3.1 reproduction

• Sexual reproduction


  The process involving the fusion of the nuclei of two gametes to form a zygote (fertilised egg cell) and the production of offspring that are genetically different from each other
• Fertilisation


  The fusion of gamete nuclei, and as each gamete comes from a different parent, there is variation in the offspring
• Gamete


  A sex cell (in animals: sperm and ovum; in plants: pollen nucleus and ovum)
• Haploid nucleus


  Gametes contain half the number of chromosomes found in other body cells
• Diploid nucleus

  The zygote (fertilised egg cell) contains the full number of chromosomes, half of which came from the father and half from the mother
• Advantages and disadvantages of sexual reproduction

  • Advantages
  • Disadvantages
• Asexual reproduction


  The process resulting in genetically identical offspring being produced from one parent
• Bacteria produce exact genetic copies of themselves in a type of asexual reproduction called binary fission
• Advantages and disadvantages of asexual reproduction

  • Advantages
  • Disadvantages
• Comparing sexual and asexual reproduction


  • Number of parent organisms
  • How offspring are produced
  • Level of genetic similarity between offspring
  • Possible sources of genetic variation in offspring
  • Number of offspring produced
  • Time taken to produce offspring
• Meiosis


  The process of cell division that produces haploid gamete cells
• Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be genetically different from any others
• Genome


  The entire set of the genetic material of an organism
• In 2003, scientists completed a 13-year project in which they sequenced the genes that make up the whole human genome. This project was named the human genome project.
• Gene


  A section of DNA that codes for a particular sequence of amino acids
• DNA (deoxyribonucleic acid)


  The genetic material found in the nucleus of a cell
• DNA


  • A polymer made up of two strands coiled around to make a double helix
  • Consists of nucleotides with a common sugar and phosphate group and one of four different bases attached to the sugar
• Nucleotide


  The individual subunits of DNA
• Complementary base pairing


  Adenine always pairs with Thymine, and Cytosine always pairs with Guanine
• Extracting DNA from fruit

  1. Mash the fruit and mix with detergent and salt solution

  2. Filter the mixture

  3. Gently add ice-cold ethanol to precipitate the DNA
• Gene


  A section of DNA with a particular sequence of bases that codes for a particular sequence of amino acids
• A sequence of three bases is the code for a single specific amino acid
• The order of bases controls the order and different types of amino acids that are eventually joined together in a polypeptide chain
• Extracting DNA from fruit

  1. Add cold alcohol to the test tube

  2. DNA precipitates out of solution

  3. DNA appears as a stringy white precipitate

  4. Precipitate can be extracted using a glass rod
• DNA is not soluble in cold alcohol
• Codon


  A sequence of three bases that is the code for a single specific amino acid
• Polypeptide chain


  The amino acid sequences that form a particular type of protein
• Types of proteins


  • Enzymes
  • Hormones
  • Structural proteins
• Transcription


  1. DNA unwinds

  2. RNA polymerase binds to non-coding DNA

  3. RNA polymerase makes mRNA copy of gene

  4. mRNA leaves nucleus
• mRNA


  Messenger RNA, a single-stranded nucleic acid molecule that is a transcript copy of a gene
• Translation


  1. mRNA attaches to ribosome

  2. Ribosome reads mRNA codons

  3. tRNA brings amino acids to ribosome

  4. Peptide bonds form between amino acids

  5. Translation stops at stop codon
• DNA cannot travel out of the nucleus, so the gene code is transcribed into mRNA which can leave the nucleus
• Genetic variant


  A different version of a gene caused by a mutation
• Mutation in non-coding DNA

  Affects binding of RNA polymerase, altering protein production
• Mutation in coding DNA


  Changes amino acid sequence, affecting protein shape and function
• Fruit flies with normal XDH enzyme have red eyes, those with genetic variants have brown eyes