Cell Division: Mitosis and Meiosis

Cell Division

• Cell division's goal is to make genetically identical daughter cells via cell replacement, growth, asexual reproduction, or gamete formation for sexual reproduction.

Prokaryotic Cell Division

• Prokaryotic cells reproduce through binary fission, also known as cloning.
• The circular chromosome of DNA in the cell is copied.
• The cell then splits.
• This results in two identical daughter cells that move away from each other.

Eukaryotic Cell Reproduction

• Eukaryotic cells undergo mitosis, where the nucleus divides, followed by cytokinesis where the cell splits into two.
• Mitosis produces two identical nuclei with identical DNA.
• Eukaryotic DNA consists of many linear chromosomes.
• Meiosis results in four genetically different cells, each containing half (haploid) of the original DNA.

The Cell Cycle

• The cell cycle includes Interphase, Mitotic (M) phase, and Cytokinesis.
• Interphase is a non-division phase including G1 (metabolism), S (DNA replication), and G2 (preparation for mitosis) phases.
• The Mitotic (M) phase is the nuclear division.
• Cytokinesis is the cell division stage.

Interphase

• Interphase consists of 3 subphases: G1, S, and G2
• There is high metabolic activity during interphase.
• The cell is serving its function and preparing for cell replication.
• During interphase, DNA is uncoiled in chromatin form, a gigantic, linear, double-stranded molecule of uncondensed DNA complexed with proteins called histones.
• The uncondensed length of human DNA within a cell's chromosomes is two meters, while the typical cell size is just 10 micrometers.

Mitosis

• Mitosis is nuclear division, where the nucleus organizes and equally divides the chromosomes

Mitosis Phases

• Prophase involves the duplicated chromosomes condensing and the nuclear envelope disappearing.
• Metaphase involves the thick, coiled chromosomes aligning in the center of the cell on the metaphase plate, where spindle fibers attach to the chromosomes.
• Anaphase involves the chromosomes separating and moving toward opposite poles.
• Telophase has the chromosomes arriving at the poles and becoming more diffuse and the reformation of the nuclear envelope.
• Cytokinesis involves the movement of the cytoplasmic membrane to physically separate the cells.
• Mitosis results in two identical daughter cells.

Control of the Cell Cycle

• Checkpoints exist in the cell cycle to allow progression only when appropriate cues are present.
• Cues can come from within the cell or from its environment, such as growth factors.
• Cancer cells can escape these controls, bypassing cell cycle checkpoints even without appropriate stimulation.
• This results in uncontrolled growth that can disrupt tissue architecture and function.
• Cancerous cells can also invade surrounding tissues or spread to other body sites via circulation.

Human Genome

• The human genome consists of 22 pairs of somatic chromosomes and one pair of sex chromosomes.
• There are a total of 46 chromosomes and 23 "homologous pairs".
• Diploid cells (2n) contain both chromosomes of a pair, such as both chromosome #21, one from each parent.
• Haploid cells (1n) contain one chromosome of a pair, such as one of the chromosome #21.
• Mitosis results in 2n -> 2n, which happens in somatic cells.

Karyotype

• A karyotype is a photo of a cell frozen in metaphase, with chromosomes organized by size

Reproduction

• Asexual reproduction results in exact copies of the parent (2N to 2N), such as in binary fission or mitosis in somatic cells.
• Variation in cells arises from mutations or environmental effects.
• Somatic tissues (2N) undergo mitosis for replacement.
• Germ-line tissues (2N) undergo meiosis for gamete formation.
• Gametes (1N) are single cells produced by meiosis (egg or sperm).
• Sexual reproduction involves meiosis for gamete production.
• Two parents contribute a set of chromosomes (one from each) resulting in 1 gamete + 1 gamete = 1 zygote.
• 1N + 1N = 2N.
• Sexual reproduction creates variety beyond mutations or the environment.

Meiosis

• Meiosis involves of two nuclear divisions.
• DNA is only replicated once.
• Functions include going from 2N to 1N, equal division, and genetic diversity.

Meiosis I

• Prophase I has tetrads of homologous pairs crossing over during "synapsis".
• Metaphase I has tetrads lining up randomly.
• Anaphase I separates pairs.
• Telophase I is the last phase of Meiosis I.
• Metaphase I involves independent assortment.

Meiosis II

• In Prophase II, it is the same as mitosis.
• In Metaphase II, each ell is haploid.
• In Anaphase II, it is the same as mitosis, except haploid.
• In Telophase II, it is the same as mitosis, except haploid
• Meiosis produces 4 haploid gametes.

Down Syndrome (Trisomy-21)

• Nondisjunction is the failure of chromosomes to separate during anaphase (more common in anaphase II).
• Aneuploidy is a different chromosome number than the parent(Trisomy-21).