ANNOUNCEMENTS
• LAB #3 MITOSIS & MEIOSIS has been moved to FRIDAY
because I’m not finished grading lab #1.
• DO THE SAMPLE FREE RESPONSE QUESTIONS FOR
HOMEWORK. I’ll post them on facebook and teacherweb.
• Do the lab bench exercise for LAB 3 mitosis & meiosis
http://www.phschool.com/science/biology_place/labbench/
• LAB 3 is now on FRIDAY- you will have a lab quiz
• Here are other websites that will help you:
•
http://users.nac.net/challoran/apbio.htm
• TEST #2 IS TUESDAY 10/9… YOUR BIG CHANCE TO
BRING YOUR GRADE WAY UP! :)
CHAPTER 13
Meiosis & Sexual Life Cycles
Asexual
reproduction
produces
clones
ex. Hydra
sea sponge
ASEXUAL REPRODUCTION
reproduction which does not involve meiosis, reduction, or fertilization.
• Only one parent is involved in asexual
reproduction.
• A lack of sexual reproduction is relatively
rare among multicellular organisms, for
reasons that are not completely
understood.
• Current hypotheses suggest that asexual
Types of asexual reproduction reproduction may have short term benefits
1.1 Binary fission
when rapid population growth is
1.2 Budding
important or in stable environments,
1.3 Vegetative reproduction
1.4 Spore formation
• while sexual reproduction offers a net
1.5 Fragmentation
advantage by allowing more rapid
1.6 Parthenogenesis
generation of genetic diversity, allowing
1.7 Agamogenesis
adaptation to changing environments.
Figure 13.x1 SEM of sea urchin sperm fertilizing egg
Sexual reproduction
= genetic recombination
= diversity
= better chance of survival
Sexual reproduction offers
a net advantage by allowing
more rapid generation of
genetic diversity, allowing
adaptation to changing
environments.
WHY SEX?
Natural selection operates on populations through
the phenotypic differences (traits) that
individuals display;
meiosis followed by fertilization provides a
spectrum of possible phenotypes on which
natural selection acts, and variation contributes
to the long-term continuation of species.
Figure 13.4 The human life cycle
Fertilization involves the fusion of two gametes, increases genetic
variation in populations by providing for new combinations of
genetic information in the zygote, and restores the diploid number of
chromosomes.
Meiosis ensures that each gamete
(sperm/egg) receives one complete
haploid (1n) set of chromosomes.
Figure 13.x3 Human female karyotype shown by bright field G-banding of
chromosomes
Figure 13.x5 Human male karyotype shown by bright field G-banding of
chromosomes
QUIZ:
1. What determines whether you
are genetically male or female?
2. What are autosomes?
3. What are gametes?
4. What is a zygote?
5. What is the difference between
a diploid cell and haploid cell?
6. What is Meiosis?
1. Sex chromosome combo
XX=female XY=male.
2. Non-sex chromosomes (1-22)
3. Sex cells- sperm or egg.
4. Fertilized egg.
5. Diploid=2n (homologous
chromosomes); hapliod=1n
(only one of each kind of
chromosome)
6. Nuclear division that includes a
“reduction” it reduces the
chromosome number by 1/2.
The creation of gametes for sexual
reproduction (MEIOSIS)
Occurs in 2 stages:
MEIOSIS I- REDUCTION
1. separates homologous chromosomes.
2. 2n ---> 1n
3. ex. 46 ---> 23
4. split up the “pairs” or TETRAD
Law of Segregation- alleles of a gene
Separate during gamete formation.
MEIOSIS II
1. separates sister chromatids
2. 1n (2 chromatids) --->
2 separate x 1n chromosomes
*similar to mitosis.
Figure 13.7 The stages of meiotic cell division: Meiosis I
meiosis1 is like separating all
the pairs of shoes in your
closet… putting one of each
kind in one room & the
other in another room.
During meiosis, homologous chromosomes are paired, with one
homologue originating from the maternal parent and the other
from the paternal parent. Orientation of the chromosome pairs
is random with respect to the cell poles.
Separation of the homologous chromosomes ensures that each
gamete receives a haploid (1n) set of chromosomes composed
of both maternal and paternal chromosomes.
Figure 13.7 The stages of meiotic cell division: Meiosis II
Major Differences between Mitosis & Meiosis:
(2 divisions- not 1 creates 4 haploid cells- not 2 diploid)
PROPHASE IS SPECIAL…
• In Prophase 1 of Meiosis 1
the homologous chromosomes
line up together.
2. SYNAPSIS is the process of
pairing homologous
chromosomes.
3. TETRAD is 4 chromatids of
a homologous pair.
4. CHIASMATA are the X shaped
regions of chromatid overlap.
5. CROSSING OVER produces
recombinant chromosomes
when pieces break off & attach.
CROSSING
OVER
• During meiosis, homologous
chromatids exchange genetic
material via a process called
“crossing over,” which
increases genetic variation in
the resultant gametes.
• Benefit to the organism’s
species because it produces
genetic variation.
• Recombinant chromosomes are
produced.
• New combinations of your
parent’s genes in your gametes.
• Diversity = better chances for
survival of a species.
Figure 13.7 The stages of meiotic cell division: Meiosis I
INTERPHASE I
• Cell grows by producing
proteins (cyclin) and
organelles
• Chromosomes replicate
• Centrosomes/centrioles
replicate
• Preparation for cell
division
PROPHASE I
•
•
•
•
•
•
Chromatin condenses
Homologous replicated
chromosomes line up
together
“SYNAPSIS”
Crossing over occurs
TETRAD consists of 4
chromatids
CHIASMA(TA)
METAPHASE I
• Spindle microtubules from
opposite poles attach to one
each of a homologous pair
of replicated chromosomes.
• Homologous pairs poised to
move to opposite poles of
the cell.
• Position of pairs on
metaphase plate leads to
variation due to
independent assortment.
• =Alleles of different genes
assort independently during
gamete formation.
Figure 13.9 The results of alternative arrangements of two homologous chromosome
pairs on the metaphase plate in meiosis I
Independent Assortment
means that when you
separate these pairs it
doesn’t matter if one side
gets the left or right shoe.
Each pair is split up
independently.
How many ways could you
split up these 12 pairs?
2 n (n=#chromosomes)
2 12 =
For humans???? 2 23=
ANAPHASE I
• Homologous pairs
of chromosomes
are separated.
• Sister chromatids
remain attached at
their centromere.
• Independent
assortment occurs
now… the
separation of
each chromosome
pair occurs
independently.
Figure 13.9 The results of alternative arrangements of two homologous chromosome
pairs on the metaphase plate in meiosis I
TELOPHASE I
• Cytokinesis occurs.
• 2 haploid daughter
cells are formed.
• Each chromosome still
consists of 2 sister
chromatids.
• Meiosis II is next.
• (INTERKINESIS)
Figure 13.7 The stages of meiotic cell division: Meiosis II
MEIOSIS II
• Essentially mitosis
• Begins with haploid cells
(unlike mitosis which starts
with diploid cells)
• Sister chromatids pulled apart
• Results in 4 haploid cells.
• Each with one set of
chromosomes.
Figure 13.8 A comparison of mitosis and meiosis
SPERMATOGENESIS
• Sperm are produced in the seminiferous
tubules of the testes of adult males.
• Each ejaculation contains 100-650 million
sperm cells. They live 4 days.
• Each diploid stem cell (spermatogonium)
produces 4 haploid sperm cells
(spermatozoa)
OOGENESIS
DIFFERENCES BETWEEN OOGENSIS & SPERMATOGENESIS:
•
Cytokinesis is unequal- almost all the cytoplasm is
taken by a single daughter cell SECONDARY
OOCYTE. This cell can go on to form the ovum
(egg).The other smaller cells are Polar bodies.
• At birth, ovary already contains all the primary oocytes.
• Oogenesis has long resting periods. Starting at puberty,
a single primary oocyte completes meiosis I each
month. The secondary oocyte completes meiosis II only
if a sperm cell enters it.
* Ootid is an undifferentiated ovum.
THE PLANTS
•
•
•
•
•
•
•
•
Alternation of Generations
Sporophytes
Spores
Spores
Haploid
Gametophytes
Mitosis
There aren’t homologous
chromosomes to separate by
meiosis (already haploid)
• DIPLOID
• HAPLOID
Figure 13.8 A comparison of mitosis and meiosis: summary
Figure 13.5 Three sexual life cycles differing in the timing of meiosis and fertilization
(syngamy)