Mitosis and the Cell cycle
Mitosis and Cell Cycle
Learning Objectives
• - Explain the significance of
mitosis
• - Explain the meaning of the
term homologous pair of
chromosomes;
• - State that mitosis occupies
a small amount of the cell
cycle
Success Criteria
• List purpose of mitosis
• Draw and annotate the cell
cycle from the video
resource
• Identify the phases of
mitosis from EM
micrographs
Chromosome number
A human somatic (body) cell contains 46
chromosomes. These consist of 23 pairs of
homologous chromosomes.
Each pair contains one chromosome
from each parent. Other species have
different numbers of these
homologous pairs.
Sex cells, or gametes, have only one copy of each chromosome: they are haploid. A
somatic cell, containing two of each, is called diploid.
Haploid gametes
All somatic cells in a multicellular organism are genetically identical because they are
the result of mitosis.
They are all descended from a single cell – a zygote.
A zygote is formed when two haploid
gametes fuse.
These gametes are genetically unique
because, unlike somatic cells, they
were formed by a special form of cell
division called meiosis.
What is the cell cycle?
All cells are formed by division of existing cells. Eukaryotic cells always divide in one
of two ways:
Mitosis: division into two daughter cells
that are genetically identical to each
other and to the parent cell.
Meiosis: division into four unique daughter
cells with half the chromosomes of the
parent cell.
All cells follow the same sequence of events between mitotic divisions, and this is known
as the cell cycle.
The cell cycle
Stages of the cell cycle
What is mitosis?
Mitosis specifically refers to the process of nuclear division that occurs before a cell
physically divides in two.
During mitosis, the cell’s DNA is copied into each of the two daughter cells. In multicellular
organisms, mitosis provides new cells for growth and tissue repair.
In eukaryotes, it can also be a form
of asexual reproduction. This most
commonly occurs in single-celled
organisms, such as yeast.
Mitosis
Cell cycle summary
The life cycle of a cell
Interphase can be
divided into 3
phases
• G1 – cell growth,
duplication of
organelles
• S – DNA replication
and chromosome
duplication
• G2 – cell growth and
preparation for
mitosis
Interphase – S phase
• Before mitosis can occur 2 copies of each
chromosome are needed.
• During interphase the chromosomes which
initially consist of a single DNA molecule are
duplicated.
•The new chromosome is made of two
identical structures called chromatids
•Each chromatids contains one DNA
molecule
•Chromatids are held together by a region
called the ‘centromere’
•DNA is made up of a series of genes
Mitosis
With mitosis, the nucleus of a cell will divide
resulting in two nuclei with the same number of
chromosomes as the parent chromosome.
•
•
Can be split into four stages:
Table: three columns Stage, Details, picture
1) Prophase
2) Metaphase
3) Anaphase
4) Telophase.
• Mitosis animation
• The chromosomes shorten and
thicken by coiling (supercoiling)
• The nuclear membrane breaks
down at the end of prophase
• The other structures important
for mitosis are also forming (i.e.
the centrioles or microtubule
organising centre MTOC).
Microtubules grow from the MTOC at the poles of the cell to the
chromosomes. They form a spindle shape. (Mitotic spindle)
•
The chromosomes are lined up along
the cell's equator .
• Spindle microtubules from each pole
are attached to each centromere on
opposite sides.
• The centromeres divide and the
sister chromatids have become
chromosomes.
• The chromosomes are pulled along
the microtubules toward opposite
poles of the cell.
• The chromosome have
migrated to the poles.
• The nuclear membrane
reforms.
• The mitotic structures
breakdown.
• The chromosomes uncoil.
• The plasma
membrane of the
cell pinches down
along the equator
creating two
separate cells. At
this time, the
chromosomes
become indistinct
(as they are during
Interphase).
Biological significance
of mitosis?
The significance of mitosis is its ability to produce daughter
cells which are exactly the same as the parent cell. It is
important for three reasons…
• If a tissue wants to get bigger by growth it needs new cells
that are identical to the existing ones.
• Damaged cells have to be replaced by exact copies of the
organism so that it repairs the tissues to their former
condition.
• It is the basis of asexual reproduction.
Stages of the cell cycle
Meiosis I and II
Meiosis is the process of cell division underlying sexual reproduction. It is a two-stage
process:
Meiosis I introduces genetic diversity by
randomly dividing a cell’s genes in two. It
results in two haploid cells.
Meiosis II is similar to mitosis. It splits each
chromosome into its two chromatids and places
one in each daughter cell. It results in four
haploid gametes.
Genetic variation
Sexual reproduction creates genetic
diversity within a population, which is
vital to a species’ survival.
Two processes during meiosis
determine the unique genetic makeup of the four daughter cells:
During meiosis I, homologous pairs of chromosomes swap parts of their genetic material.
This is crossing over.
The chromosomes from each pair are randomly allotted to the daughter cells by
independent assortment.
Meiosis: true or false?
Variation from meiosis
Chromosome mutations