Inheritance, genes and cell division

• Chromosomes

  Thread-like structures of DNA, carrying genetic information in the form of genes
• Gene

  Short length of DNA found on a chromosome that codes for a specific protein
• Alleles

  Different versions of a particular gene
• Sex chromosomes

  • Females have XX, males have XY
• Inheritance of sex

  1. Father produces sperm with X or Y chromosome

  2. If X chromosome fertilises egg, female

  3. If Y chromosome fertilises egg, male
• The DNA base sequence determines the amino acid sequence in protein</b>
• Protein synthesis


  1. Transcription (rewriting DNA base code into RNA)

  2. Translation (using RNA to build amino acids into protein)
• Different sequences of amino acids give different shapes and functions to protein molecules
• Transcription


  Rewriting the base code of DNA into bases of RNA
• Translation


  Using RNA base sequence to build amino acids into sequence in a protein
• DNA cannot travel out of the nucleus, so the base code is transcribed onto mRNA which moves out to the ribosomes
• Ribosomes 'read' the mRNA code in groups of three bases, each triplet coding for a specific amino acid
• Although all body cells contain the same genes, many genes are not expressed in a particular cell as the cell only makes the specific proteins it needs
• Diploid


  Cells with two sets of chromosomes
• Haploid

  Cells with one set of unpaired chromosomes
• Mitosis


  Chromosomes double and then split to form two genetically identical cells
• Mitosis


  • Used for growth, repair, replacement of cells, and asexual reproduction
  • Produces genetically identical cells
• Stem cells


  • Unspecialised cells that divide by mitosis to produce specialised cells
• Meiosis


  Chromosomes double and then divide twice to form four genetically different haploid cells
• Meiosis


  • Produces gametes (sex cells)
  • Halves the chromosome number from diploid to haploid
  • Increases genetic variation in offspring
• Differences between mitosis and meiosis

  • Mitosis produces genetically identical cells, meiosis produces genetically different cells
  • Mitosis has one cell division, meiosis has two cell divisions
  • Mitosis maintains the diploid chromosome number, meiosis halves the chromosome number
• Meiosis


  1. Cells are pulled apart

  2. Four haploid daughter cells are produced
• Importance of meiosis

  • Production of gametes (sperm, egg, pollen, ovum)
  • Increases genetic variation of offspring
• Meiosis produces variation by forming new combinations of maternal and paternal chromosomes
• Questions on cell division often ask for differences between mitosis and meiosis
• You should know the reasons for a specific type of cell division taking place and the types of cells where each happen
• Inheritance


  Transmission of genetic information from one generation to the next
• Gene


  Short length of DNA on a chromosome that codes for a particular characteristic
• Alleles


  Variations of the same gene
• Individuals have two copies of each gene and therefore two alleles for each gene
• One allele is inherited from the mother and the other from the father
• Phenotype


  Observable characteristics of an organism
• Genotype


  Combination of alleles that control each characteristic
• Dominant allele


  Only needs to be inherited from one parent for the characteristic to show up in the phenotype
• Recessive allele


  Needs to be inherited from both parents for the characteristic to show up in the phenotype
• Homozygous


  Individuals have two copies of the same allele
• Heterozygous


  Individuals have two different alleles
• Alleles are abbreviated using letters, with the dominant allele in uppercase and the recessive allele in lowercase
• Phenotype associated with a dominant allele will be seen in both a dominant homozygous and a dominant heterozygous genotype
• Homozygous individuals are 'pure breeding' and will always produce offspring with the same characteristics