Cell Cycle
the sequence of events that occur between the completion of one cell division and the end of the next division
karyokinesis
nuclear division
Cytokinesis
cytoplasmic division
Two types of nuclear division
mitosis & meiosis
Mitosis
chromosome replication before cell division (replication division)
1. forms somatic cells
2. daughter cells are identical to parental cells
3. resulting daughter cells are diploid
Subdivided into 4 stages:
1. prophase
2. metaphase
3. anaphase
4. telophase
Prophase
Chromosomes become thicker, shorter, and more visible
Sister chromatids join at the centromeres
Centrosomes split into two centrioles
metaphase
Chromosomes are aligned at the equator of the cell
Chromosomes are at their thickest and shortest structure
anaphase
Sister chromatids separate and migrate to poles of the cell
Chromatids are now called chromosomes
Equal segregation of chromosomes into two developing nuclei
telophase
Formation of two nuclei each containing daughter chromosomes at cell poles
Cytokinesis takes place
Cell membrane forms between the two daughter nuclei; known as a cleavage furrow in animal cells
Interphase
Begins when cell division is completed
Divided into 3 stages
1. G1: nucleus and cell increase in size & chromosomes are extended
2. S: DNA synthesis occurs
3. G2: protein synthesis occurs
Meiosis
specialized cell division (reduction division)
1. forms reproductive cells
2. daughter cells are similar but not identical to parental cells
3. resulting daughter cells are haploid
Consists of 2 rounds of cell division
Each division consist of the same 4
Meiosis i: Prophase 1
Chromosomes begin to shorten and thicken
Homologous chromosomes come together at the synapse
Crossing over occurs
Meiosis i: Metaphase I
These homologous chromosomes once again align at the equator
One member of each pair faces the opposite pole of the cell
Meiosis i: Anaphase I
Homologous chromosomes begin to separate and migrate to cell poles
Entire chromosomes migrate to the poles not sister chromatids
Each pole randomly receives maternal or paternal chromosome of each homologous pair
Meiosis i: Telophase I
Chromosomes arrive at the poles of the cells
Nuclear membrane forms
Reorganization of the nucleolus occurs
Meiosis II: Prophase II
Nucleolus disappears
Nuclear membrane disappears
Mitotic apparatus and spindles begin to form
Meiosis II: Metaphase II
Chromosomes organize at the midpoint (equator)
Meiosis II: ANAPHASE ii
Sister chromatids begin to separate
Sister chromatids migrate to the poles
Meiosis ii: Telophase II
Chromosomes lengthen
Nucleus reforms
Nucleolus reorganizes
Cytokinesis occurs
Crossing over
exchange of genetic material between homologous chromosomes
Crossing over results in recombinant chromosomes
Crossing over also contributes to genetic diversity
There are 3 cell cycle checkpoints
1. G1(restriction) checkpoint: determines if cell is divided, delayed, or put into resting stage
2. G2 checkpoint: DNA replication is checked
3. M-checkpoint: checks for proper assembly of mitotic spindles (microtubules) to the kinetochores
Chromosomal Disorders
Occurs due to loss of control during cell cycle
2 types of chromosomal aberrations:
1. Numerical aberrations: are caused by non-disjunction (anaphase I or II)
2. Structural aberrations
aneuploidy
When cells have abnormal chromosome numbers
trisomic cells
One extra chromosome
monosomic cells
cell is lacking or missing a chromosome