Biology: Cell Division - Karyotype

What is a Karyotype

• A karyotype is an ordered display of the pairs of chromosomes from a cell.
• The two chromosomes in a pair are called homologous chromosomes, which are the same shape and size, carry genes, but are alleles of each other.

Cell Division: Meiosis

• Meiosis occurs in germ line cells to produce gametes.
• Meiosis I divides the homologous chromosome pairs, creating two cells, each with 23 chromosomes (comprising 46 chromatids).
• Meiosis II splits the sister chromatids, resulting in four daughter cells, each with 23 individual chromosomes.

Meiosis Stages

• Prophase: chromatin condenses into condensed chromosomes, spindle fibers emerge from centrosomes, nuclear envelope breaks down, and nucleolus disappears.
• Prometaphase: nuclear envelope dissolves and spindle fibers attach to the kinetochore.
• Metaphase: chromosomes line up along the equator of the cell, each sister chromatid is attached to a spindle fiber originating from opposite poles.
• Anaphase: cohesin proteins binding the sister chromatids together break down, sister chromatids are split and are pulled to opposite poles of the cell.
• Telophase: nuclear envelope reforms, cytoplasm begins to cleave, and chromosomes arrive at opposite poles and decondense.
• Cytokinesis: splitting of the cytoplasm which results in the formation of two daughter cells.

Genetic Variations within Gametes

• Three ways of genetic variations within gametes: crossing over, independent assortment, and random fertilization.
• Independent Assortment of Chromosomes: homologous pairs of chromosomes orient randomly at metaphase I of meiosis, resulting in 2^n possible combinations.
• Crossing Over: produces recombinant chromosomes, combining DNA inherited from each parent, contributing to genetic variation.
• Random Fertilisation: any sperm can fuse with any ovum, producing a zygote with any of about 70 trillion diploid combinations.

Compare Mitosis and Meiosis

• Mitosis: two diploid daughter cells with identical genetic information.
• Meiosis: division that can produce sperm and egg cells (4 unidentical haploid daughter cells).

Aneuploidy and Chromatin

• Aneuploidy: a condition characterized by the presence of an abnormal number of chromosomes.
• Chromatin: the entire complex of DNA and proteins, which condenses during cell division.

Cohesin Proteins and Chromosome Structure

• Cohesin proteins: proteins that bind to DNA to keep strands/sister chromatids together.
• Chromosome structure: p arm (short arm), q arm (long arm), and centromere (specialized DNA sequence which links the sister chromatids).

Cell Cycle

• The cell cycle consists of 90% Interphase and 10% Mitotic Phase.
• Interphase: G0 (resting phase), G1 (cell growth), S (DNA replication), and G2 (cell preparation).

Meiotic Non-Disjunction and Mosaicism

• Meiotic Non-Disjunction: failure of homologous chromosomes to segregate properly during anaphase 1 and/or sister chromatids in anaphase 2, resulting in the production of gametes with an improper chromosome number.
• Mosaicism: some of the cells are aneuploid.

Polyploidy and Trisomy

• Polyploidy: gain of a whole haploid set of chromosomes.
• Trisomy: gain of an extra chromosome, resulting in conditions such as Down syndrome, Edward's syndrome, and Patau syndrome.

Turner's Syndrome and Klinefelter's Syndrome

• Turner's syndrome: a condition where only one X chromosome is inherited, resulting in a sterile female, short in stature, and heart and kidney defects.
• Klinefelter's syndrome: a condition where an individual has an extra X chromosome, resulting in a male with abnormal physical and reproductive characteristics.