Cell Growth

1
Cell Growth

• Most organisms grow by producing more cells, not
  by producing larger cells

2
There are two main reasons why cells divide

• The larger a cell becomes, the more demands the
  cell places on its DNA
• Also, a larger cell has more trouble keeping up
  with the needs of the cell
• moving nutrients in and
• exporting wastes out
• through the cell membrane

3
The process by which cells divide

• Cells divide to form two daughter cells
• This is called cell division
• allows organisms to grow larger while, allowing
  cells to remain small
• Before a cell divides it must copy its genetic
  information, so each daughter cell can get a copy

4
The cells genetic information is carried by the
chromosomes

• Chromosomes are composed of bundles of DNA
• Cells of different organisms have different
  numbers of chromosomes
• Fruit fly cells have 8 chromosomes
• Carrot cells have 18 chromosomes
• Human cells have 46 chromosomes

5
The DNA of a cell is not visible until it begins
to condense into chromosomes for cell division
Before DNA condenses into chromosomes it is
replicated
The copies condense into two identical sister
chromatids
Attached at an area called the centromere
When the cell divides these sister chromatids
separate so that each new cell formed gets one
set of chromosomes
6
The cell cycle

• The steps that a cell goes through as it grows
  and divides
• During this cycle the cell grows
• Prepares for cell division
• Divides to form two identical daughter cells
• These new cells begin the cycle again
• The cell cycle consists of four phases

7
The first group of phases is called interphase
and has three stages
G1 phase is where cell growth occurs
S phase follows and is where DNA gets replicated
and proteins associated with chromosomes are
synthesized

• G2 phase is where organelles and molecules
  required for cell division are produced

8
The time frame for the cell cycle
Most of the cell cycle is spent in interphase
The last phase is called the M phase
This phase consists of mitosis and cytokinesis
Mitosis is divided into four steps
9
Step one of mitosis is prophase
Centrioles produce spindle fibers that separate
the sister chromosomes later on in mitosis
Nucleolus disappears and nuclear envelope breaks
down
Chromosomes condense and become visible and form
structures like this
10
Step 2 of mitosis is metaphase
The chromosomes line up across the center of the
cell
Spindle fibers are connected to each chromosome
11
Step 3 of mitosis is anaphase
Centromeres that join the sister chromatids splits
Chromatids separate and are pulled to either side
of the cell by the spindles
12
Step 4 of mitosis is telophase
Condensed chromosomes begin to disperse
Spindle fibers break down
Nuclear envelope begins to develop around each
set of separated chromosomes
13
At this time mitosis is complete but cell
division in not completed

• Because DNA is replicated in interphase
• Each copy is condensed and separated in mitosis
• If the original parent cell has 4 chromosomes,
  then mitosis will produce 2 daughter cells each
  containing 4 chromosomes

14
The second part of the M phase is called
cytokinesis

• This process divides the cytoplasm between the
  two newly formed nuclei
• In animal cells, the cell membrane is pinched in
  the center until it touches to form two separate
  cells
• In plant cells, a cell plate forms between the
  two new nuclei and then a cell wall forms around
  it

15
What controls cell growth and division so
carefully?
Cells in a petri dish begin to divide
If some cells are removed from the center
Until they come in contact again
The cells bordering the open space begin to
divide again
Until they fill the dish and come in contact on
all sides and then stop dividing
16
The controls on cell growth and division can be
turned on and off by the cells

• One protein that regulates the timing of the cell
  cycle in eukaryotes is cyclins
• Cyclins is an internal regulator
• Several different regulatory proteins regulate
  different steps of cell division
• Proteins that respond to events outside the cell
  are called external regulators
• Growth factors are important external regulators
  during embryonic development and wound healing

17
Micro-experiments lead to the understanding of
proteins that regulate cell division
A protein called p53 halts the cell cycle until
all chromosomes have been properly replicated
18
Cancer

• A disorder in which cells lose the ability to
  control growth
• Cancer cells do not respond to the signals that
  regulate growth
• This can result in un-controlled growth
• Can lead to tumors
• Many different cancers have different causes but
• All have lost control over the cell cycle

19
Meiosis, the production of sex cells

• Genes are located on the chromosomes in the cell
  nucleus
• All cells have two sets of genes, a single copy
  from each parent
• These are homologous (the same) and the pair make
  a diploid cell
• Reproductive cells are called gametes and contain
  only one set of genes so they are haploid

20
Meiosis is the process of reduction division that
cuts the number of chromosomes in half
It does this by separating the homologous
chromosomes in a diploid cell.
Meiosis has two distinct divisions
Meiosis 1 is similar to mitosis, this example
follows two chromosomes
The big difference between meiosis 1 and mitosis
is that during prophase 1 of meiosis, the
replicated homologous chromosomes pair up to form
a tetrad
There are 4 chromatids in a tetrad that contain
one identical copy of each of the original pairs
of chromosomes from the diploid cell
21
In anaphase 1 of meiosis, the homologous
chromosomes separate and two new cells are formed
If the original cell was heterozygous this can
create two unique cells
Crossing over is a process that occurs when
chromosomes of the tetrad rub into each other
while being separated (Occurs during Prophase 1)
Crossing over can also make the new cells unique
as the chromosomes exchange alleles while rubbing
together
22
The two new cells formed in meiosis I now have a
set of chromosomes that are different from each
other. The next step is meiosis II
Here, replication does not occur before the next
cell division
In anaphase II, the paired chromatids will
separate, this reduces the number of chromosomes
in the new gamete cells by half
23
The cells will go through telophase and
cytokenesis to produce four haploid cells called
gametes
These will be the reproductive cells
The reproductive gamete cells produced in males
are called sperm
The reproductive gamete cells produced in females
are called eggs
Meiosis results in the formation of 4 different
haploid cells
24
Linked genes

• Chromosomes contain many genes
• These traits remain linked during meiosis
• It is the chromosomes that assort independently,
  not the individual genes
• The rate at which genes are separated due to
  crossing over is called recombination frequency
• This can be used to produce a map of the location
  of genes on the chromosomes