Mitosis
I.
INTRODUCTION
A. Chromosomes
1. At cell division, chromatin (long threads of DNA and protein) coils, folds, and
twists into a highly compact structure termed chromosomes
2. Each organism has a characteristic number of chromosomes (humans 2N= 46)
a) In the human, the 46 pairs of chromosomes are divided into 44 pairs of
autosomes (non-sex chromosomes) and one pair of sex chromosomes
(1) XX in females and XY in males
3. Prior to cell division, each chromosome is composed of 2 genetically identical
parts termed sister chromatids held together at a region called the centromere
B. Binary fission is another type of asexual reproduction
1. Binary fission is used by most bacteria
a) And also in chloroplast and mitochondria(?)
2. The chromosome is replicated and partitioned into daughter cells
II.THE
LIFE CYCLE OF HUMANS
A. Meiosis
1. Occurs in the gonads (testes and ovaries) to produce gametes (sperms, egg),
which are N, or haploid (1 set of chromosomes)
2. When the sperm fertilizes the egg, a zygote (diploid or 2N) is produced
B. Mitosis
1. Definition
a) The partitioning of chromosome before asexual reproduction
III.MITOSIS
A. Introduction
1. Each body cell is genetically identical as a result of mitosis
a) They all have the same number and kinds of chromosomes
2. Functions
a) Growth from a zygote (fertilized egg) to an adult organism
b) Repair (of a wound) in multicellular organisms
c) Asexual reproduction in other organisms
B. Cell cycle
1. Consists of mitosis followed by interphase.
2. Interphase
a) Cells have a distinct nuclear envelope, nucleoli and chromatin; metabolically
active
b) If a cell is to divide, it leaves the G1 phase and enters the S stage
(1) DNA replicates to form 2 genetically identical sister chromatids and the
centrioles duplicate to form 2 pairs outside the nucleus
(2) The cell then enters the G2 stage
c) If a cell is not to divide, it remains in the GO stage
C. Overview of Mitosis
1. Mitosis produces 2 daughter cells, each with the same number and kinds of
chromosomes as the parent cell
a) Therefore, the parent cell and the daughter cells are genetically identical or
clones
2. Interphase
a) Before mitosis, each chromosome duplicates to form 2 sister chromatids
(1) Sister chromatids separate during mitosis and go to daughter cells
b) Stages on interphase
(1) G0
(a) Highly differentiated cells that are not going to divide remain in this stage
(b) Includes cells like neurons and muscles
(2) G1
(a) During this stage a cell can perform its normal function and grow
(b) Also during this stage a cell repairs its DNA
(i) If the DNA cannot be repaired a cell will undergo apoptosis
(3) S
(a) DNA polymerases will make a copy of the genome
(b) Each cell will still have only 46 chromosomes as the copies (sister
chromatids) remain attached to each other at the centromere
(4) G2
(a) A cell will perform its normal function at this stage and continue to grow
3. Karyokinesis and cytokinesis
a) Karyokinesis
(1) The nuclear membrane disintegrates into small vessicles at the onset of
mitosis and fuse together to reform the nuclear membrane at the end of
mitosis
b) Cytokinesis
(1) The is the division of the mother cell into daughter cells
(2) Though technically not part of the mitotic cycle, in humans this usually occurs
during telophase
D. The Stages
1. Prophase
a) Nuclear envelope disintegrates into small vesicles
b) Chromosomes (composed of sister chromatids) condense and are randomly
scattered throughout the cell
c) Mitotic spindle apparatus forms
(1) Spindle fibers are composed of microtubules, capable of assembling and
disassembling that is associated with the centrioles at the poles
2. Prometaphase
a) Two pairs of centrioles move away from each other and spindle fibers
appears between them
3.
4.
5.
6.
(1) Non-kinetochore microtubules
(a) Reaches from the poles to the equator and attach to non-kinetochore
microtubules from the other pole
(b) These are able to lengthen which elongate to elongate the cell
(2) Kinetochore microtubules
(a) These attach to the chromosome centromeres
(b) These are able to lengthen or shorten thereby moving the chromosomes
during the mitotic stages
b) Chromosomes begin their movement towards the equator
Metaphase
a) Chromosomes move to and align at the equator
b) At end of metaphase, the centromeres split
Anaphase
a) Chromatids separate and (now called daughter chromosomes) migrate to
opposite poles of the spindle
Telophase
a) Begins when chromosomes arrive at the poles
b) The spindle apparatus disappears
(1) Mictotubules disassemble
c) The nuclear envelope and nucleoli reappear
d) Chromosomes relax to form indistinct chromatin
Cytokinesis
a) Cytoplasmic division
(1) Occurs in animal cells by furrowing (indentation of cell membrane) to form 2
daughter cells
(2) Occurs in plants by fusion of vesicles from the Golgi and usually the formation
of a cell wall