Lecture 5: Mitosis and Meiosis in Cell Division

Prokaryote

A unicellular organism lacking a nucleus and membrane-bound organelles

• Includes Bacteria and Archaea

• Lacks a nuclear membrane — DNA is in close contact with other components of the cell

• Bacteria contain a single circular chromosome

• Cell may contain small, circularized plasmid DNA that is not part of the bacterial genome

Eukaryote

A complex organism with a nucleus and membrane-bound organelles

• Contains a nuclear membrane surrounding the DNA — DNA is spatially segregated from other components of the cell

• Contain a complex of DNA and histone proteins known as chromatin that is condensed to form tightly-packed chromosomes

• Contain multiple linear chromosomes

Humans have __ chromosomes per cell.

46

Humans have __ pairs of homologous chromosomes per cell.

23

Homologous pair

Most eukaryotic cells have two sets (2n) of chromosomes, one set from the mother and one from the father

• The two chromosomes in a homologous pair are usually similar in structure and size, and carry genetic information for the same set of hereditary characteristics

Diploid organism

• Has cells that contain two sets of chromosomes (2n)

Haploid organism

• Has cells that contain a single set of chromosomes (n)

Polyploid organism

• Has cells that contain more than two sets of chromosomes (n > 2)

Human reproductive cells (eggs and sperm) are __-ploid.

haploid

A fertilized human zygote is __-ploid.

diploid

Chromosome

A structure of tightly-packed chromatin that contains three essential elements:

1) Centromere

2) Pair of telomeres

3) Origins of replication

Centromere

A constricted region of the chromosome that serves as the attachment point for spindle microtubules to move chromosomes during cell division

• Where the kinetochore complex assembles

Telomeres

Specific DNA sequences and associated proteins located at the tips of eukaryotic linear chromosomes that serve to protect and stabilize the chromosome ends

Sister chromatids

Two genetically identical copies formed after a chromosome replicates

Identify the type of chromosome and the stage of the cell cycle:

• 1 pair of homologous chromosomes

• 2 chromosomes

• 2 sets of chromosomes, one copy from each parent (2n, diploid)

• In the G1 phase, before the cell has replicated its DNA

Identify the type of chromosome and the stage of the cell cycle:

• 1 pair of homologous chromosomes

• 2 chromosomes

• 2 sets of chromosomes, two copies from each parent (2n, diploid)

• After the S phase when the cell has replicated its DNA

• The cell has double the amount of DNA but the same number of chromosomes

Describe each phase of the cell cycle.

Interphase (cell growth)

• G1 — cell grows and synthesizes proteins necessary for cell division

  • G0 — stable, nondividing state if a cell exits the active cell cycle

• S — cell replicates its chromosomes

• G2 — cell continues to grow and prepares for cell division

M phase (active nuclear and cell division)

• Mitosis — nuclear division, the division of replicated chromosomes

• Cytokinesis — cytoplasmic division, where 1 cell divides into 2 cells

What must happen for successful cell division?

• Cell must copy its DNA, and the copies must be separated from each other so that both daughter cells receive the same number of chromosomes

• Cell must divide properly — if cell division is disrupted, an organism may end up with too few cells (ex. hair loss) or too many cells (ex. warts, tumors)

Prophase

Chromosomes condense and mitotic spindle begins to form

Prometaphase

Nuclear envelope disintegrates and spindle microtubules anchor to kinetochores

Metaphase

Chromosomes align on the metaphase plate, and the mitotic spindle must pass a checkpoint to ensure that it is properly assembled

Anaphase

Sister chromatids separate to become individual chromosomes that migrate towards the spindle poles

Telophase

Chromosomes arrive at spindle poles, the nuclear envelope reforms, and the chromosomes decondense

Cytokinesis

Cytoplasm divides (in plant cells, a cell wall forms)

Cell cycle checkpoints

Cellular “stop signs” that can either stimulate or inhibit cell division depending on whether conditions are right to proceed to the next phase of the cell cycle

G1 phase checkpoint

• Does the cell have enough nutrients and resources to keep growing and divide?

• Does the cell have damaged DNA?

If the cell passes, it goes to the S phase
If the cell does not meet the requirements, it either dies or goes to G0

S phase checkpoint

• Are there any errors in DNA replication?

• Is there any DNA damage (double-stranded breaks)?

If the cell passes, it proceeds to the G2 phase
If the cell does not meet the requirements, it uses HDR to repair damage

BRCA1

One of the proteins involved in HDR (homology-directed repair) — detects a break in dsDNA and removes some nucleotides on both strands of the DNA

BRCA2

One of the proteins involved in HDR (homology-directed repair) — helps find the sister chromatid copy to use as a template for DNA repair

M phase checkpoint

• Are all sister chromatids attached to the mitotic spindle?

If the cell passes, the cell proceeds to mitosis
If the cell does not meet the requirements, the cell halts the cell cycle

If the M phase checkpoint is faulty or disabled…

• The cell will proceed to mitosis despite not having all sister chromatids attached, and will divide unevenly

• The daughter cells will have unequal number of chromosomes (nondisjunction — cell could gain or lose a chromosome)

If the G1 phase checkpoint is faulty or disabled…

• The cell may proceed onto S phase and the rest of the cell cycle despite having damaged DNA

• However, the cell can still halt the cell cycle if the S and M phase checkpoints are working properly

If the S phase checkpoint is faulty or disabled…

• The cell proceeds to G2 phase, and may end up dividing without properly replicating its DNA

• However, the cell can still halt the cell cycle before mitosis if the M phase checkpoint is working properly

Nondisjunction

The failure of 1 or more pairs of chromosomes or sister chromatids to separate normally during nuclear division in mitosis or meiosis, usually resulting in an abnormal division of chromosomes in the daughter nuclei

Mitosis

Somatic cell division that consists of 1 cycle and produces 2 diploid (2n) somatic cells that are genetically identical to each other and the original parent cell

Meiosis

Germ cell division that consists of 2 cycles and produces 4 haploid (n) gametes that are genetically unique from each other and the original parent germ cell

Fertilization

The fusion of haploid gametes

Compare the effects of germline vs. somatic mutations.

• Mutations that occur in the germline (the lineage of cells that gives rise to the gametes) can be inherited by the offspring.

• Mutations that occur in somatic cells (body cells) are typically not transmitted to the offspring

• A mutation that occurs early in development is more likely to affect a greater portion of the body than a mutation occurring later in life

Meiosis I

• The separation of homologous chromosomes

• Chromosome number is reduced by half (2n → n)

• Reduction division

Meiosis II

• Separation of sister chromatids

• Chromosome number is not affected (n → n), but the daughter cells have half the amount of DNA compared to the mother cells

• Equational division

Prophase I

• Crossing over — the exchange of genetic information between sister chromatids to form recombinant chromosomes

  • Crossing over shuffles alleles on the same chromosome into new combinations

Metaphase I

• Homologous pairs of chromosomes align on the metaphase plate

Anaphase I

• The random separation of pairs of homologous chromosomes

  • Shuffles alleles on different chromosomes into new combinations

• Because there are different ways the homologous pairs can align, each daughter cell of meiosis I can inherit a different set of chromosomes

• There are 2ⁿ possible combinations, where n is the number of homologous pairs

Metaphase II

• Individual chromosomes align

Anaphase II

• Sister chromatids separate

• Because the chromosomes have crossed over and aligned in one of many possible combinations in anaphase I, each of the four daughter cells produced in anaphase II will have a different combination of alleles

Describe the sources of genetic variation in meiosis.

• Crossing over during prophase I

• Random separation of homologous chromosomes during anaphase I

• Random separation of sister chromatids during anaphase II

When did nondisjunction occur in meiosis if starting with one homologous chromosome, the gamete chromosome ratio is 2:0:1:1 (n+1 : n-1 : n : n)?

During meiosis II

When did nondisjunction occur in meiosis if starting with one homologous chromosome, the gamete chromosome ratio is 2:2:0:0 (n+1 : n+1 : n-1 : n-1)?

During meiosis I

Aneuploidy

Having too many or too few chromosomes in a cell

Monosomy

A type of aneuploidy characterized by loss of a single chromosome, 2n-1

Trisomy

A type of aneuploidy characterized by gain of a single chromosome, 2n+1

How do prokaryotes replicate?

Prokaryotes replicate through binary fission:

• Prokaryotes contain a single circular chromosome made up of double-stranded DNA, and as the chromosome replicates itself, the two origins of replication move to opposite sides

• The origins of replication anchor to opposite sides of the cell

• The cell divides, resulting in two prokaryotic cells with identical copies of the original chromosome