE
  - - - in a chromosome
        part of the DNA
        unwinds
        
        RNA nucleotides line up
        along the DNA template strand
        G - C; C - G; T - A; A - U
        
        RNA nucleotides
        link up to form mRNA
        
        bonds formed between
        ribose & phosphate groups
        to make backbone of molecule
        
        until all DNA segment transcribed
        
        mRNA leaves
        the DNA
        
        passes out of nucleus
        to cytoplasm
      - translation
        (at ribosomes)
        
        3 bases in mRNA = codon
        
        1 codon = 1 amino acid
        
        mRNA attaches to ribosome
        
        tRNA molecules
        
        anticodon 3 bases
        at one end
        
        site for specific amino
        acid attachment
        
        a particular tRNA
        for each amino acid
        
        carries amino acid to the ribosome
        
        20 codons and anticodons
        one for each amino acid and
        sometimes more than one
        
        first tRNA binds at start codon
        always AUG = methionine
        
        next tRNA
        brings its amino acid
        
        next amino acid
        bonds to methionine
        
        tRNA released to
        collect another amino acid
        
        more tRNA arrive
        with their amino acids
        
        1 more item...
    - - is a single strand
      - contains ribose
        (not deoxyribose)
      - contains uracil (U)
        instead of thymine (T)
      - 1 x ribose
        1 x phosphate
        1 x base (C G A U)
      - messenger RNA
        transcription
      - transfer RNA
        translation
- - - - duplication
        
        nucleotide inserted twice
        instead of once
        ATT TTC CGT TAT C
        entirely different protein
      - deletion
        
        nucleotide is missed out
        ATT CCG TTA TC
        entirely different protein
      - substitution
        
        different nucleotide is used
        ATC TCC GTT ATC
        may code for different amino acid
      - inversion
        
        sequence in a triplet reversed
        ATT CCT GTT ATC
        may code for a different amino acid
- - - - DNA replicates
      - chromosomes form exact copies
        chromatids
      - chromatids become visible
      - joined at centromere
      - nuclear membrane breaks down
    - - spindle forms
      - chromosomes line up
        at spindle 'equator'
      - attached to spindle by
        centromeres
    - - spindle fibres shorten
      - chromatids pulled to poles
      - chromatids separate
    - - two new nuclei form at poles
      - cytoplasm divides
      - 2 daughter cells created
  - - - chromosomes pair up into
        homologous pairs
      - members of each pair
        separate
      - parent cell full set of chromosomes
      - 1st cell division
        2 cells with 1/2 chromosomes
    - - chromatids separate
        (now 'full set' of chromatids)
      - each cell divides in 2 again
      - 4 cells each with one set of chromatids
      - = 4 haploid gametes
      - chromosomes have alleles
- - - - each individual
        has 2 x alleles per feature
      - each gamete
        has 1 x allele per feature
      - multiple alleles in population
    - - gene is on
        chromosome
        
        each chromosome
        has 2 copies in non-gamete
        body cells
        
        there are two alleles
        of each gene
        
        Complete Dominance
        
        one allele
        is recessive
        
        one allele
        is dominant
        
        genotype describes
        the alleles for a feature
        eg. blue or brown eyes
        
        phenotype describes
        the feature resulting from
        the genotype
        
        2 x same allele
        = homozygous
        
        different alleles
        = heterozygous
        
        Codominant
        
        each allele contributes
        
        most genes
      - sex controlled by
        sex chromosomes
        
        Female: 2 x X
        
        Male: 1 x X, 1 x Y
    - - polygenic
        inheritance
  - - - F2 generation
        3:1 ratio of types
        3/4 same as F1
        1/4 with other parental feature
  - - - this favours one form
        = selective advantage
        
        favoured form increases
        it is selected for
        
        survival of the fittest
        
        unfavoured form decreases
        it is selected against