DNA & cell division

• the genome is the entire DNA of an organism

  • it contains about 20,000-25,000 genes coding for proteins
• a gene is a small section of a molecule of DNA that codes for a specific protein 

  • it is the proteins that cells make which determines the features of an organism
• DNA is the chemical basis of inheritance in all organisms

  • it is found in the nucleus of a cell in the chromosomes
• each chromosome contains one double-stranded DNA molecule

  • the DNA is folded & coiled so that it can be packed into a small space
  • the DNA is coiled around proteins called histones
• each chromosome is made up of one particular DNA molecule

  • therefore it also contains the genes that make up that DNA molecule
  • another chromosome is made of a different DNA molecule, and will therefore contain different genes
• human cells contain 46 chromosomes (23 pairs)

  • male sex chromosomes do not match = they have an X and Y chromosome
  • females have a matching pair of X chromosomes
• pairs of matching chromosomes are called homologous pairs

  • these pairs contain genes for the same features
  • these genes are arranged at the same position & sequence along the chromosome
• Diploid cells contain both chromosomes of each homologous pair (matching chromosomes)

  • Body cells are diploid cells
• Haploid cells contain one chromosome from each homologous pair

  • Gametes are haploid cells
• after fertilisation, the zygote must divide by mitosis to form an embryo

  • when a cell divides by mitosis, two identical cells are made
  • the resulting cells have the same number & type of chromosomes as the original cell
  • mitosis forms all of the cells in the body, except gametes
  • it is therefore used for growth & the replacement of worn out or damaged cells
• division by mitosis = two genetically identical cells are made
• gametes = an organism's reproductive cell which contains half the number of chromosomes

  • eg egg or sperm cell
• prophase

  • DNA has replicated & formed two exact copies of each chromosome called chromatids
  • chromatids condense & become visible
  • duplicate chromatids joined at the centromere
  • the nuclear envelope breaks down

  A) centromere

  B) two chromatids

  
• metaphase

  • during metaphase, the spindle forms & attached to the centromeres of the duplicated chromatids
  • the chromosomes line up at the equator (centre of the cell)

  A) spindle

  
• anaphase

  • the spindle fibres contract & shorten, pulling the chromatids in opposite directions
  • identical chromatids are separated to become the chromosomes of the new cell

  
• telophase

  • nuclear envelopes form around the two identical sets if chromosomes
  • the cytoplasm starts to divide to produce 2 genetically identical daughter cells

  
• during meiosis:

  • each chromosome must be copied during DNA replication
  • the cell must divide twice
  • each daughter cell must receive one chromosome from each homologous pair
• meiosis process

  
• meiosis division 1

  
• meiosis first division:

  • one chromosome from each homologous pair goes into each daughter cell
  • DNA replicates
  • chromosomes pair up in their homologous pairs
  • spindle fibres attach to the centromeres in each pair, contract & pull the members of each pair to opposite sides of the cell
  • cell divides so that each cell now has half the number of chromosomes
• meiosis division 2

  
• meiosis second division:

  • chromosomes separate into 2 parts
  • spindle fibres attach to the centromeres of the replicated chromosomes
  • they contract & the replicated chromosomes are pulled to opposite sides of the cell
  • this results in 4 genetically different haploid gametes with half the number of chromosomes
• mitosis:

  • 1 cell forms 2 daughter cells
  • 1 cell division takes place
  • all cells formed are diploid
  • genetically identical
  • used for growth, cell replacement, sexual reproduction
• meiosis:

  • 1 cell forms 4 daughter cells
  • 2 cell divisions take place
  • 4 haploid cells formed (with half the number of chromosomes)
  • genetic variation
  • used to produce gametes for sexual reproduction