Download Mitosis in plant cells

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Biology
Preliminary Course
Stage 6
Patterns in nature
Part 8: Growth and repair
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Contents
Introduction................................................................................ 2
Cell division ............................................................................... 3
Sites of mitosis .....................................................................................4
What happens during mitosis? ............................................................6
Mitosis in plant cells .............................................................................7
Cytokinesis .........................................................................................11
Additional resources ................................................................ 12
Suggested answers ................................................................. 15
Exercises–Part 8 ..................................................................... 17
Student evaluation of module
Part 8: Growth and repair
1
Introduction
Maintenance of organisms requires growth and repair.
In this part you will be given opportunities to learn to:
•
identify mitosis as a process of nuclear division and explain its role
•
identify the sites of mitosis in plants, insects and mammals
•
explain the need for cytokinesis in cell division
•
identify that nuclei, mitochondria and chloroplasts contain DNA.
In this part you will be given opportunities to:
•
perform a first–hand investigation using a microscope to gather
information from prepared slides to describe the sequence of changes
in the nucleus of plant or animal cells undergoing mitosis
Extract from Biology Stage 6 Syllabus © Board of Studies NSW, originally
issued 1999. The most up-to-date version can be found on the Board's website
at http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_lista.html
This version November 2002.
Materials required:
•
microscope and lamp
•
two slides and a cover slip
•
onion with fresh roots (you need to soak onion base in water at least a
week in advance)
•
methyl green pryonin or aceto–orcein stain.
Alternatively use prepared slides of a root tip (if available).
If you do not have access to a microscope or prepared slide, use the
photographs provided.
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Cell division
A multicellular organism such as a human, begins life as a single cell
formed from the union of two sex cells. From this microscopic beginning
the organism grows to become an adult. This is achieved by the process
of cell division. One cell divides forming two cells and then each of these
cells divide forming more cells to continue the process of cell division.
Mitosis is a type of cell division that results in the replication of identical
cells. Meiosis is another type of cell division which produces gametes or
sex cells. Meiosis produces cells that have half the number of
chromosomes of the parent cell.
Mitosis results in growth of an organism, is involved in the healing of
wounds and the replacement of cells eg. red blood cells and skin cells.
With the exception of gametes (ova and sperm), all the body cells or
somatic cells come from pre–existing cells by mitosis.
1
Write a definition, in your own words, for mitosis.
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2
How is mitosis different from meiosis?
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3
What is the role of cell division in multicellular organisms?
_____________________________________________________
_____________________________________________________
Check your answers.
Part 8: Growth and repair
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Sites of mitosis
Mitosis occurs in areas of rapid growth in organisms. These sites are in
different places in different types of organisms. This is usually due to the
need for rapid replication such as growth points or sites where repair to
damaged tissue is required. Multicellular organisms may also have stages
in their life cycle during which mitosis may occur at a greater rate such as
within a developing foetus.
Plants
Mitosis in plants occurs in special cells called meristematic cells and in
the layer of cells in the stem called cambium. These cells are responsible
for the growth in length and width.
In the root there is a protective area called the root cap. Behind this area
is the apical meristem where active cell division is occurring. This is
followed by an area of elongation where the newly formed cells increase
in size.
cortex
growth zone
xylem
cell
division
phloem
cell
elongation
root hair
root cap
Diagram of a root tip showing the growth zone.
In the stems, secondary growth occurs in the cambium. The vascular
cambium forms phloem and xylem cells.
At the tip of the shoots there is the apical meristem where mitosis is
occurring rapidly forming new cells. Buds are another structure that
contain meristematic tissue which is capable of rapid growth.
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Insects
Insects have a multiple staged life cycle. During a larval stage the
organism increases in size. This is due to cell enlargement and not cell
division. Increased rates of mitosis occur in the epidermal cells before a
moult during the pupal stage. Metamorphosis results in the breakdown of
the larval tissue and the development of the adult insect.
Mammals
In mammals mitosis is occurring in many parts of the body. Skin, hair
and nails are continually growing. Blood cells are made daily to replace
those that have died. Any injury results in rapid mitosis to repair the
damage. Young mammals are growing rapidly and at this stage of life
mitosis rates are high.
1
Complete the following table by matching the sites of mitosis in either
plant or mammal.
Site of mitosis
Plant/animal
root tip
skin
digestive tract
shoot tip
bone marrow
hair and nails
stems
2
Explain why mitosis is important to insects during metamorphosis.
_____________________________________________________
_____________________________________________________
Check your answers.
Complete Exercise 8.1.
Part 8: Growth and repair
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What happens during mitosis?
When a cell divides, a series of changes occur in the nucleus of cells.
The most important parts of the nucleus involved in the process are the
chromosomes. Chromosomes determine the characteristics of an
organism. Genes are found along chromosomes and consist of sections of
DNA (deoxyribose nucleic acid). Most of the DNA in a cell is found in
the nucleus.
1
What is DNA? Why is it important?
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2
The DNA part of chromosomes carries the genetic code as genes.
Each time a cell divides by mitosis, new daughter cells end up with
chromosomes, and hence DNA, which is identical to those of the
original parent cell.
The discovery of the structure of DNA and the way it is replicated
during cell division, has been one of the most exciting and important
events in 20th century biology. Mitosis is essentially the replication
of chromosomes and their separation into daughter cells.
What major developments in technology do you think assisted in the
identification of DNA?
______________________________________________________
______________________________________________________
Check your answers.
Self replicating organelles
Plastids and mitochondria are self–replicating organelles. This means
that when mitosis is occurring these organelles reproduce independently
of the nuclear division.
Chloroplasts (a type of plastid) and mitochondria both posses genetic
material (DNA) that enables them to replicate. It is thought that they may
be descendants of ancient procaryotic cells that have since become part of
other cells.
1
List the cell organelles that contain DNA.
_____________________________________________________
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2
Most of the cell’s DNA is present in the nucleus. What parts of the
nucleus are made of DNA?
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3
What is the role of DNA in the cell?
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Check your answers.
Complete Exercise 8.2.
Mitosis in plant cells
The process of mitosis in plants is similar to that in animals. However,
there are two differences:
•
there are no centrioles in most plants
•
the cell does not become constricted in the last stage of the process.
In plant cells, the partition usually starts in the centre of the cell and
grows outwards to meet the existing right cell wall.
1
Why do cells undergo mitosis?
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2
What are the differences between the parent cells undergoing mitosis
and the resulting daughter cells?
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3
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What is the significance of division after replication for a cell?
_____________________________________________________
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Check your answers.
Part 8: Growth and repair
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Microscopic examination of mitosis in plant cells
As you have already read the root tip is a site of rapid mitosis in plants.
In this experiment you will examine a root tip for the stage of mitosis.
Materials required:
•
microscope and lamp
•
two slides and a cover slip
•
onion with fresh roots (you need to soak onion base in water at least a
week in advance)
•
methyl green pryonin or aceto–orcein stain.
Alternatively use prepared slides of a root tip (if available).
If you do not have access to a microscope or prepared slide, use the
photographs provided following.
Procedure:
1
Remove a new root from the base of the onion and place it in the
centre of a clean slide (only the top portion is required if it is very
long).
2
Place another slide on top and gently squash the two slides together.
This should grind the root tip.
3
Remove the top slide, ensuring the squashed material remains on the
lower slide.
4
Add one drop of stain to the material and cover with the cover slip.
5
Allow to stand for about 20 minutes and then examine under the
microscope.
Observe the cells in the slide of the root tip. You are looking for the
different stages of mitosis.
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6
Identify cells that have undergone mitosis.
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Draw diagrams showing the stages in mitosis.
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Slide of root tip showing various stages of mitosis. How many stages can you
pick out? (Photo Jane West)
Interphase (animal
cell)
Interphase plant cell
Your drawing of a plant cell
nucleus
Part 8: Growth and repair
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Prophase (animal
cell)
Prophase plant cell
Your drawing of a plant cell
Metaphase (animal
cell)
Metaphase plant cell
Your drawing of a plant cell
Anaphase (animal
cell)
Anaphase plant cell
Your drawing of a plant cell
Telophase (animal
cell)
Telophase plant cell
Your drawing of a plant cell
Patterns in nature
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Cytokinesis
Mitosis refers to the changes involving the chromosomes during cell
division. Cell division, however, usually includes the division of the
cytoplasm and certain organelles within the cytoplasm. The division of
the cytoplasm is called cytokinesis.
In animal cells cytokinesis is usually achieved by the formation of a
cleavage furrow which deepens to constrict the two parts. In plant cells, a
cell wall forms across the middle, separating the two parts.
1
At what point in mitosis does cytokinesis occur?
_____________________________________________________
2
Why is cytokinesis important in cell division?
_____________________________________________________
_____________________________________________________
Check your answers.
Complete Exercise 8.3.
You have come to the end of the module Patterns in nature. You will
have come to recognise that there are patterns in living things as they
adopt similar methods of solving the problems of surviving.
Part 8: Growth and repair
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Additional resources
Phases in mitosis
You do not need to learn the names of the stages of mitosis.
1
Interphase
This stage is sometimes misleadingly called the ‘resting’ stage.
In fact, the cell is very active. It is during this stage that each
chromosome becomes replicated. A cell with four chromosomes would
end up with eight at this stage. Cells with 46 chromosomes (a human
cell) would end up with 92 and so on.
Organelles, such as mitochondria, ribosomes (and chloroplasts in plants)
are also replicated although how they are replicated is not clearly
understood. As well, the centrioles, which are outside the nucleus, begin
to separate. In most of the more complex plants there are no centrioles.
2
Prophase
During this stage, the chromosomes are visible, first as long, thin strands.
As the process continues, the chromosomes become shorter and thicker.
Each chromosome and its replica are held together by a structure called
the centromere. The identical chromosomes at this stage are called
chromatids. The centrioles move to opposite ‘poles’ of the cell and
spindle fibres start to form. This stage ends with the breakdown of the
nuclear membrane.
3
Metaphase
The spindle consists of long molecules of protein lying across the cell
from pole to pole. The chromosomes move through the cytoplasm to the
spindle and become fastened to it by their centromere. The centromere
becomes attached along a plane about halfway between the poles.
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This plane is called the equator. At this middle stage the chromosomes
are in the middle of the cell.
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Anaphase
The centromeres divide so that each chromatid has its own centromere.
Each chromatid now is a daughter chromosome. The daughter
chromosomes move apart, each member of a pair moving to opposite
poles of the cell. Each group of daughter chromosomes forms a densely
packed group at each pole. Remember: at anaphase the chromosomes are
moving apart. Notice that four chromosomes move to opposite parts
(poles) of the cell. Notice also that the cells start to become constricted in
the centre.
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Telophase
Nuclear membranes form around each group of daughter chromosomes.
The chromosomes uncoil to become slender threads. A new cell
membrane forms at the equator. The cytoplasm divides and two new
daughter cells now exist, where there was originally only one parent cell.
Part 8: Growth and repair
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2 early prophase
1 interphase
3 late prophase
nucleus
chromatid
centromere
4 metaphase
spindle
6 telophase
5 anaphase
Mitosis in an animal cell with two chromosomes.
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Suggested answers
Cell division
1
Mitosis is a type of cell division resulting in the replication of
identical cells.
2
Meiosis produces cells that have half the number of chromosomes
compared to the parent cell. This process produces the gametes or
sex cells.
3
Cell division is responsible for growth, repair and reproduction of
multicellular organisms.
Sites of mitosis
1
2
Site of mitosis
Plant/animal
root tip
plant
skin
animal
digestive tract
animal
shoot tip
plant
bone marrow
animal
hair and nails
animal
stems
plant
Cells need to be produced rapidly when undergoing metamorphosis
compared to other stages in which no mitosis occurs.
Part 8: Growth and repair
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What happens during mitosis?
1
DNA makes up the material of inheritance or the genetic material in
a cell. Every cell needs to have its own DNA code for its specific
structure and function.
2
The development of the electron microscope and staining techniques.
Self replicating organelles
1
The nucleus, chloroplasts and mitochondria all contain DNA.
2
The chromosomes in the nucleus are made of DNA. A section of
DNA with specific information is called a gene. Genes are part of
chromosomes.
3
DNA contains information that is transferred when cells replicate.
Mitosis in plants
1
Cells undergo mitosis for growth and repair of body tissue.
2
The parent cells are usually larger than daughter cells initially.
3
Cells must divide after replication otherwise they would end up with
double the amount of genetic material.
Cytokinesis
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1
After the chromosomes have separated into two nuclei the cytoplasm
divides so that the cells are able to enter interphase.
2
Cytokinesis is important because after mitosis the nucleus has
divided and separate nuclear membranes of the daughter cells have
formed. Then the cytoplasm must divide (cytokinesis) to produce
two new cells.
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Exercises - Part 8
Exercises 8.1 to 8.3
Name: ________________________
Exercise 8.1: Mitosis
Mitosis is a very significant process in any living thing.
Write a short report to explain the significance of the process of mitosis
for plants and animals.
Your report should include reference to:
•
the role of cell division in multicellular organisms
•
the activities of chromosomes during mitosis (describe the sequence
of change)
•
where mitosis occurs in plants, mammals and insects.
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Part 8: Growth and repair
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Exercise 8.2: What happens during mitosis?
Identify the parts of a cell that contain DNA.
_________________________________________________________
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_________________________________________________________
Exercise 8.3: Cytokinesis
Explain the importance of cytokinesis.
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