Biology

• Chromosomes are organised into pairs, with humans having 23 pairs (46 in total)
• Chromosomes are visible when the cell is about to divide, otherwise, DNA is less condensed and called chromatin
• After replication, a chromosome looks like an 'X' and is made up of two identical chromatids
• Mitosis is the process of cell division that creates two identical daughter cells from a single parent cell
• Mitosis is essential for growth, repair, and replacing damaged or worn-out cells
• A karyotype is a picture of chromosomes arranged by size, used to show chromosomal disorders and determine the sex of an individual
• DNA replication is necessary for mitosis to ensure correct DNA amount in daughter cells
• Steps of DNA replication:

  • DNA helicase unzips the 2 strands
  • Free nucleotides bind through complementary base pairing
  • DNA polymerase joins nucleotides forming the phosphate-sugar backbone
  • Replication results in two DNA molecules, each with one new and one original strand
• Homologous chromosomes are grouped in pairs, with one inherited from the mother and one from the father
• Humans have 23 pairs of chromosomes, making a total of 46 chromosomes
• Diploid cells have 46 chromosomes, while haploid cells have 23 chromosomes
• Sex cells (eggs and sperm) are haploid with 23 chromosomes, produced by meiosis
• Different somatic cells have different functions due to specific genes being switched on or off, leading to the production of essential proteins for tissue functioning
• During cell division, the daughter cell must receive an exact copy of genetic material, requiring DNA replication
• Meiosis is the process of producing sex cells (eggs and sperm)
• One parent cell divides into four, non-identical daughter cells, each with half the number of chromosomes (haploid)
• Meiosis is necessary because when an egg and sperm combine, they form a zygote
• The zygote will divide via mitosis until it forms an embryo and then a foetus
• Each cell of the foetus must be diploid, so the original zygote must also be diploid (2n = 46 chromosomes)
• If the egg and sperm were diploid too, the resulting zygote would have 92 chromosomes, which is too many
• Therefore, the egg and sperm need to be haploid (n), so when they combine, the resulting zygote will be (2n) and diploid
• Phases of meiosis:

  • PMAT x2
  • Prophase I, metaphase I, anaphase I, telophase I
  • Prophase II, metaphase II, anaphase II, telophase II
• Crossing Over:

  • During prophase I, homologous chromosomes pair up
  • Homologous chromosomes are similar
  • Some DNA is exchanged between chromosomes
  • Crossing over leads to more variation in the offspring
• Variation in Meiosis:

  • Reproducing sexually allows for more variation in the offspring
  • This variation increases the likelihood of survival in changing environments
  • Asexual reproduction results in identical offspring, similar to cloning, leading to no variation
• Compare and Contrast Mitosis and Meiosis:

  • Mitosis:
  • No. daughter cells produced: 2
  • No. nuclear divisions: 1
  • Purpose: Growth and repair
  • Where the process occurs: All somatic (body) cells
  • Number of chromosomes in daughter cells: 46
  • Crossing over: No
  • Daughter cells are identical: Yes
  • Meiosis:
  • No. daughter cells produced: 4
  • No. nuclear divisions: 2
  • Purpose: Sexual reproduction
  • Where the process occurs: In the testes and ovaries
  • Number of chromosomes in daughter cells: 23
  • Crossing over: Yes
• Mitosis is the process of cell division that creates two identical daughter cells from a single parent cell
• All the cells in your body divided from a single fertilised egg, and they all contain identical copies of your DNA
• Mitosis is essential for growth and repair
• As you grow up, you need more cells

  If you have an injury, cells become damaged and have to be replaced

  Cells can become worn out and also need to be replaced
• Phases of Mitosis:

  • After DNA replication has occurred, mitosis can begin
  • It is a continuous process, but can be divided up into distinct stages
  • Phases: Interphase, Prophase, Metaphase, Anaphase, Telophase
• Interphase:

  • Not really part of mitosis
  • DNA is replicating
  • DNA is not visible (chromatin)
  • The cell undergoes its normal daily functions
• Prophase:

  • Chromosomes become visible as double-stranded chromosomes
  • Each double strand is made up of two sister chromatids
  • The spindle begins to form
  • The nuclear membrane begins to break down
• Metaphase:

  • Chromosomes line up along the equator of the cell
  • Each chromosome is attached to a spindle fibre at the centromere
• Anaphase:

  • Spindle fibres contract
  • Chromosomes are pulled apart by the spindle fibres and move to opposite ends of the cell
• Telophase:

  • The nuclear membrane reforms around the chromosomes
  • The cell starts to split into two (cytokinesis)
  • Chromosomes de-condense back into chromatin
  • By the end of telophase, there will be two identical daughter cells
• Chromosomes are organised into pairs, with humans having 23 pairs (46 in total)
• Chromosomes are visible when the cell is about to divide, otherwise, DNA is less condensed and called chromatin
• After replication, a chromosome looks like an 'X' and is made up of two identical chromatids
• Mitosis is the process of cell division, creating two identical daughter cells from a single parent cell
• Mitosis is essential for growth, repair, and replacing damaged or worn-out cells