Mitosis and Meiosis Notes

Specific Learning Objectives

• Why do we need proteins in the body?

• What is a nucleic acid?

• What are the similarities and differences between DNA and RNA?

• What is a gene?

• What is the connection between DNA, RNA, and protein synthesis?

• Describe transcription and translation.

• What is meant by the terms phenotype and genotype?

• What is the difference between a chromosome and a chromatid?

• What are the stages of mitosis?

• What are the stages of meiosis?

• Which cell types undergo each type of cell division?

Textbook Pages: Cells

• Somatic cell:

  • A biological cell forming the body of a multicellular organism.

  • Most cells.

  • 46 chromosomes (diploid number).

  • Mitosis.

• Germ cell:

  • Cells that give rise to gametes.

  • Located in the gonads - ovaries and testes.

  • Meiosis.

• Gamete:

  • Cells that fuse during sexual reproduction.

  • Sperm or egg (spermatocyte or oocyte).

  • 23 chromosomes (haploid number).

Cell Life Cycle

• Cells spend the majority of their life in interphase.

• Interphase: Phase between cell divisions.

  • Ongoing normal cell activities (e.g., makes hormones, transmits action potentials, contracts).

  • Replication of DNA and preparation for division.

• Mitosis:

  • Series of events that leads to the production of two somatic cells by division of one mother cell into two daughter cells.

  • Cells are genetically identical.

    • Prophase

    • Metaphase

    • Anaphase

    • Telophase

• Cytokinesis: Division of cell cytoplasm.

• Chromatin: DNA complexed with proteins (histones).

• During cell division, chromatin condenses into pairs of chromatids called chromosomes.

• Each pair of chromatids is joined by a centromere.

Chromosomes

• Humans:

  • 23 pairs of chromosomes.

  • 46 - Diploid number.

  • 22 autonomic pairs.

  • 1 sex-determining pair.

    • XX - female

    • XY - male

• Karyotype: map of chromosomes.

• Homologous:

  • Pairs of chromosomes - where one is from the father, and the other is from the mother (gametes).

• Locus: the location of a gene on a chromosome.

• Allele: different forms of the same gene.

Interphase – DNA Replication

• DNA replication occurs.

• Each chromosome becomes doubled, consisting of 2 identical strands of DNA.

• After replication, each chromosome consists of 2 sister chromatids (strands) connected by a centromere.

• Two individual chromosomes before replication form one homologous pair.

Structure of a Mitotic Chromosome

1. The DNA of a chromosome is dispersed as chromatin.

2. The DNA molecule unwinds, and each strand of the molecule is replicated.

3. During mitosis, the chromatin from each replicated DNA strand condenses to form a chromatid. The chromatids are joined at the centromere to form a single chromosome.

4. The chromatids separate to form two new, identical chromosomes. The chromosomes will unwind to form chromatin in the nuclei of the two daughter cells.

Mitosis

• Mitosis – produce 2 identical daughter cells.

• Prophase:

  • Chromatin condenses to form chromosomes.

  • Centrioles migrate to ends of each cell.

  • Spindle fibres attach to centromeres.

  • Nuclear envelope disintegrates.

• Metaphase – chromosomes are aligned at the nuclear equator.

• Anaphase:

  • Spindle fibres separate the chromatids.

  • 2 identical sets of chromosomes are moved to separate ends of the cell.

  • Cytokinesis begins.

• Telophase:

  • Nuclear envelope reforms around each set of chromosomes.

  • Chromosomes decondense into chromatin.

  • Cytokinesis continues.

• Cytokinesis – cytoplasmic division.

Centrioles & Spindle Fibers

• 2 centrioles, located in centrosome.

• Center of microtubule (spindle fiber) formation.

• Before cell division, centrioles divide, move to ends of cell, and organize spindle fibers.

Mitosis - I.P.M.A.T.

• Interphase is the time between cell divisions.

  • DNA is present as thin threads of chromatin in the nucleus.

  • DNA replication occurs during the S phase of interphase.

  • Organelles, other than the nucleus, and centrioles duplicate during interphase.

• Prophase:

  • The chromatin condenses into chromosomes.

  • Each chromosome consists of two chromatids joined at the centromere.

  • The centrioles move to the opposite ends of the cell, and the nucleolus and the nuclear envelope disappear.

  • Microtubules form near the centrioles and project in all directions.

  • Some of the microtubules end blindly and are called astral fibers; others, known as spindle fibers, project toward an invisible line called the equator and overlap with fibers from opposite centrioles.

• Metaphase:

  • The chromosomes align in the center of the cell in association with the spindle fibers.

  • Some spindle fibers are attached to kinetochores in the centromere of each chromosome.

• Anaphase:

  • The chromatids separate, and each chromatid is then referred to as a chromosome. Thus, when the centromeres divide, the chromosome number is double, and there are two identical sets of chromosomes.

  • The chromosomes, assisted by the spindle fibers, move toward the centrioles at each end of the cell.

  • Separation of the chromatids signals the beginning of anaphase, and by the time anaphase has ended, the chromosomes have reached the poles of the cell.

  • Cytokinesis begins during anaphase as a cleavage furrow forms around the cell.

• Telophase:

  • Migration of each set of chromosomes is complete.

  • The chromosomes unravel to become less distinct chromatin threads.

  • The nuclear envelope forms from the endoplasmic reticulum.

  • The nucleoli form, and cytokinesis continues to produce two cells.

• Mitosis is complete, and a new interphase begins.

  • The chromosomes have unraveled to become chromatin.

  • Cell division has produced two daughter cells, each with DNA that is identical to the DNA of the parent cell.

Mitosis Examples

• Skin – keratinised stratified squamous epithelium.

• High abrasion:

  • Skin, gastrointestinal tract - oral cavity & anus, female repro – cervix & vagina

• Normal growth:

  • In-utero (pregnancy), puberty.

• Wound healing.

• Red blood cells.

• Mitosis inhibitors used in cancer treatment – stop tumour growth.

• Brain, heart, skeletal muscle = slow mitosis.

Meiosis

• Germ cells divide and produce gametes.

• Specialized for sexual reproduction.

• DNA replication followed by two cell divisions.

• Produces 4 genetically different daughter cells, which are gametes, haploid (n) = only one homolog from each homologous pair.

• Resulting gametes (egg, sperm) unite to form a zygote – a new ‘genetically unique’ human being.

Meiosis - Homologous Crossing-Over

• Prophase 1:
  *Homologous chromosomes line up next to each other.
  *DNA is exchanged between the adjacent homologous chromatids.
  *Sister chromatid strands of each chromosome are no longer identical.
  *Results in exchange of genetic material between maternal and paternal chromosomes.
  *New gene combinations = human variation.

Meiosis - End of Meiosis 1

• 2x 23 chromosome cells.

• Replicated DNA; Non-identical.

• There is NO DNA replication before meiosis 2.

• Meiosis 2 starts with 2 non-identical cells, both with 23 chromosomes, DNA is replicated.

Spermatogenesis & Oogenesis- Where is Meiosis Happening?

• Spermatogenesis:

  • In the gonads; the testes make gametes (sperm) via meiosis.

  • 4 functional sperm cells per division; non-identical; 23 chromosomes.

  • Lifelong process in testes.

• Oogenesis:

  • The ovaries make gametes (oocytes) via meiosis.

  • At birth, the ovaries contain all the oocytes they will ever have – stalled in prophase 1.

  • 1 functional oocyte per division, 3 polar bodies, non-identical, 23 chromosomes.

Summary

• Cells spend the majority of their life in interphase.

  • Interphase: phase between cell divisions.

• Mitosis: Somatic cells divide and produce 2 identical daughter cells. Cells are genetically identical (I.P.M.A.T).

• Cytokinesis: division of cell cytoplasm.

• Meiosis: Germ cells divide and produce haploid gametes; produces 4 genetically different daughter cells.