B13 - Reproduction

• Sexual reproduction


  • two parents
  • cell devision through meiosis
  • fusion of male and female gametes
  • sperm and egg in animals, pollen and ovule in plants
  • produces non-identical offspring that are genetically different to parents
  • results in wide variation within offspring and species
• Advantages of sexual reproduction


  • produces variation in offspring
  • if the environment changes, the offspring may have a survival advantage by natural selection due to their genetic variaton
• Disadvantages of sexual reproduction


  • finding a mate and reproducing is time consuming and requires a lot of energy
  • much slower than asexual reproduction
• Asexual reproduction


  • one parent
  • cell divison through mitosis
  • no fusion of gametes
  • produces offspring that are genetically identical to parent (clones)
  • no mixing of genetic information
• Advantages of asexual reproduction

  • only one parent needed
  • time and energy efficient as do not need to find a mate
  • faster than sexual reproduction
  • many identical offspring can be produced when conditions are favourable
  • successful traits passed on as offspring are identical
• Disadvantages of asexual reproduction


  • reduced genetic variation - if the environment changes, the offspring may have a survial disadvantage
  • harmful mutations in parent would be passed on to all offspring
• Organisms that reproduce sexually and asexually 


  • Malaria parasites
  • reproduce asexually in human hosts, but sexually in mosquitos
  • Fungi
  • many reproduce asexually by spores, but also sexually to give variation
  • Plants
  • many produce seeds sexually, but also reproduce asexually by bulb division or runners
• Meiosis 


  • a type of cell division
  • produces gametes in the reproductive organs
  • halves the number of chromosomes in gametes, and fertilisation restores the full number of chromosomes
  • the fertilised cell divides by mitosis, producing more cells
  • as the embryo develops, the cells differentiate
• Gamete


  specialised sex cell formed by meiosis
• Chromosome


  long molecule found in the nucleus of cells made from DNA
• Gene


  part of a chromosome that codes for a protein - some characteristics are controlled by a single gene, but most are controlled by genes interacting
• Allele


  different forms of the same gene
• Dominant


  allele that only needs one copy to be expressed
• Recessive


  allele that needs to copies present to be expressed
• Homozygous


  when an individual carries two copies of the same allele for a trait
• Heterozygous


  when an individual carries two different alleles for a trait
• Genotype


  combination of alleles an individual has
• Phenotype


  physical expression of the genotype - the characteristic shown
• DNA 


  • Genetic material in the nucleus of a cell is composed of DNA
  • DNA is a polymer made up of two strands forming a double helix
  • DNA is contained in structures called chromosomes
• Gene


  A small section of DNA on a chromosome that codes for a specific sequence of amino acids to produce a specific protein
• The genome


  • The entire genetic material of that organism
  • The whole human genome has been studied, which has allowed scientists to:
  • The entire genetic material of that organism
  • The whole human genome has been studied, which has allowed scientists to:
  • search for genes linked to different diseases
  • understand and treat genetic disorders
  • trace human migration patterns from the past
• Structure of DNA


  • a polymer made from four different nucleotides
  • a nucleotide is a molecule made of a phosphate, a sugar, and one of four organic bases
  • A, C, G, T
  • a sequence of three bases codes for a particular amino acid
  • the order of the bases determines the order in which amino acids are assembled to produce a specific protein
  • in complementary DNA strands, a C base is always linked to a G ase on the opposite strand, and T to an A
• Protein synthesis 


  • Proteins are synthesised in the ribosomes using a template of DNA
  • Carrier molecules bring amino acids to add to the protein chain in the correct order
  • When the protein is complete it folds up to form a specific shape, and this shape allows proteins to do specific jobs
  • Non-coding parts of DNA can control the expression of genes by switching them on and off
• Mutations and genetic variation


  • Mutations occur continuously and change the base code of DNA
  • in coding DNA, they may alter the activity of a protein
  • Most do not alter the appearance or function of the protein the DNA produces
  • A change in DNA structure may change the amino acid order, causing a gene to synthesise a different protein
  • Some mutations alter the shape of a protein, so the protein may no longer fit the substrate binding site, or lose its strength if it is structural
  • In non-coding DNA, mutations may affect how the genes are expressed
• Inherited disorders


  • Some disorders are due to the inheritance of certain alleles
  • Polydactyly
  • extra fingers or toes
  • caused by a dominant allele
  • Cystic fibrosis
  • a disorder of cell membranes
  • caused by a recessive allele
  • Embryo screening and gene therapy may alliviate suffering from these disorders, but there are ethical issues surrounding their use
• Genetic crosses


  • When the result of two known parents is considered
  • Punnett squares are used to predict the outcome of a genetic cross, for both the genotypes and the phenotypes of the offspring
• Sex determination


  • Normal human body cells contain 23 pairs of chromosomes
  • one of these pairs determines the sex of the offspring
  • In human females, the sex chromosomes are XX (homozygous)
  • In human males, the sex chromosomes are XY (heterozygous)
  • A punnett square can be used to determine the probability of the sex of an offspring
  • The probability in humans is always 50%