B6

• Asexual reproduction:

  Involves one parent

  No fusion of gametes

  Produces genetically identical offspring

  Mitosis
• Many fungi reproduce asexually through spores

  Malarial protists reproduce asexually when there are in a human host
•  Sexual reproduction involves the fusion of male and female gametes:

  Sperm and egg cells in animals

  pollen on excels in flowering plants
• the formation of gametes involves meiosis which is:

  cell with two pairs of chromosomes (diploid cell)

  each chromosome replicates itself

  chromosomes part company and move to opposite poles

  so divide for the first time

  copies now separate and the second cell division takes place

  4 haploid cells (gamete) each with half the number of chromosomes of the parent cell
• Cell divides by meiosis:

  copies of the genetic information all made

  the cell divides twice to form 4 gametes each with a single set of chromosomes

  all gametes are genetically different from each other
• meiosis is important because it half the number of chromosomes in gametes. This means that fertilisation can restore the full number of chromosomes. What fertilised the resulting cell divides rapidly by meiosis and cells become specialised by differentiation
• the advantages of sexual reproduction are:

  it produces variation in the offspring

  if the environment changes any variation means that at least some organisms will be suited to survive

  it allows humans to selectively breed plants and animals and increase food production
• Sperm [23] + egg [23] = fertilised egg cell [46]
• the advantages of asexual reproduction are:

  only one parent is needed

  it is more time and energy efficient as the Organism does not have to find a mate

  it is faster than sexual reproduction

  many identical offsprings can be produced to make the best use of good conditions
• some plants can reproduce both sexually and asexually:

  many plants can produce seeds by sexual reproduction and can also reproduce asexually EG using bulbs an runners

  many fungi can make spores by asexual or by sexual reproduction

  malaria parasites reproduce sexually in the mosquito as well as asexually in humans
• strawberry plants send out long shoots called runners which touched the ground and grow a new plant.
•  The genetic material in the nucleus of a cell is made-up of a chemical called DNA which contained in structures called chromosomes. And a gene is a small section of DNA on a chromosome
• each gene codes for a particular sequence of amino acids to make a specific protein
• the genome of an Organism is its entire genetic material
• the whole human genome has now been studied and this may have some important uses in the future:

  doctors can search for genes linked different types of disorders

  can help understand and treat inherited disorders

  can show scientists how humans have changed over time
• DNA is a polymer made-up of repeating units called nucleotides
• a nucleotide consists of:

  a sugar

  a phosphate

  one of four bases this could be A, C, G or T
• Through nucleotides are joined together to form 1 long strand each molecule has two alternating sugars and a phosphate strands which are twisted to form a double Helix. Attached to eat sugar is one of four bases.

  It is an attraction between the different bases that holds the two strands together:

  a C on one strand always links with a G on the opposite strand

  a T on one strand always links with on a on the opposite strand
• the order of bases on DNA controls the order in which amino acids are joined together to make a particular protein a sequence of three bases codes for one amino acid.
• Proteins are synthesised on ribosomes using a template that has been taken from the DNA and carried out of the nucleus. Carrier molecules then bring specific amino acids to add to the growing protein chain in the correct order. When the protein chain is finished it folds up to form a unique shape. This unique shape allows the proteins to do their job as enzymes, hormones or structural proteins such as collagen.
• A change in DNA structure is called a mutation and this produces A genetic variant if any bases in the DNA are changed then it may change the order of amino acid in the protein coded by this gene. Mutations occur all of the time. Most do not alter the protein or only alter it slightly so it still works.
• A few mutations may cause the proteins to have a different shape if it is an enzyme in the substrate may no longer fit into the active site. If it is a structural protein it may lose its strength.
• Not all parts of DNA code for proteins:

  non coding parts of DNA can switch genes on / off so that they can / cannot make specific proteins

  mutations in these areas of DNA may affect how genes are expressed
• Until the mid 19th century, most people thought that sexual reproduction produced a blend of characteristics, which was disproven by Gregor Mendel through his breeding experiments on pea plants.
• Gregor Mendel found that characteristics are determined by units that are inherited and do not blend together.
• In the 19th century, the behaviour of chromosomes during cell division was observed.
• In the early 20th century, scientists realized that chroma zones, which Mendel's unit behaved in a similar way, were located on chromosomes.
• The important all Mendel’s discovery was not recognised during his lifetime because:

  he was a monk working in a monastery not a scientist at a university

  he did not publish his work in a well known book or journal

  the structure of genes had not yet been worked out
• In the mid 20th century, scientists worked out the structure of DNA and the mechanism by which genes work.
• the development of the gene theory explaining how characteristics are passed on is a good example of how ideas gradually change and develop. The illustrates how scientists make new observations and discoveries over time.
• Some characteristics are controlled by a single gene e.g. Red green colour blindness in humans
• each gene may have a different form called allele
• an individual always have two alleles for each gene:

  one comes from the mother

  one comes from the father
• the combination of alleles present in a gene is called genotype e.g. BB
• how alleles are expressed -what characteristic appears - is called a phenotype e.g. Blue eyes
• alleles can be either dominant or recessive
• if the two alleles which are present are the same the person is homozygous for that gene e.g. BB or Bb

  if the alleles are different they are heterozygous e.g. Bb
• a dominant allele is always expressed even if only one copy is present and a recessive allele is only expressed if 2 copies are present meaning if there is no dominant allele present.
• Most characteristics are controlled by several genes working together
• if only one gene is involved it is called monohybrid inheritance