Organisms grow, repair tissues and replace worn out cells by cell division.
Instructions for cell development and function are coded for in the DNA of
the cell, which must be replicated before the cell can divide. Humans have
46 DNA molecules in each cell. These molecules are organised into
Mader Fig 5A
In order for cells to divide, they must go through several stages. Firstly there is
a growth period (G1) in which organelles begin to double in number. The
nuclear then DNA replicates (S phase), followed by a second growth phase
(G2). Finally mitosis occurs, resulting in two cells with identical DNA. In
mammals one round of division takes about 16 hours.
Mader Fig 5.2 The cell cycle
Control of cell division
There are two control points in the cell cycle: transition between G1 and S, and
transition between G2 and mitosis. At each point the transition is triggered by
phosphorylation of a regulator protein by a cyclin/kinase complex. By
preventing the S phase transition, cells can be prevented from dividing again.
M cyclin M kinase S kinase S cyclin
protein triggers protein triggers S
Mader Fig 5.3
Mitosis is a cell division that produces two daughter cells with the same DNA
content as the original cell. Various stages in this continuous process are called
Chromosomes normally consist of a single long molecule of double stranded DNA.
But we can only see chromosomes when they are condensed and ready for mitosis.
At this stage, the DNA is replicated, so there are actually two identical DNA
molecules joined together. To make matters even more confusing, each cell also
has two similar, but not identical, copies of each chromosome.
Each chromatid is a single DNA
chromosome A homologous chromosome (similar but not
Animals and Plants are diploid organisms, that is, they have two copies of each
kind of chromosome. Diploid organisms receive one copy of each chromosome
from each parent during sexual reproduction. Bacteria reproduce asexually, and
are haploid organisms with a single copy of each chromosome.
Chromosomes of a normal human female. Note the 46 chromosomes consist of 23 pairs.
Human chromosomes Human lymphocyte cell
(metaphase) stained to reveal the (metaphase) with chromosomes
chromosome ends (telomeres) labelled different colours
Human metaphase Each chromatid is a single DNA
chromosomes: scanning molecule
Centrioles in centrosomes have duplicated, chromatin is condensing
Centrosomes move apart, nuclear envelope and nucleolus disappears, Two
chromatids may be visible on each chromosome
Chromosomes are further condensed, Spindle is forming and attaching to
Chromosomes fully condensed and lined up at the center of the fully formed
Centromeres split and daughter chromosomes move to opposite ends of the
Daughter cells form by cytokinesis, nuclear envelope and nucleolus reform, Chromatin
Mitosis in onion root cells All x 1000
URLs for videos
Lily Cell mitosis:
Mitosis in newt cell, showing spindle apparatus
Other cell division videos are available at these sites
Prior to mitosis, cell organelles and DNA replicate. These events occur in G1
and S phases respectively.
Transition between G1/S and G2/Mitosis is under regulatory control.
Mitosis proceeds continuously through four stages: prophase, metaphase,
anaphase and telophase.
Chromosomes are moved during mitosis by a spindle composed of
After mitosis the two new cells are separated by cytokinesis.
Mitosis results in two daughter cells with identical DNA content to the parent
Each sexually reproducing diploid organism has two parents that contribute
equal amounts of DNA. Hence in the formation of reproductive (sex) cells there
must be a way of halving the DNA content of the parental cells. The cell
division process of meiosis accomplishes this.
Mader Fig 5.9
Meiosis: first division
During Meiosis I, crossing over occurs between homologous chromosomes, and
the chromosome number is halved from the diploid number (2n) to the haploid
number (n). In other words at the end of Meiosis I there are two cells with half
the chromosome number of the parent cell. The parental chromosomes may also
have been rearranged.
Mader Fig 5.12
In prophase of meiosis I, homologous chromosomes pair up (synapsis), and
exchange portions. This means that offspring do not necessarily get
chromosomes identical to their parent’s chromosomes.
Chromosome from mother
Chromosome from father Chromosomes going into sex cells
Mader Fig 5.10
Sex cells have different combinations of homologous chromosomes because of
independent assortment during Meiosis I. This is because maternally and paternally
derived chromosomes can line up in different ways prior to anaphase I. Also the
chromosomes may have crossed over, further increasing diversity. In this way, no
two sex cells are likely to ever have exactly the same DNA content.
Maternal and Paternal chromosomes can line up in different ways at
Anaphase I, leading to diversity in subsequent products of meiosis.
Mader Fig 5.11
Meiosis: second division
In the second division of meiosis, chromatids separate and move to opposite
ends of the cell. This results in a total of four cells, each of which contains half
the number of chromosomes in the parental cells.
Mader Fig 5.13
URL for video
Other cell division videos are available at this site
Sexual reproduction proceeds through meiosis, gamete production, and
Animals and plants have a diploid number of chomosomes, that is, two copies
of each chromosome per nucleus.
Meiosis reduces this to the haploid number, that is, one copy of each
chromosome per sex cell.
Meiosis generates diversity through crossing over and independent
Uniting two sex cells results in offspring with the restored diploid
Offspring receive half their DNA from their mother and half from their
father, although the actual chromosomes they receive from both may not be
identical to those in the parent’s cells (because of crossing over).
In humans, meiosis leads to haploid oocytes in females, and haploid
spermatozoa in males. Fertilisation leads to a diploid zygote that grows
by mitosis into a baby and eventually another adult.
Mader Fig 5.16
The haploid phases of human existence:
egg and sperm
Meiosis in humans
Mader Fig 5.16