Chapter 2 Notes – Mitosis & Meiosis
• Genetic material of eukaryotes is often distributed among multiple linear chromosomes.
• Chromosome (definition): In eukaryotic cells, a linear structure composed of a single DNA
molecule complexed with protein. Each species has a characteristic number of chromosomes in
the nucleus of its cells.
Haploid and Diploid
• Eukaryotes that have two copies of each type of chromosome in their nuclei
– Diploid or 2n
• Eukaryotes that have only one set of chromosomes…usually the gametes
– Haploid or “n” (actually 1n but we leave off the 1 since it is understood).
• Example: Human somatic cells have 46 chromosomes, while our gametes have 23.
– 2n=46, n=23
• In diploids, chromosomes that contain the same genes and that pair during meiosis are called
– Each member is called a homolog
– One homolog is inherited from each parent.
• In humans, one is the paternal homolog while the other is the maternal
• Chromosomes that contain different genes and that do not pair during meiosis are called
Autosomes and Sex Chromosomes
• Chromosomes that differ in number or morphologically between two sexes are called sex
– One sex may have a matched pair of sex chromosomes, while the other sex may have
an unmatched pair or just a single sex chromosome.
– Example: in humans, females have two X chromosomes (XX) while males have only one
X and a Y chromosome (XY).
Chromosomes other than sex chromosomes are called autosomes
• A complete set of all the metaphase chromosomes of a cell is called a karyotype.
• Notice for this human karyotype there are 22 autosomes and one of each of X and Y.
– Largest chromosome is #1
– Homologues are paired Additional Nomenclature for Chromosomes
• When displayed in a karyotype the chromosomes are arranged from largest to smallest, and
each chromosome is orientated with the largest chromosomal arm down, and the smallest on
• The short arm is designated as p and the long arm is designated as q.
– Used for the location of markers along the chromosomes
– Eg. BRCA1 is at location 17q21 (on chromosome 17 on the long arm in region 21).
Interphase and the Cell Cycle
• In both unicellular and multicellular eukaryotes cellular reproduction is necessary for growth.
• This is a cyclical process that includes growth and cell division…the cell cycle.
• The cell cycle is divided into two stages:
– Interphase and cell division Interphase
• Interphase is the interval between cell divisions and is subdivided into:
– G (gap 1) Presynthesis stage where the cell prepares for DNA replication and grows.
– S (synthesis) Stage of DNA synthesis or replication.
– G 2gap 2) Postsynthesis stage where the cell prepares for the onset of mitosis or cell
– G 0 nondividing quiescent state some cells enter.
• These cells are nondividing.
• The length of G 1an vary among the different cell types
– Minutes in rapidly dividing cells such as cancer cells and fetal cells, to years for some
differentiated cells such as nerve cells.
• In contrast S and G 2re the same for most cell types.
– Once started a cascade of events occur to transition the cells through the stage.
• Series of controls determine if cells continue through G to1S, or withdraw from cell cycle into
• G0cells are metabolically active and may re-enter cell cycle if stimulated.
• Cancer cells avoid G 0 or spend only a short time in G co0pared to other cells.
Cell Cycle Checkpoints
• The cell cycle has 3 major checkpoints in the mitotic cell cycle.
• Controlled by the activity of kinases and cyclins.
• Mitosis occurs in both haploid and diploid cells.
– Outcome--two cells, each with the same number and type of chromosomes as the
original parent cell.
– The process is divided into four cytologically distinguishable phases:
– Prophase, metaphase, anaphase and telophase. Mitotic Spindle
• During prophase, the mitotic spindle assembles outside of the nucleus from microtubule
organizing centres (MTOCs).
– In animal cells, the MTOCs are the centrioles which divide prior to S phase.
• The pair of centrioles separate during prophase aligning themselves at opposite
poles of the cell.
• From each centriole a sunburst of microtubules are projected across the
• The centrioles establish the axis for the upcoming mitotic division.
• These microtubules bind to the centromere regions of the chromosomes forming