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• Sexual reproduction - the joining of male and female gametes

  • Mixing of genetic information - leads to variety in offspring
  • Formation of gametes involves meiosis
• Fertilisation - the fusion of gamete nuclei

  • Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis
  • As the embryo develops, cells differentiate
• Asexual reproduction - process resulting in genetically identical offspring from one parent

  • No mixing of genetical information - leads to genetically identical offsprings
  • Only mitosis is involved
• Sexual reproduction

  Advantages

  • Produces variation in the offspring
  • If the environment changes, variation gives a survival advantage by natural selection
  • Natural selection can be sped up by humans in selective breeding to increase food production

  Disadvantages

  • Takes time and energy to find mates
  • Difficult for isolated members of the species to reproduce
• Asexual reproduction

  Advantages

  • Only one parent needed
  • More 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

  Disadvantages

  • Limited genetic variation in population
  • Population is vulnerable to changes in conditions, and is only suited for one habitat
  • Disease is likely to affect the whole population as there is no genetic variation
• Some organisms reproduce both asexually and sexually

  Malarial parasites

  • Reproduce asexually in human host
  • Sexually in mosquito

  Fungi

  • Reproduce asexually by spores
  • Sexually to give variation

  Plants

  • Many produce seeds sexually
  • Can reproduce asexually by runners such as strawberry plants, or bulb division such as daffodils
• Why is meiosis important for sexual reproduction?

  
  

  • Cells in reproductive organs divide by meiosis to form gametes
  • It increases genetic variation
  • It ensure that the zygote formed at fertilisation is diploid
• Process of meiosis

  1. Each chromosome is duplicated, forming X-shaped chromosomes
  2. First division - the chromosome pairs up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome
  3. Second division - the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
  4. A total of 4 haploid daughter cells will be produced
• Production of gametes

  1. Copies of the genetic information are made
  2. The cell divides twice to form four gametes, each with a single set of chromosomes
  3. All gametes are genetically different from each other
• Chromosomes from gamete to embryo

  1. Meiosis halves the number of chromosomes in gametes.
  2. Gametes join at fertilisation to restore the normal number of chromosomes.
  3. The new cell divides by mitosis - the number of cells increases
  4. As the embryo develops, cells differentiate
• Importance of meiosis

  • Produces gametes
  • Increases genetic variation in offspring
  • Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made - each offspring from fertilisation will be different from any others
  • It ensures that the zygote formed at fertilisation is diploid
• Mitosis vs Meiosis

  Mitosis

  • 1 division
  • 2 genetically identical daughter cells
  • Diploid daughter cells (23 pairs)
  • Body cells

  Meiosis

  • 2 divisions
  • 4 genetically different daughter cells
  • Haploid daughter cells (23)
  • Gametes
• Genome - the entire set of the genetic material of an organism
• Gene - small section of DNA on a chromosome

  • Each gene codes for a particular sequence of amino acids, to make a specific protein
• DNA

  • The molecule that contains the instructions for growth and development of all organisms
  • Polymer made up of two strands forming a double helix, with many repeating units (monomers) called nucleotides
  • DNA is contained in structures called chromosomes
  • Chromosomes are located in the nucleus of cells
• Human Genome Project

  • Human Genome Project - the international, collaborative research effort to determine the DNA sequence of the entire human genome and record every gene in human beings
  • Importance:
  • It improves our understanding in searching for genes linked to different types of diseases
  • Helps understand and find treatments for inherited disorders
  • Has made it possible to study human migration patterns from the past
• Expain how a gene codes for a protein

  • A sequence of 3 bases is the code for a particular amino acid
  • The order of bases controls the order and different types of amino acids that are joined together
  • These amino acid sequences then form a particular type of protein
• Protein Synthesis

  • Proteins are made in the cell cytoplasm on ribosomes, according to a template
  • Ribosomes use the sequence of bases contained within DNA to make proteins
• Steps of protein synthesis

  1. DNA cannot travel out of the nucleus to the ribosomes (far too big to pass through a nuclear pore) so the base code for each gene is transcribed onto an RNA molecule called messenger RNA (mRNA)
  2. mRNA can move out of the nucleus and attach to a ribosome (acts as a messenger between DNA and ribosome)
  3. The correct sequence of amino acids are then brought to the ribosome and joined together
  4. This amino acid sequence then forms into a protein, which folds up to form a unique shape, enabling the proteins to do their job as enzymes, hormones or form structures such as collagen
• Effect of DNA structure on proteins

  • A change in DNA structure may result in a change in the protein synthesised by a gene
  • If there is a change in the order of the bases in a section of DNA (gene) then a different protein may be produced
  • This protein may not function in the same way as the original protein would have
• Function of Ribosomes

  1. The ribosome 'reads' the code on the mRNA in groups of 3. Each triplet of bases codes for a specific amino acid
  2. Carrier molecules bring specific amino acids to add to the growing protein chain in the correct order. In this way, the ribosome translates the sequence of bases into a sequence of amino acids that mae up a protein
  3. Once the amino acid chain has been assembled, it is released from the ribosome so it can fold and form the final structure of the protein
• Protein structure

  • When the protein chain is complete it folds up to form a unique shape. The unique shape enables the proteins to fulfil a specific function, e.g:
  • Enzymes - biological catalysts to speed up chemical reactions occuring in the body
  • Hormones - proteins that carry messages around the body
  • Structural proteins - proteins that provide structure and are physically strong
  • E.g. collagen is a structural protein that strengthens connective tissues such as ligaments and cartilage
• Mutations

  • Mutations - random changes that occur in the sequence of DNA bases in a gene or chromosome
  • Mutations occur continuously
  • AS the DNA base sequence determines the sequence of amino acids that make up a protein, mutations in a gene can sometimes lead to a change in the protein that the gene codes for
  • Most mutations do not alter the protein or only alter it slightly so that its appearance or function is not changed
• Types of mutations

  Insertions

  • A new base is randomly inserted into the DNA sequence
  • Changes the amino acid that would have been coded for by the group of 3 bases in which the mutation occurs
  • Insertion mutation has a knock-on effect by changing the groups of 3 bases further on in the DNA sequence

  Deletions

  • A base is randomly deleted from the DNA sequence
  • Effects like insertions

  Substitutions

  • A base in the DNA sequence is randomly swapped for a different base
  • Will only change the amino acid for the group of 3 bases in which mutation occurs - no knock on effect
• Effects of Mutations

  • Most do not alter the protein or only alter it slightly so that its appearance or function is not changed
  • But a small number of mutations code for a significantly altered protein with a different shape
  • This may affect the ability of the protein to perform its function
  • E.g. Shape of the active site on an enzyme changes so the substrate may no longer be able to bind to the active site
  • Structural proteins like collagen may lose its strength if its shape changes
• Gene switching

  • Not all parts of DNA code for proteins
  • Some non-coding parts of DNA can switch genes on and off
  • This means they can control whether or not a gene is expressed
  • Variations in these areas of DNA may affect how genes are expressed
  • If a mutation occurs in a section of non-coding DNA that controls gene expression, the expression of these genes may be altered or in some cases, the mutation may cause them not to be expressed at all
• • Gamete - sex cells
  • Chromosome - thread-like structures of DNA, carrying genetic information in the form of genes
  • Gene - short lengths of DNA which code for specific proteins
  • Allele - different versions of a particular gene
  • Genotype - combination of alleles that control each characteristic
  • Phenotype - observable characteristics of an organism
• Monohybrid inheritance - characteristics conctrolled by a single gene

  • E.g. fur colour in mice, red-green colour blindness
• Multiple Gene Inheritance

  • Polgyenic - when most characteristics are a result of multiple genes interacting, rather than a single gene
• Cystic Fibrosis

  • Genetic disorder of cell membranes, resulting in the body producing large amounts of thick, sticky mucus in the air passages
  • Over time, this may damage the lungs and stop them from working properly
  • Cycstic fibrosis is caused by a recessive allele
• Polydactyly

  • Genetic disorder that causes someone to be born with extra fingers or toes
  • Polydactyly is caused by a dominant allele
• Polydactyly

  • Genetic disorder that causes someone to be born with extra fingers or toes
  • Polydactyly is caused by a dominant allele
• Embryo Screening

  • During IVF, a cell can be taken from the embryo before being implanted and its genes can be analysed / DNA is taken from the cell of an embryo already in the womb
  • Genetic disorders can be detected during this analysis
• Reasons for embryo screening

  • Can avoid suffering by stopping children from being born with genetic disorders
  • Treatment for disorders costs governments and taxpayers lots of money
  • Laws have been put in place to stop embryo screening being abused
• Reasons for embryo screening

  • Can avoid suffering by stopping children from being born with genetic disorders
  • Treatment for disorders costs governments and taxpayers lots of money
  • Laws have been put in place to stop embryo screening being abused
• Reasons against embryo screening

  • Process could imply that people with genetic disorders are 'undesirable' which is not fair or true
  • Embryo screening is a very expensive process and therefore is not available to all potential parents
  • Could be abused to produce 'desirable' offspring
• Gene therapy

  • Gene therapy - process by which normal alleles are inserted into the chromosomes of an individual who carries defective alleles
  • Is a developing technology and is not always successful
  • Concerns:
  • Many believe that gene alteration is unnatural
  • Many believe it is a good idea as it can alleviate suffering in people with genetic disorders