Transcript Chap 4 - CRCBiologyY11

Cell replication
Chapter 4
How long is a cell cycle?
The time taken for a newly formed cell to
mature and then give rise to two new cells
is called the cell cycle. The time taken for
this cycle to occur varies greatly, from as
short as 20 minutes to as long as several
weeks. But generally the time lasts
• 10 to 30 hours in plants and
• 18 to 24 hours in animals.
S Phase
The phase between successive mitosis is called interphase
and it is during a restricted period of interphase, termed
the S (for synthesis) period, that DNA is replicated in
preparation for reproduction of the cell.
Synthesis (S) phase
G Phase
The S period is flanked by G (gap) phases during which cell
growth takes place. The G phase also seems to be at
which the cell checks its DNA for mistakes (shown as
‘checkpoints’ in fig 4.9)
Gap (G) periods
G Phase
Gap 1 consists of an examination for any mistakes in
DNA that may have occurred in replication.
In Gap 2, cells check for mistakes that may have
occurred during the S phase.
If the cell doesn’t get the ok to continue, it exits the
cycle.
How many chromosomes?
Humans body cells contain 46 chromosomes. In most species
of mammals, the males and females have the same number
of chromosomes.
It is important to note that the size of an animal and its
chromosome number are not related!
Quick check questions 1-5 on page 84
Mitosis – the purpose
We now know that mitosis is a division of the nucleus by
which genetic information is accurately copied and passed
onto both daughter cells.
But what is the purpose of cell
division???
How many chromosomes?
The collection of chromosomes includes a pair of sex
chromosomes (X and Y in males and X and X in females).
The remaining chromosomes are called autosomes, and
comprise of 22 homologous pairs. When chromosomes of a
cell are paired in this way, the cell is said to be diploid.
Mitosis – the purpose
In multicellular organisms, the purposes of cell replication
are growth, development, maintenance and repair. In
unicellular eukaryotes, cell replication involving mitosis
and cytokinesis is a simple means of reproduction – the
cell’s production of genetically identical copies of itself.
Growth and development
Multicellular organisms grow in size by further
increasing the number of their cells through repeated
cell replications. As the new individual continues to
develop, new cells become specialised for different
purposes, such as muscle, blood and bone in animals,
or photosynthesis and transport in plants. More
replications follow and the cells become organised
into the tissues that form the body of the organism.
Growth and development
Development involves a balance between cell replication and
cell death. Some cells are designed to undergo a
‘programmed cell death’, like the skin tissues that form
the webbing between the fingers of a developing feotus.
This topic will be discussed later.
Maintenance and repair
The activities of maintenance and repair of tissues require
that new cells are produced to replace those damaged
either during their normal function, such as the regular
death of the surface layer of the lining of the gut, or
through injury.
Maintenance and repair
These cells are produced by mitotic cell division. The extent
to which cells can be generated differs greatly between
organisms.
For example, starfish can produce an entirely new organism
by cell replication of a single arm. On the other hand, a
nematode worm, once hatched, cannot produce any new
cells at all.
Growth, specialisation and death
Mitotic cell division is programmed to occur rapidly in
embryos. As growth continues, a stage is reached where
individual cells begin to become different from one
another, specialised for particular functions. This
process, which is called specialisation (also called
differentiation), is under control of the genes.
Growth, specialisation and
death
The zygote contains all the genes required to produce
every type of cell, but in specialised cells, only some
genes are active.
Cell specialisation is found in all multicellular organisms,
as cells are more efficient if they focus on a single
function rather than many. And obviously, the
function of specialised plant cells differs greatly
from the function of specialised animal cells.
Growth, specialisation and
death
For example;
• the gene hemoglobin is active in developing red blood
cells
• in gland cells, the gene that codes for a particular
enzyme or hormone is active
• in nerve cells, genes that control the production of
neurotransmitter molecules are active
• in lymphocytes, genes that produce antibodies are
active.
Where does mitosis occur?
Read pages 85-88 and briefly summarise where mitosis
occurs in the following organisms.
- In mammals
- In fungi
- Some plants (Bryophytes)
- Liverworts
- Insects and other invertebrates
Programmed cell death
Cells have controlled internal mechanisms which give the
instruction was to when the cell is to ‘self destruct’.
This programmed cell death is called apoptosis.
Apoptosis and cell replication are regulated by the body of a
multicellular organism so that cell death and cell
replication are in balance.
Cancer can be the result of uncontrolled cell replication.
Chapter questions
• Quick-check questions 6-8 page 88
• Biochallenge questions 1 and 2 only page 90
• Chapter review questions
3 and 6.