Transcript cell

CHAPTER 9
Why do cells divide?
DNA Overload: the genetic material is only capable of
“managing” a limited amount of cellular activity
Materials Exchange: As a cell get bigger, the
volume increases faster than the surface area,
making it difficult to get material in and wastes out at a pace
fast enough to allow the cell to survive.
Surface Area to Volume Ratio
How does cell division help cells?
• Keeps surface area to volume ratio high.
• Before cell becomes too large it splits, maintaining
nuclear control and efficient transport of materials
Chromosomes
What are they?
• Segments of
condensed DNA.
• DNA is coiled
around proteins for
organization.
• Only visible during
division.
Structure:
The Cell Cycle
– Events a cell goes through between one division and the next
– During this time a cell:
1. Grows & Develops
2. Prepares for Division
3. Divides to form two daughter cells
Phases of the Cell Cycle (visual)
G1
S
G2
Phases of the Cell Cycle (words)
Stage
(in order)
Description
G1
Initial growth
Increase in size & make new organelles etc.
S
DNA replication
G2
Prepare for division
M
Nuclear (mitosis) & Cell division (cytokinesis)
*G0
Resting stage following G1
“Waits” here until it is ready to divide again
Some cells (nerve cells) cannot come out of G0
Interphase
• Non-dividing stage
• Cell engaged in metabolic
activity & preparing for
mitosis
• DNA is not condensed
(chromatin)
• Nucleolus may be visible
Prophase
a. Nuclear membrane
breaks down
a. Chromosomes condense
and become visible
a. Centrioles move to
opposite ends and begin
to form spindle
Metaphase
a. Chromosomes line up in
the middle
b. Microtubules connect the
centromere of each
chromosome to the poles
of the spindle
Anaphase
a. Chromatids split at
centromere (each is now
an individual
chromosome)
a. The new, singlestranded, chromosomes
begin to move to
opposite ends
Telophase
a. Chromosomes reach
opposite ends and begin
to de-coil.
a. 2 new nuclear
membranes begin to
form.
Cytokinesis
a. Division of the cell
b. Often occurs during or
directly after telophase
c. Mitosis can occur
independently of
cytokinesis, this results in a
multinucleated cell, such
as muscle cells.
Cytokinesis: Plants v. Animals
1. Plants: Cell plate forms and gives rise to a cell
wall. No centrioles involved.
2. Animals: membrane pinches in and cell splits.
Controls on Cell Division
 Some cells stop dividing when mature (stay in G0)
ex) many neurons (nerve) & muscle cells
 Others divide for growth & repair
 Normally, cells will stop dividing when they come in contact with other
cells
Cell Cycle Regulators:
•
•
Proteins called cyclins start/stop
the cell cycle.
The cycle is regulated by various
factors:
1. Intracellular factors:
- All chromosomes are replicated
- All spindles are attached
2. Extra-cellular factors:
- Growth factors
- Cell to cell contact
Uncontrolled Cell Growth: CANCER
• Tumor Suppressor Genes: (p53)
- genes that normally halt the cell cycle
- stuck OFF (cells keep dividing)
• Oncogenes:
- genes that tell cells to divide
- “stuck ON” (cells keep dividing)
Stem Cells & Development
• Cells take on specialized roles through the process of
differentiation
• Differentiated cells develop from undifferentiated cells called
stem cells
• Fully undifferentiated stem cells are totitpotent
– (ex. Fetilized egg through first few divisions)
• Mostly undifferentiated are pluripotent
– (ex. Early embryonic cells)
• Partially differentiated are multipotent
– (ex. Adult marrow cells)
• When/Why do organisms rely on mitosis/cytokinesis?
• How do the daughter cells formed as a result of
mitosis/cytokinesis compare to the original parent
cell?
• Can you think of a time when new cells need to be
formed that do not compare to the parent cell in this
way?
Gamete Production
• Sexual reproduction begins
with the fusion of egg and
sperm to produce a zygote.
• Each organism must inherit a
single copy of each gene from
each parent.
• Therefore, when an organism
produces its gametes (egg or
sperm) their gene pairs must
be separated.
Chromosome Number
1. The fruit fly (drosophila) has 8 chromosomes
2. 4 come from the mom and 4 from the dad
2. These two sets of chromosomes are called homologous
chromosomes.
2. A cell with both sets is called diploid (2n).
Chromosome Number (con’t)
5. Gametes (egg/sperm) only have one set of
chromosomes and are referred to as haploid (1N)
6. The process of forming haploid (N) cells,
{gametes}, from diploid (2N) cells {body/somatic
cells} is called meiosis.
5. Meiosis involves two distinct stages meiosis I and
meiosis II
Crossing Over
• Genetic material is exchanged
between homologous
chromosomes during
Prophase 1
• This is referred to as genetic
recombination.
• During “synapsis” the crossed
over homologous
chromosomes are referred to
as a tetrad.
What is the benefit of crossing over?
HINT: Think from an evolutionary perspective.
Meiosis I v. Meiosis II
• During Meiosis I, homologus pairs are separated.
• During Meiosis II, sister chromatids of double-stranded
chromosomes are separated to form single-stranded
chromosomes.
Gamete Formation
Males
“spermatogenesis”
• All 4 haploid cells
produced become
sperm
Females
“oogenesis”
• Only one of the
cells becomes an
egg
• Egg is larger,
needs the
cytoplasm
Male v. Female Gamete
Formation
Mitosis v. Meiosis
• Mitosis:
– 2 genetically identical diploid cells
– produces somatic (regular body) cells
– Method of asexual reproduction
• Meiosis:
– 4 genetically different haploid cells
– Reduction division
– Produces gametes (reproductive cells) for
sexual reproduction