Biology Unit 2

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  • Created by: Ali
  • Created on: 20-05-12 12:15


Investigating Variation

Interspecific variation - variation between different species

Intraspecific variation - variation between the same species

Causes of variation

  • Mutations - sudden changes to genes which may or may not be passed onto the next generation
  • Meiosis - nuclear division forming the gametes
  • Fusion of gametes - random fusion at fertilisation

Environmental Influences

  • climate
  • pH
  • food availability
  • soil conditions

In most cases, variation is due to both genetic and environmental factors

Standard Deviation

  • Mean - measurement at the maximum height
  • Standard deviation - range of values around the mean

DNA and Meiosis

Structure of DNA

  • a nucleotide is made up of a phosphate, a deoxyribose sugar and an organic base
  • DNA is made up of two strands of nucleotides which are very long, held together by hydrogen bonds
  • the bases contain nitrogen
  • adenine and guanine are double ring bases
  • thymine and cytosine are single ring bases
  • adenine and thymine join by two hydrogen bonds
  • guanine and cytosine join by three hydrogen bonds

Double Helix

  • phosphate and deoxyribose sugar wind around one another to form a double helix
  • each complete turn contains 10 base pairings


  • hereditary material responsible for passing genetic material
  • infinite sequences of bases causing immense inter/intraspecific diversity

The Triplet Code


  • sections of DNA that code for making polypeptides and proteins

Triplet Code

  • 3 bases code for an amino acid
  • each amino acid has it's own code of bases
  • introns are sections of DNA that don't code for amino acids



  • centromere in the middle surrounded by chromatids
  • proteins hold the DNA in place in chromosomes

Homologous proteins

  • chromosomes come in pairs because one comes from the egg and the other from the sperm
  • total number of homologous pairs is the diploid number


  • different forms of a gene
  • differences in the base sequences of an allele may result in a different amino acid being coded for
    • lead to the production of a different polypeptide
    • protein formed from polypeptide will have different tertiary structure so not complementary of substrate



  • homologous chromosomes pair with their chromatids and wrap around each other
  • chromatids can cross over and recombine
  • homologous pairs separate and one chromosome from each pair goes to one of the two daughter cells
  • chromatids move apart making 4 daughter cells

Independent Segregation

  • chromosomes randomly segregate and assort resulting genetically unique gametes

Crossing over

  • chromatids become twister around each other
  • portions of the chromatids break off
  • broken portions rejoin with the chromatids of its homologous partner

Genetic Diversity

Selective Breeding

  • deciding who should breed with who depending on the desired characteristics

Founder Effect

  • a few members of a population colonise a new region
    • when they breed there won't be much variation due to a small range of alleles

Genetic Bottlenecks

  • population can suffer massive drop in numbers
  • survivors don't have wide range of alleles

The Variety of Life



  • primary - 4 polypeptide chains
  • secondary - each chain is coiled into a helix
  • tertiary - each chain is folded into a precise shape to carry oxgyen
  • quaternary -




Very nicely done. Thankyou :)

Thierry de Lange





really good!!!



Got rid of all the waffle in the books, well done and thank you so much :D

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