Cell Division, Mitosis, and Meiosis

Cell Division

• Cells replicate DNA and split into two cells leading to reproduction or growth.
• Prokaryotes: binary fission.
• Eukaryotes: mitosis (breaks down nuclear membrane for DNA division).
• Mitosis: daughter cells are genetically identical to the parent.

Binary Fission

• Prokaryotes, asexual reproduction.
• Separation into two new bodies, offspring inherit genes from a single parent.

Chromosomes

• DNA molecule containing genetic information.
• Humans: 46 chromosomes.
• Replicated chromosome: two chromatids attached by a centromere.

Interphase

• Longest phase, normal cell life.
• DNA: loosely packed chromatin.
• Cell grows, produces proteins and organelles, duplicates each chromosome.
• Sister chromatids bound by centromere.
• Centrosome duplicated.

Mitosis Stages

• Prophase:
  • Chromatin condenses into visible chromosomes.
  • Centrosomes move, release spindle fibers.
  • Nuclear membrane breaks down.
• Metaphase:
  • Spindle fibers connect to centromeres, align chromosomes at the cell center.
  • Each cell gets half the genetic information.
• Anaphase:
  • Spindle fibers contract, pull sister chromatids apart.
  • Cell elongates, chromatids move to opposite ends of the cell.
  • Each side is genetically identical.
• Telophase:
  • Nuclear envelope forms around each set of chromosomes.
  • Chromosomes and spindle fibers decondense.
  • Cytokinesis divides cytoplasm and organelles evenly, each cell receives a centrosome.

Mitosis vs. Meiosis

• Mitosis: clones body cells (diploid → diploid).
• Meiosis: Makes distinct gametes (diploid → haploid).

Meiosis

• Two stages, specialized cell division for sperm & egg cells.
• Haploid cells (gametes: sperm/egg).
• Fertilization: gametes form a zygote which grows to an embryo.

Chromosomes - Sex Differences

• Karyotype: 23 pairs of chromosomes, last pair determines sex.
  • XX = female, XY = male.
  • Errors: 47(XXX), 47(XXY), 46(X), etc.

Errors in Meiosis

• Gametes with extra/missing chromosomes.
• Consequences depend on affected chromosome.
• Examples: Turner syndrome, trisomy 21 (Down syndrome).

Meiosis I

• Interphase (prior to Meiosis):
  • Cell grows and replicates (DNA Replication).
  • DNA in cell is copied - resulting in two identical full sets of chromosomes.
• Homologous chromosomes cross over recombining DNA, chromatids have DNA from both parents cells.
• Centrosomes release spindle fibers to attach to each chromosome.
• Each chromosome migrates to the center of the cell.
• Two cells are now formed, each with half of a homologous pair, but each half with a sister chromatid.

Meiosis I Stages

• Prophase I:
  • Chromosomes condense into X shaped structures, each with two sister chromatids.
  • Pairs of chromosomes exchanged “bits” of DNA in a process called crossing over.
  • Nuclear membrane dissolves.
• Metaphase I:
  • Chromosome pairs align along the centre equator.
  • Centrosomes are at opposite poles of cell
  • Spindle fibers attach to one chromosome (at the centromere) of each pair.
• Anaphase I:
  • Pair of chromosomes are then pulled apart by spindle fibers.
  • Sister chromatids stay together.
• Telophase I / Cytokinesis:
  • Chromosomes complete their move to opposite poles of the cell.
  • Membrane forms around each set of chromosomes
  • Two haploid daughter cells

Meiosis II

• Sister chromatids condense
• Sister chromatids migrate to center of cells
• Centromeres cleave as sister chromatids split - one copy of each chromosome for each pole of the cells
• Each cell cleaves creating four daughter cells with the haploid number of chromosomes in each

Meiosis II Stages

• Prophase II:
  • Two daughter cells (23 chromosomes)
  • Condense into X shaped structures
  • Membrane dissolves
  • Centrosomes duplicate
• Metaphase II:
  • Chromosomes line up end to end along the equator cell
  • Spindle fibers at each pole of the cell attach to each of the sister chromatids (at the centromere)
• Anaphase II:
  • Sister chromatids are pulled to opposite poles
  • Separated chromatids are now individual chromosomes
• Telophase II / Cytokinesis
  • Chromosomes complete their move to opposite poles of cell
  • Membrane forms around each set to create four new cells
  • Haploid (n)