MEIOSIS
Why don’t cells get bigger forever??
• Let’s do a lab!
– With some math!
• Surface area = 6 (l x w)
• Volume = l x w x h
Calculations
• Cell 1
• Sides: .5 cm
• SA=
• Volume =
• Cell 2
• Sides: 1 cm
• SA=
• Volume =
• Cell 3
• Sides: 2 cm
• SA=
• Volume =
Limits to cell growth
• The larger a cell becomes, the more demands
the cell places on its DNA.
• In addition, the cell has more trouble moving
enough nutrients and wastes across its cell
membrane
– Activity
Surface Area to volume ratio
• We want surface area to be bigger then
volume
– WHY? (Think about the lab)
So how do cells solve this problem?
• Before they get too large…
– They divide!
– Into two “daughter” cells
– Called CELL DIVISION
What steps may be involved?
Two stages
• Stage 1: MITOSIS
– Dividing nucleus
• Stage 2: CYTOKINESIS
– Dividing cytoplasm
Chromosome
• We have 46!
• Super condensed DNA
• In mitosis, each
chromosome gets
copied
– Now called two SISTER
CHROMATIDS held
together by a
centromere
Animation 1
Animation 2
Sketch it out!
The Cell Cycle
• Series of events that
cells go through as they
grow and divide
• Grows, prepares to
divide, and divides
• Two main parts
– Interphase
– Mitosis
Divided into 2 parts
•
Interphase
– G1 - “Gap” - Cell grows from last cell cycle
– S – Synthesis of DNA (DNA makes a copy of
itself)
– G2 – Cell grows and gets ready for cell division
– G0 – No cell division – typical of cells like nerve
cells that stop dividing at maturity
• Mitosis – M phase
– Prophase
– Metaphase
– Anaphase
– Telophase
– Cytokinesis
Interphase
• Longest phase in the
Cell Cycle
– over half the time is
spent in this phase
Mitosis
•
Prophase
– longest phase in Mitosis
1. Copied chromosomes pair up. Held together by
Centromere (each is called a chromatid)
2. Shortening up of chromosomes, makes them visible
3. Replicated Centrioles (of animal cells) migrate to
opposite poles
4. Nuclear envelope & nucleolus start to break down
5. Spindle fibers (microtubules) extend from centrioles to
centromeres
• Metaphase
– shortest part of
Mitosis
– Chromosomes line up
on the equator (middle)
• Anaphase
– “And away they go!”
– Chromatids separate and go to opposite
poles. Now called chromosomes!!
– Spindle fibers shorten up
• Telophase
– “the end” – Final phase
– Chromosomes reach opposite ends
– Cell membrane pinches in (cleavage furrow) or
Cell plate forms (if plants)
– Chromosomes uncoil
– Nuclear membrane & nucleolus reappear
– Spindle fibers disassembles
• Cytokinesis
– Dividing cellular contents. Occurs
simultaneously with Telophase
MITOSIS OVERVIEW
Difference between Plant and Animal mitosis
• No centrioles in plant cells
• A cell plate forms in plant cells
Difference between Plant and Animal mitosis
• Animal cells
have a
cleavage
furrow that
splits the
two cells
How do cells…
• Know when to grow?
• Found important factors
– Contact prevents growing
• Cyclin is a protein that regulates the timing of the cell
cycle.
– Amount of cyclin in a cell rises and falls with the steps in the cell
cycle.
– There are other internal and external proteins that are also
involved the cell cycle.
• Cancer
– uncontrolled cell growth.
Cells do not respond to normal signals and grow out of control.
Cause masses called tumors.
Cancer
• Cancer occurs
when cells do
not respond to
signals that tell
them to stop
growing
Count those chromosomes
• We have 46
• If we made a baby, how many would they
have?
– 46 from mom
– 46 from dad
– 92 for baby
– No longer human!!
46
+ 46
What do we do?
• We go through MEIOSIS
– Process that takes our two sets of chromosomes
(diploid number{2N}) and cuts them into one set
(haploid number{1N})
– 4623
– 23 from mom + 23 from dad = 46!
• Normal baby!
Meiosis Steps
• Unlike mitosis, meiosis has two stages of
division
– Meiosis I
– Meiosis II
Meiosis I:
Interphase I:
•
Replication of chromosomes. Like chromosomes are called
homologous chromosomes
Prophase I:
•
The homologous chromosomes come
together (Synapsis) and form a group of 4
called a tetrad.
Twisting & exchange parts of adjacent
chromatids will occur – Crossing over.
This results in genetic recombination and
unlinks genes that are located on the same
chromosome.
•
•
–
HUGE GENETIC VARIETY!!!!!
Anaphase I & Telophase I
• Chromosomes separate independently of each
other.
• This is known as Independent assortment.
• Results in…
– Random separation of chromosomes
– Genetic recombination!!!!!
At the end of Meiosis I, there are
2 - 1N cells
Meiosis I
Results in 2 - 1N cells
Meiosis II
Results in 4-1N cells
Meiosis II
• Occurs immediately following Telophase I.
• NO INTERPHASE II !!
• 1N cells immediately go into Prophase II,
then Metaphase II, Anaphase II, and
Telophase II
• Split into four 1N cells, each with unpaired
chromosomes
–Haploid
At the end of
Meiosis II, there are
four 1N cells.
Meiosis Animations
• http://www.sumanasinc.com/w
ebcontent/animations/content/
meiosis.html
• John Kryk
• Cells Alive
• You Tube
• http://www.csuchico.edu/~jbell
/Biol207/animations/meiosis.ht
ml
• McGraw
• Arizona
• How Mitosis and Meiosis Differ
Gametogenesis
Creation of gametes (sex cells)
• Spermatogenesis (testes)
– Makes 4 haploid sperm cells from 1 - 2N primary sperm
cell. Looks like normal Meiosis
• Oogenesis (ovaries)
– Produces ovum (eggs) from 1 diploid primary egg cell.
Difference occurs during cytokinesis during meiosis I and
meiosis II.
– Unequal distribution of the cytoplasm resulting
in 1 large ovum and 3 small polar bodies.
• Polar bodies serve no function in animals but are
needed in to plant embryos.
Comparing mitosis and meiosis
• Be sure to know…
– How many divisions for each?
– How many starting cells?
– How many cells at the end?
– What N are those cells?