chapter 08

Chromosomes and cell division

All cells arise from pre-existing cells
- Cell theory
  1. All living organisms are made up of one or more cells.
  2. All cells arise from pre-existing cells.
Prokaryotes reproduce by DNA replication followed by binary fission.
- Binary fission
  To reproduce, a prokaryotic cell initiates replication of its circular DNA, copying the DNA in both directions to yield two identical DNA molecules.
  The cell elongates and grows a new membrane, then undergoes binary fission and divides into two daughter cells.
  This form of asexual reproduction produces daughter cells that are genetically identical to the parent cell.
  Bacterial growth:
The complementary nature of the DNA double helix allows for easy replication.
- The two strands of a DNA double helix are complementary to each other.
  The base on one strand always has the same pairing partner on the other, following base-pairing rules:
  - A pairs with T
  - G pairs with C
  To replicate DNA, the double helix is separated into single strands, each of the parental strands serving as a template to build a complementary strand, using the enzyme DNA polymerase.
  At the end of this process, two identical daughter double helixes are formed.

Human DNA is packaged into chromosomes composed of diploid pairs.

DNA in living cells are organized into molecules called chromosomes.

Prokaryotic cells such as bacteria have a single circular chromosome.	Eukaryotic cells have linear chromosomes, with 1 or 2 chromatids.

Before a cell divides, each chromosome consists of a DNA double helix, called a chromatid.
Normally, the double helix is not easily visible under a microscope.
To prepare for division, each chromatid must be replicated: a copy of each chromatid is now held to its sister chromatid by a centromere.
Before a cell divides, the chromosomes condense into tighter (coiled) forms that can be seen under a microscope.

Human karyotype
Human somatic cells (cells other than sex cells - sperm and eggs) have 46 chromosomes with sizes and shapes that can be arranged in a karyotype.
44 of the 46 are occur in homologous pairs.
The other 2 chromosomes are sex chromosomes: females have 2 X chromosomes; males have 1 X and 1 Y chromosome.
Thus somatic cells are diploid: chromosomes occur in pairs. One in each pair is inherited from the mother, the other from the father.

Homologous chromosomes
44 of the 46 human chromosomes occur in homologous pairs: each of the pair have the same genes, inherited from the parents.
Before cell division, each chromatid is replicated: a copy of each chromatid is now held to its sister chromatid by a centromere.
The sister chromatids are genetically identical, while homologous chromosomes have the same genes, but possibly different alleles.

Sexual reproduction involves alternation between mitosis , which produces diploid cells, and meiosis , which produces haploid cells.
- Sexual life cycle
  Most of our body cells are somatic cells that are diploid: they have 2 sets of chromosomes, one set inherited from each parent.
  Humans have a diploid number of 46: 23 chromosomes inherited from each parent.
  Some somatic cells undergo meiosis to form sex cells called gametes (eggs and sperm).
  Gametes are haploid and can pass on 1 set (23 for humans) of chromosomes to offspring.
  Fertilization from the fusion of haploid gametes creates the diploid zygote, or fertilized egg.
  The zygote undergoes many rounds of mitosis to produce the somatic cells of the organism.
- Gender in humans is determined by the sperm from the father of an individual.
  After meiosis, all gametes are haploid and contain 1 sex chromosome.
  Female eggs always contain the X as its sex chromosome.
  Half of a male's sperms carry a X, half carry a Y chromosome.
  - If a X-carrying sperm fertilizes an egg, the offspring is XX and female.
  - If a Y-carrying sperm fertilizes an egg, the offspring is XY and male.
Mitosis is part of a cell cycle, which is regulated at several checkpoints to control cell growth.
- The cell prepares to divide during interphase of a cell cycle.
  Interphase is where the cell grows and grows and prepares to divide.
  - Gap 1 and Gap 2 are growth phases.
  - G₀ is a "resting" phase of no growth; some cells stay here forever.
  - S phase is where DNA synthesis occurs.
  The nucleus divides during mitosis (M), then the cytoplasm divides during cytokinesis (C).
  Review and mitosis preview:
- The cell cycle is regulated by checkpoints.
  Interphase contains two growth sub-phases (G₁ and G₂) separated by a S (DNA synthesis) sub-phase.
  The checkpoints at G₁, G₂, and M (mitosis) ensure that a particular phase is completed before advancing to the next phase.
  If a cell does not pass any of these checkpoints, mitosis will not proceed.
  Breakdown in checkpoint regulation during the cell cycle may lead to cancer.
- Cancer is a growth disorder of cells.
  Uncontrolled cell division may occur if a cell fails to stop at all the checkpoints of the cell cycle.
  The initial result is a growing cluster of cells called a tumor.
  If a tumor continues to grow, it may become malignant and starts to spread to other areas, the result is cancer.
Mitosis
- Cell division
  - Interphase
  Mitosis stages:
  1. prophase
  2. metaphase
  3. anaphase
  4. telophase
  Cytokinesis: cytoplasm divides.
  This form of asexual reproduction produces daughter cells that are genetically identical.
  Review:
- Interphase
  Cell grows and makes copies of its components.
  DNA replication occurs.
  The cell prepares for division.
- Prophase
  The nuclear membrane breaks down.
  Chromosomes start to condense.
  A network of protein fibers called the spindle forms from microtubules.
- Metaphase
  The sister chromatids line up at the center of the cell.
- Anaphase
  Sister chromatids separate and are pulled to opposite sides of the cell by microtubules of the spindle fibers.
  Each chromatid is now a full-fledged chromosome.
  - A human cell can briefly possess how many chromosomes at this point?
    - 92
- Telophase
  The chromosomes de-condense (uncoil).
  The nuclear membrane reappears.
  Mitosis is complete and the cell enters cytokinesis.
- Cytokinesis
  The cytoplasm divides and two daughter cells form.
  Each cell is a replicate of the parent cell and is diploid.

Meiosis

Meiosis consists of two rounds of cell division and produces 4 haploid cells from each parent cell.
Meiosis I is the reduction division: the daughter cells are haploid where each chromosome has 2 sister chromatids.
Meiosis II is where the sister chromatids of each chromosome separate.
Review:


Prophase I Homologous chromosomes pair up (synapsis). Crossing over (DNA exchange) occurs.	Metaphase I Homologous chromosomes align along center of cell (metaphase plate), the pairs oriented toward opposite poles of the cell.	Anaphase I Homologous chromosomes separate, moving to opposite poles. Sister chromatids are no longer identical: genetic recombination has occurred.	Telophase I/Cytokinesis I One set from the paired chromosomes arrives at each pole. The cell divides into two haploid cells.


Prophase II Chromosomes condense again; spindle fibers form again.	Metaphase II Chromosomes align in the center of the cell.	Anaphase II Sister chromatids separate and move to opposite poles.	Telophase II/Cytokinesis II Chromatids arrive at each pole. Nuclear membranes reform. Each cell pinches into two haploid cells.

Nondisjunction may occur during meiosis, resulting in gametes with incorrect number of chromosomes.
- Nondisjunction is the unequal distribution of chromosomes during cell division.
  This can occur in meiosis I when homologous chromosomes fail to separate during anaphase I.
  Nondisjunction can also occur in meiosis II when sister chromatids fail to separate during anaphase II.
  This produces gametes with incorrect number of chromosomes.
  Most zygotes from such gametes are not viable; survivable cases include trisomy 21 and sex chromosome abnormalities.
  Meiosis II nondisjunction animation:
- Down syndrome is caused by trisomy 21, nondisjunction resulting in the existence of three copies of chromosome 21.
  This condition can be detected by a karyotype.
- Nondisjunction of a sex chromosome is usually survivable.
  In humans, the presence of a Y chromosome determines gender and makes the individual male.
  - Turner syndrome (X_) individuals are females: missing the small Y is not serious.
  - Klinefelter syndrome (XXY) individuals are males: the extra X is inactivated.
  - XYY individuals are males: an extra small Y is not too harmful.
  - XXX individuals are females: the two extra X are inactivated.
  Calico cat and X inactivation
  - Can a calico cat be male?
    - Yes, if he is Klinefelter (XXY).