Topic 2.5 Cell Division

IB Biology SL

� 2.5.1 Outline the stages in the cell cycle, including interphase (G1, S, G2), mitosis and cytokinesis.

� 2.5.2 State that tumours (cancers) are the result of uncontrolled cell division and that these can occur in any organ or tissue.

� 2.5.3 State that interphase is an active period in the life of a cell when many metabolic reactions occur, including protein synthesis, DNA replication and an increase in the number of mitochondria and/or chloroplasts.

� 2.5.4 Describe the events that occur in the four phases of mitosis (prophase, metaphase, anaphase and telophase).

� 2.5.5 Explain how mitosis produces two genetically identical nuclei.

� 2.5.6 State that growth, embryonic development, tissue repair and asexual reproduction involve mitosis.

� As you will recall from Topic 1.1.3, all cells

come from pre-existing cells.

� All cells go through a cycle over and over

again known as the cell division cycle.

� It involves:

� Interphase

� Mitosis

� Cytokinesis

� Some cells divide constantly: cells in the embryo, skin cells, gut lining cells, etc. Other cells divide rarely or never: only to replace themselves.

� Actively dividing cells go through a cycle of events that results in mitosis. Most of the cycle was called “interphase” by the microscopists who first studied cell division. During interphase the cell increases in size, but the chromosomes are invisible.

� The cell lifecycle is well defined and can be divided into four stages:� Gap 1 (G1) - The growth phase in which

most cells are found most of the time

� Synthesis (S) - During which new DNA is synthesized

� Gap 2 (G2) - The period during which no transcription or translation occurs and final preparations for division are made

� Mitosis - Cell division

G1G1

MM

G2G2

SS

The Cell Life Cycle

Gap 1 - Doubling

of cell size.

Regular cellular

activities. Making

proteins and

energy etc.

Synthesis of DNA -

Regular cell

activities cease and

a copy of all

nuclear DNA is

made so the

chromosomes are

now doubled and

look like Xs

Gap 2 - Final

preparation for

division

Mitosis - Cell

division

� During mitosis an exact copy of the genetic material in the “mother” cell must be distributed to each “daughter” cell

� Each stage of mitosis is designed to achieve equal and exact distribution of the genetic material which has been copied during the S phase of the cell cycle

� http://www.youtube.com/watch?v=0oJZDK

dperU&feature=related

� Cancer is often referred to as mitosis gone wrong and this is because cancer is abnormal cell division.

� Normal cells will grow and divide until a certain point.

� Cells will experience contact inhibition which is basically contact with an adjacent cell that stops cell division.

� Cancer cells do not respond to contact inhibition so they will continue to grow and divide, piling one on top of another until a tumor is formed. A tumor is a mass of cancer cells.

� Tumors can grow very large in size and then spread to other areas of the body.

� The spreading of cancer is referred to as metastasis and the cancer will be described as having metastasized in the body.

� Terry Fox is an example of

an individual who

experienced metastasis.

His cancer began in his

leg and eventually

metastasized in his

lungs.

� http://www.youtube.com/watch?v=DD3IQ

knCEdc&feature=related

� Remember this from Junior High!???!?!?

� Here’s one with creepy music:

� http://www.youtube.com/watch?v=NR0md

DJMHIQ&feature=related

� Interphase is a very active stage and also

the longest phase of the cell cycle.

� There are many metabolic reactions

occurring including

protein synthesis

which includes

transcription and

translation.

� DNA is also being replicated during the

interphase stage of the cell cycle in

preparation for the cell to divide once it

reaches a certain size.

� Organelles also increase in number to be

divided among the two daughter cells.

� The process of mitosis in the cell cycle is when

the nucleus of the cell divides and forms two

new genetically identical nuclei.

� This takes place over the four phases of mitosis

and towards the end there is a division of the

cytoplasm and all its contents.

� Then the cell begins to form two separate new

daughter cells, each containing one nucleus

and they are an exact copy of each other.

� stage is often divided into:

� early and late prophase

� or there is prophase and then prometaphase.

� the chromatin (DNA) begins to condense by

super-coiling.

� Spindle fibres begin to form from the pair of

centrioles in animal cells that begin to move

to opposite poles of the cell.

� The shorter spindle fibres resemble stars so

at this point they are referred to as “asters”.

� The nuclear envelope surrounding the

nucleus begins to dissolve.

� the DNA has condensed to form dark chromosomes consisting of two identical sister chromatids which are very visible.

� The sister chromatids are joined in the center by a centromere.

� The centrioles have each arrived at either pole and the spindle fibres which are composed of microtubules extend out to the equator (middle) of the cell.

� The nuclear envelope has now completely disappeared.

� During this stage of mitosis, the spindle fibres attach to the chromosomes (sister chromatids) at the centromere.

� The chromosomes are guided by the spindle fibres to line up along the equatorial plate in the cell.

� One spindle fibre microtubule from each pole is attached to each of the sister chromatids.

� This is the longest phase in mitosis lasting about 20 minutes.

� During this phase of mitosis the spindle

fibres begin pulling the sister chromatids to

opposite poles.

� The chromatids separate at the centromere

and are pulled toward opposite ends of the

cell as the spindle fibres shorten.

� This phase is very short lasting only a few

minutes.

� During this phase in mitosis, the

chromosomes have arrived at each end and

a nuclear membrane begins to form around

them.

� The chromosomes uncoil and become less

dense which makes them no longer visible.

� The spindle fibres begin to break down and

the process of cytokinesis begins.

� This involves the division of the cytoplasm

and all its contents.

� In animal cells a cleavage furrow starts to

develop while a cell plate begins to form in a

plant cell which divides the cell into two

new daughter cells.

� The process of cytokinesis is often referred

to as late telophase.

� http://www.sumanasinc.com/webcontent/

animations/content/mitosis.html

� Mitosis and Cytokinesis:

http://highered.mcgraw-

hill.com/sites/0072495855/student_view0

/chapter2/animation__mitosis_and_cytokin

esis.html

� Mitosis in plants is very similar to

animals with 2 exceptions:

� plant cells lack centrioles

� plant cells form a cell plate between the

two daughter cells in telophase.

� During the mitosis the nucleus of the parent

cell is divided equally to produce two

daughter cells with identical genetic

information.

� The DNA, referred to as chromosomes

during mitosis, was replicated during

interphase of the cell cycle.

� A cell will replicate its DNA when it is

preparing to divide.

� This ensures that each new cell produced

during mitosis gets an equal and identical

copy of the cell’s DNA.

� Eukaryotic cells use the process of mitosis when identical cells are required.

� This can be during growth of an organism, when there has been tissue damage that needs to be repaired, or when an organism reproduces asexually.

� This also occurs during embryonic development when the zygote begins mitotic cell division and the organism begins to grow and develop.

� http://www.youtube.com/watch?v=HYKesI

9jL8c

� Draw Labelled Diagrams of the four stages

of mitosis in an animal cell with 4

chromosomes. (5 marks)

� A chemical called colchicine disrupts the

formation of microtubules. What effect

would this drug have on a cell going through

mitosis?

� If a parent cell has 24 chromosomes, how

many chromatids would be present during

metaphase of mitosis?

� Explain where cytokenisis occurs in the cell

cycle.