Download Cell Growth & Division

Cell Growth & Division
• Do cells grow?
– Yes but with limitations
– if a cell grows to be too large it requires more
energy and resources to maintain metabolism and
homeostasis
– As a cell becomes too large DNA (the brain of the
cell) becomes overloaded much like a computer of
limited memory trying to run a very large
program.
• As multicellular organisms grow, their cells
multiply rather than growing indefinitely.
• Before a cell becomes too large it undergoes
cell division forming two new daughter cells.
• This is a more efficient use of cell resources.
• These new cells each get a full set of genetic
information through the replication of DNA
Types of Cell Division
• Prokaryotes divide by a process called Binary
Fission.
• In Eukaryotes there are 2 types of cell
division
– Mitosis:
Results in 2 new identical cells
each with a full set of chromosomes
– Meiosis:
Results in 2 cells with only half
the original number of chromosomes (sex
cells)
Binary Fision
1. The DNA of the
single
chromosome is
copied.
2. The cell grows to
2X its size
3. A cell membrane
forms dividing
the cell in half
4. the cell splits
into 2 new cells
each with its
own
chromosome
• Prokaryotes such as Bacteria divide using
binary fision
• They have only singular circular DNA
• Can divide and form new organisms
every 20-30 minutes
The Role of Chromosomes
• To understand cell division in eukaryotes it is
important to first understand the role of
Chromosomes.
• During cell division DNA is tightly coiled with
proteins into compact rod shaped structures
called chromosomes within the nucleus.
• The DNA is tightly coiled around proteins
called histones which give the chromosome its
shape.
• During the time of cell life between divisions
the DNA is less tightly coiled allowing its
information to be “read” and processed by the
cell.
• In this loose form the DNA/protein complex is
called chromatin.
• As the DNA replicates it forms 2 identical
halves of the chromosome called chromatids.
• These halves are joined by a constricted area
known as the centromere.
• When the cell divides each new cell receives 1
chromatid or 1 copy of the DNA.
• Each different species has a unique number of
chromosomes.
–man
–cats
–gorillas
–fruit fly
46
32
48
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There are two types of human and
animal chromosomes
• sex chromosomes - these determine the sex of
an organism but may also carry genes for
other traits.
In humans these are the X / Y chromosomes
• autosomes – all of the other chromosomes
that are not sex chromosomes.
The majority of inherited trait are contained
on these chromosomes.
Humans –
44 Autosomes
2 Sex chromosomes
46 total chromosomes
• Every organism produced by sexual
reproduction has 2 copies of each autosome.
• One set is received from each parent.
• These pairs of chromosomes are called
Homologous chromosomes.
• Each one of the chromosomes in a
homologous pair is
– the same size,
– shape,
– and carries genes for the same traits.
Humans –
22 homologous pairs of autosomes
1 pair of sex chromosomes
23 total pairs of chromosomes
• Cells that contain 2 full sets of autosomes are
said to be diploid.
• All normal human cells are diploid meaning
that they have a full 23 pairs of chromosomes.
• The only human cells which are not diploid are
the egg and sperm cells which have only half
the number of chromosomes.
Diploid cells are abbreviated as 2n
• Reproductive or sex cells are called haploid
cells because they have only 1 set of
chromosomes and only 1 sex chromosome (X
or Y).
• Haploid cells do not have homologous pairs
but merely 1 chromosome from each original
pair
Haploid cells are abbreviated as 1n
• In sexual reproduction 2 haploid cells create a
diploid cell.
1n + 1n = 2n
• If sex cells were 2n the new cell formed from
their union would have too many
chromosomes.
Karyotype of Human Chromosomes
Cycle of Life
Cell Cycle
• Cell division may be more efficient but the
phase of a cell’s life when it is dividing is
relatively small compared to that of growth
and development
• This is the repeating set of events in the life of
a cell is called the cell cycle.
• Cell division is only a small part of that cycle.
• The period of time between cell divisions is
referred to as Interphase.
There are 4 phases of the cell cycle:
1. G1 phase – This is the time when new cells grow
and mature
2. S phase – This is the period of time when DNA is
replicated to prepare for cell division
3. G2 phase – The cell grows to 2x its size in
preparation for cell division
4. M phase – consists of Mitosis,the process of
nuclear division, and Cytokinesis, the process of
cytoplasmic division.
The first 3 phases of the cell cycle make up the
period we refer to as interphase.
• There are many times in which mature cells
will leave the cell cycle during the G1 phase.
• They enter what is known as the G0 phase.
• Here cells merely exist, but many may return
to the cell cycle in order to repair or replace
damaged cells.
• Mature human nerve cells will leave the cell
cycle and stop dividing when they reach
maturity. These cells generally do not return
to the cycle.
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