6.1 Reproduction

• Structure of a nucleotide

  A) Phosphate

  B) Deoxyribose Sugar

  C) Base

  
• What is sexual reproduction?

  The process involving the fusion of the nuclei of two gametes to form a zygote and the production of offspring that are genetically different
• What is fertilisation?

  The fusion of gamete nuclei, leading to variation in offspring as each gamete comes from a different parent
• What does the formation of gametes involve?

  Meiosis
• What happens during fertilisation?

  • Gametes join together to restore the normal number of chromosomes and the new cell then divides by mitosis
  • As the embryo develops, cells differentiate
• What is asexual reproduction?


  Process involving only one parent and no fusion of gametes so there is no mixing of genetic information, leading to genetically identical offspring (clones)
• What process is involved in asexual reproduction?

  Only mitosis
• What are the advantages and disadvantages to 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
• What are the advantages and disadvantages to 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
• Explain how malarial parasites, fungi and plants 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
• Explain the 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
• Explain how gametes are produced


  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
• Explain how the number of chromosomes change 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
• Why is meiosis important?

  • Produces gametes
  • Increases genetic variation in offspring
  • 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
• Compare mitosis with 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
• What is a genome?

  the entire set of the genetic material of an organism
• What is a gene?

  A small section of DNA on a chromosome

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

  • The molecule that contains the instructions for growth and development of all organisms
• What is the structure of DNA like?

  Polymer made up of two strands forming a double helix, with many repeating units (monomers) called nucleotides
• Where is DNA located?

  DNA is contained in structures called chromosomes which are located in the nucleus of cells
• What is the 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
• Give 3 ways the Human Genome Project is important

  • Diagnosis of inherited / genetic disorders
  • Understanding of evolution, ethnic origins or ancestry
  • Trace human migration patterns in the past
  • Helps understand and find treatments for inherited disorders
• How does a gene code for a protein?

  • A sequence of 3 bases is the code for a particular amino acid
  • The specific order of amino acids determines the shape of the protein, and the shape of the protein determines its function
  • The order of amino acids in a protein is determined by the sequence of bases in the gene for that protein
• Explain the transcription stage of protein synthesis

  1. Transcription - DNA cannot travel out of the nucleus to the ribosomes so the base code for each gene is transcribed onto a complementary template (RNA) molecule - messenger RNA (mRNA)
  2. mRNA can move out of the nucleus and into the cytoplasm
• Explain the translation stage of protein synthesis

  1. Translation - mRNA attach to ribosome and the correct sequence of amino acids are then brought to the ribosome on carrier molecules (tRNA)
  2. The ribosome reads the triplets of bases on mRNA and uses this to join together the correct amino acids in the correct order.
  3. Once the protein chain is complete, it folds up to form a unique shape, enabling the proteins to do their job
• What happens to the protein made when a change in the DNA structure occurs?

  • 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
• Explain the translation phase of protein synthesis

  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
• Why is 'folding' of amino acids important in proteins such as enzymes?

  The folding of amino acids forms a unique shape, enabling the proteins to fulfil a specific function. E.g. the shape of the active site must be highly specific to the substrate
• Give 3 different types proteins

  • 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
• What are mutations?


  Random changes that occur in the sequence of DNA bases in a gene or chromosome continuously
• Describe the effect of mutations on coding DNA

  • 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
• Describe the 3 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
• What are non-coding parts of DNA

  Parts of DNA that can switch genes on and off, controlling whether or not a gene is expressed
• What happens if a mutation occurs in a section of non-coding DNA?

  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
• What is monohybrid inheritance?


  Characteristics controlled by a single gene
• What is polygenic inheritance?

  When most characteristics are a result of multiple genes interacting, rather than a single gene
• What is polydactyly?


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