Chapter 12
Cell Continuity
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Cell Continuity
All cells develop from pre-existing cells
3 steps to form a new cell
1. Produce materials it will need
2. Grows larger
3. Reproduces to form a new cell
Cell continuity is the ability of cells to
divide and survive from one generation to
the next.
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Chromosomes
• Structures in Nucleus, made of DNA &
Protein
• Not dividing = Chromatin (long thin threads)
• When dividing = Chromatin forms a numbers
of clearly distinguishable Chromosomes
• Each species has a definite no. of
Chromosomes, Humans = 46 chromosomes
• Each Chromosomes has 1000s of genes
• What’s in a Nucleus
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TERMS
• Chromatin - When the cell is not
dividing, chromatin exists as a mass of
very long, thin fibers that are not visible
with a light microscope.
• Sister chromatids - Replicated forms of
a chromosome joined together by the
centromere and eventually separated
during mitosis or meiosis.
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Learning Check
1. What does cell continuity mean?
2. What 3 stages are involved in cell
continuity?
3. What is a chromosome?
4. When chromosomes are not dividing what
are they known as?
5. How many chromosomes are found in a
typical human cell?
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Haploid
• A Haploid cell has one set of
chromosomes, i.e. has only one set of
each type of chromosomes in the
nucleus.
• Haploid is symbolised by letter ‘n’ and
number of chromosomes in the cell is
given as e.g. n=23
• In humans, eggs and sperm are haploid
n=23
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Diploid
• A Diploid has two sets of chromosomes,
i.e. It has two of each type of chromosome
in the nucleus
• Chromosomes are in pairs in diploid cell,
called homologous pairs.
• Diploid is symbolised as ‘2n’ and total no.
of chromosomes in cell is given as e.g.
2n=46
• In humans body / somatic cells are diploid
2n=46
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In diploid cells, 1
chromosome
from the
homologous pair
comes from the
mother and the
other from the
father.
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Learning Check
1. What is meant by haploid?
2. What is meant by diploid?
3. How many chromosomes does a haploid
human cell have?
4. What is the correct way to write this?
5. How many chromosomes does a human
diploid cell have?
6. What is the correct way of writing this?
7. If a cell has a diploid number 2n=6 what is
its haploid number?
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Cell Cycle
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Cell Cycle
• The cell cycle describes the cell’s
activities in the state of non-division
(interphase) and division (mitosis).
• At Interphase when the cell is not
dividing, there is only one cell, but after
Mitosis there are two identical cells called
daughter cells .
• Mitosis is a form of nuclear division in
which one nucleus divides to form two
nuclei, each containing identical sets of
chromosomes.
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Interphase
Longest phase in cell cycle as before
mitosis can take place, the cell needs to
store enough energy to drive the chemical
processes during the cell division.
1. chromosomes become visible in the
nucleus short, thick and duplicated(DNA )
2. Produces new organelles e.g.
mitochondria, centrioles
3. Produces chemicals needed for growth.
4. Chromosomes under microscope
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INTERPHASE
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Learning Check
1. How many stages are there in the cell
cycle?
2. What part of the cell cycle does the cell
spend most of its time in?
3. What happens in this stage?
4. What is the stage when the cell is
actually dividing called?
5. What kind of cells are produced by
mitosis?
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Stage 1 - Prophase
Each chromosome has replicated during
interphase and is therefore composed of
two sister chromatids containing
identical genetic information together at
region called the centromere
1. The nuclear membrane and nucleolus
are no longer visible.
2. The Centrioles / spindle apparatus has
migrate to opposite poles of the cell.
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PROPHASE
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Stage 2 - Metaphase
• Chromosomes migrate towards the cell
centre.
• The spindle fibres attach themselves to the
centromeres of the chromosomes, 1 from
each side of the cell.
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METAPHASE
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Stage 3 - Anaphase
• The spindle fibres shorten and
• the centromere splits,
• separated sister chromatids are pulled
to opposite ends of cell.
• Hence, identical set of genes pulled to
each end of the cell.
• Stages of Mitosis
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ANAPHASE
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Stage 4 - Telophase
• The chromosomes reach the poles of their
respective spindles.
• Nuclear envelope forms around each of the 2
sets of chromosomes
• Chromosomes elongate within each nucleus
• Cleavage furrow formation in animal cells,
• cell plate formation in plant cells.
• Mitosis is complete
• Once complete, original cell divides to form 2
cells
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TELOPHASE
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Mitosis Sequence of Stages
1.
2.
3.
4.
5.
Interphase
Prophase
Metaphase
Anaphase
Telophase
I Party Monday And Tuesday
Mitosis Song
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• Mitosis Rap song
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Learning check
1. How many stages are there in
mitosis?
2. What happens in each stage 1?
3. What happens in each stage 2?
4. What happens in each stage 3?
5. What happens in each stage 4?
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Parts of the chromosome involved
in mitosis
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Functions of Mitosis
In Unicellular Organisms
• mitosis allows the organisms to multiply
e.g. Amoeba
In Multicellular Organisms
• mitosis is responsible for growth,
renewal and repair e.g. production of
new cells.
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Learning Check
1. What is the function of mitosis in
Multicellular organisms?
2. What is the function of mitosis in
unicellular organisms?
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Everyone together
• The Mitosis Song 'I Will Divide'
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Cancer
• Rate of cell division (mitosis) is carefully
controlled.
• Sometimes a cell or group of cells lose
the ability to control the rate of cell
division.
• They form a mass of cells called a
tumour which can be benign or
malignant.
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Benign tumours
• Benign means kind, they are not life
threatening and do not invade other
tissues. E.g. warts, skin tags.
• Cells stop dividing after some time
• Cancer is a group of disorders in which
certain cells lose their ability to control
both the rate of mitosis and the number
of times mitosis takes place.
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Malignant tumours
• Uncontrolled multiplication of abnormal
cells
• Malignant tumours (cancers) invade
other cells and move around the body
• Movement of these cells called
Metastasis.
• Cancer cells divide indefinitely.
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Causes of Cancer
• Caused when normal genes are altered to
form cancer-causing genes called
oncogenes
• Brought about by cancer causing agents
called carcinogens e.g. cigarette smoke,
asbestos fibres, ultraviolet radiation and
some viruses
• Most cancers can be cured with Radiation
(burn out cancer), Chemotherapy (Chemicals
slow down mitosis) and surgery.
• Cancer
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Learning Check
1. How do tumours form?
2. What is the difference between benign
and malignant tumours?
3. When cancer cells spread it is called
m......................?
4. Can you name 3 possible causes of
cancer?
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Meiosis
• Is a form of nuclear division in which the
daughter nuclei contain half the
chromosome number of the parent
nucleus.
• Human cells have 46 chromosomes
• Meiosis occurs in the ovaries and testes
to produce gametes called eggs and
sperm so there are 23 chromosomes in
each egg and sperm
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THE STAGES OF MEIOSIS
• Involves 1 replication and 2 cell divisions,
producing four haploid (n) cells:
Meiosis 1
Prophase 1: - nuclear membrane dissolves
– centrioles move to poles
– spindle fibres forming
– DNA has been replicated and forms chromosomes
made of 2 identical chromatids
– Homologous chromosomes line up
– Where chromatids overlap it allows for CROSSING
OVER of genetic information between
chromosomes
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• Metaphase 1: - homologous chromosomes (4
chromatids) line up on the equator
• Anaphase 1: - homologous chromosomes
separate and each chromosome (2
chromatids) move towards opposite poles
(SEGREGATION)
• Telophase 1: - cytoplasm divides to form 2
new cells with 46 chromosomes each
– the nuclei may not form
– NOTE: the cells are not identical
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Meiosis 2
• Interphase 2: no doubling of DNA, it is just the
time between meiosis I/II
• Prophase 2: - spindle fibres forming
• Metaphase 2: - chromosomes (2 chromatids)
line up on the equator
• Anaphase 2: - chromatids separate, each one
moves to opposite pole
• Telophase 2: - nuclear membrane forming
– 4 daughter cells are produced, haploid, (n)
– 4 sperm but 1 egg (3 polar bodies)
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Functions of Meiosis
In Multicellular Organisms:
1. Allows sexual reproduction without
increasing the number of chromosomes
in the offspring
2. Allows new combinations of genes
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Compare Mitosis & Meiosis
Mitosis
Meiosis
No. Of Chromosomes in
resulting cell
Same as parent cell i.e. kept
constant - Diploid
Half the number of the
parent cell - HAPLOID
Genes present on the
Chromosomes in resulting
cell
Identical to the parent cell
Different to the parent cell
Result
2 identical daughter cells
4 haploid cells
Role
Asexual Rep in unicellular
organisms,
Growth/replacement in
multicellular organisms
Produce haploid gametes
for reproduction,
genetic variation
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Learning Check
1.
2.
3.
4.
What is meiosis?
How does meiosis differ to mitosis?
What is the function of meiosis?
What parts of a human would you expect
meiosis to occur in?
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Syllabus Can You?.................. (OL)
• Explain the terms: cell continuity & chromosomes.
• Define the terms: haploid & diploid number.
• Describe the cell activities in he state of non-division:
Interphase and Division (mitosis).
• Define the term: mitosis.
• Explain the process in simple terms with some
diagrams.
• Define cancer and state causes.
• State the primary function of mitosis for single-celled
vs multi-cell. Organisms.
• Define the term: meiosis.
• State the functions of meiosis.
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Syllabus Can You?............... (HL)
• Explain the terms: cell continuity & chromosomes.
• Define the terms: haploid & diploid number.
• Describe the cell activities in the state of non-division:
Interphase and Division (mitosis).
• Define the term: mitosis.
• Define cancer and state causes.
• State the primary function of mitosis for single-celled vs multicell. Organisms.
• Define the term: meiosis.
• State the functions of meiosis.
• Explain the process in more detailed terms with diagrams (be
able to name each stage and chromosome parts)
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http://biology.about.com/library/blmi
tosisanim.htm
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• Mitosis in a Winter Wonderland
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