Chapter 12
Meiosis and
Sexual
Reproduction
Question?
Does
Like really beget Like?
The offspring will “resemble”
the parents, but they may not
be “exactly” like them.
This chapter deals with
reproduction of life.
Heredity
The
transmission of traits
from parents to offspring.
Comment - Humans have
been aware of heredity for
thousands of years.
Genetics
The
scientific study of
heredity.
Comment - Genetics is only
about 150 years old.
Genes
The
DNA for a trait.
Locus - the physical location of a
gene in a chromosome.
Top
part of
chromosome is the
“p” and the
bottom is the “q”
Reproduction
A
method of copying genes
to pass them on to offspring.
Two main types:
Asexual
reproduction
Sexual reproduction
Asexual Reproduction
Parent
passes all of its genes
to its offspring.
Uses mitosis.
Also known as cloning.
Comment - many organisms
reproduce this way.
Asexual Bud
Advantages
Only
need 1 parent.
Offspring are identical to the
parent.
Good genetic traits are
conserved and reproduced.
Disadvantages
No
new DNA combinations
for evolution to work on.
Clones may become extinct if
attacked by a disease or pest.
Sexual Reproduction
Two
parents contribute DNA
to an offspring.
Comment - most organisms
reproduce this way, but it
hasn’t been proven in some
fungi and a few others.
Advantages
Offspring
has a unique
combination of DNA which
may be an improvement over
both parents.
New combination of DNA for
evolution to work with.
Disadvantages
Need
two parents.
Good gene combinations can
be lost.
Offspring may not be an
improvement over the
parents.
Question ?
Do
parents give their whole
DNA copy to each offspring?
What would happen to
chromosome number if they
did?
Chromosome Number
Is
usually constant for a
species.
Examples:
Humans
- 46
Corn - 20
Onions - 16
Dogs - 72
Life Cycle - if Mitosis
Female 46
egg 46
Mitosis
Zygote
mitosis
Male 46
sperm 46
92
mitosis
Result
Chromosome
number would
double each generation.
Need a method to reduce the
chromosome number.
Life Cycle - if Meiosis
Female 46
egg 23
Meiosis
Zygote
mitosis
Male 46
sperm 23
46
mitosis
Result
Chromosome
number will
remain the same with each
sexual reproduction event.
Meiosis is used to produce
the gametes or sex cells.
Meiosis - Purpose
To
reduce the number of
chromosomes by half.
Prevents doubling of
chromosome numbers during
sexual reproduction.
Sexual Life Cycle
Has
alternation of meiosis
and fertilization to keep the
chromosome numbers
constant for a species.
Ploidy
Number
of chromosomes in a
"set" for an organism.
Or, how many different kinds
of chromosomes the species
has.
Usually shown as N = ……
Humans
N = 23
Diploid
2
sets of chromosomes.
Most common number in
body or somatic cells.
Humans
2N = 46
Corn 2N = 20
Fruit Flies 2N = 8
Haploid
1
set of chromosomes.
Number in the gametes or
sex cells.
Humans
N = 23
Corn N = 10
Fruit Flies N = 4
Polyploids
Multiple
sets of
chromosomes.
Examples
3N = triploid – Ex: seedless
watermelons – get from crossing a
diploid male with a tetraploid female
4N
= tetraploid
 Common
animals.
in plants, but often fatal in
Life Cycle Variations
Meiosis/Mitosis
Preview of differences
Two
cell divisions, not one.
Four cells produced, not two.
Synapsis and Chiasmata will
be observed in Meiosis
Meiosis/Mitosis
Preview of differences
1st
division separates PAIRS
of chromosomes, not
duplicate chromosomes.
Interkinesis is present.
Meiosis
Has
two cell divisions.
Steps follow the names for
mitosis, but a “I” or “II” will
be added to label the phase.
Prophase I
Basic
steps same as in
prophase of Mitosis.
Synapsis occurs as the
chromosomes condense.
Synapsis - homologous
chromosomes form bivalents
or tetrads.
Prophase I
Chiasmata – (a point of overlap of
paired chromatids at which fusion and
exchange of genetic material take place
during prophase of meiosis)
That’s
observed.
fancy for “crossing over”
Longest
phase of division.
Metaphase I
Tetrads
or bivalents align on
the metaphase plate.
Centromeres of homologous
pairs point toward opposite
poles.
Anaphase I
Homologous
PAIRS separate.
Duplicate chromosomes are
still attached at the
centromeres.
Anaphase I
Maternal
and Paternal
chromosomes are now
separated randomly.
Telophase I
Similar
to Mitosis.
Chromosomes may or may
not unwind to chromatin.
Cytokinesis separates
cytoplasm and 2 cells are
formed.
Interkinesis
No
DNA synthesis occurs.
May last for years, or the cell
may go immediately into
Meiosis II.
May appear similar to
Interphase of Mitosis.
Meiosis II
Steps
are the same as in
Mitosis.
Prophase
II
Metaphase II
Anaphase II
Telophase II
Meiosis - Results
4
cells produced.
Chromosome number halved.
Gametes or sex cells made.
Genetic variation increased.
Sexual Sources of
Genetic Variation
1. Independent Assortment of
Chromosomes during
Meiosis.
2. Random Fertilization.
3. Crossing Over.
Independent
Assortment
There
are 23 pairs of
chromosomes in humans.
The chance to inherit a single
chromosome (maternal or
paternal) of each pair is 1/2.
Gamete Possibilities
With
23 pairs of chromosomes,
the number of combinations of
chromosome types
(paternal and maternal) are:
223 or 8,388,608
Random Fertilization
The
choice of which sperm
fuses with which egg is
random.
Random Fertilization
Therefore,
with 8,388,608
kinds of sperms and
8,388,608 kinds of eggs, the
number of possible
combinations of offspring
is over 64
million kinds.
Result
Is
it any wonder that two
offspring from the same
human parents only resemble
each other and are not
identical twins?
Crossing-Over
The
exchange of sister
chromatid material during
synapsis.
Occurs ONLY in Prophase I.
Chiasmata
The
point of contact where
two chromosomes are
crossing-over.
Importance
Breaks
old linkage groups.
Creates new linkage groups
increases genetic variation.
Importance
Very
common during
meiosis.
Frequency can be used to
map the position of genes on
chromosomes.
Comments
With
crossing over, offspring
can never be 100% like a
parent if sexual reproduction
is used.
Multiple cross-overs are
common, especially on large
chromosomes
Comments
Genes
near the centromere
do not cross-over very often.
Summary
Know
how the chromosomes
separate during Meiosis.
Know how Meiosis differs
from Mitosis.
Know how sexual
reproduction increases
genetic variation.
Sordaria
Sordaria
Sordaria
Sordaria
Sordaria
Sordaria
Sordaria
Mitosis/Meiosis
Lab Report
Calculate
the % of cells in
each mitosis phase.
Answer questions 1 & 2.
Use Sordaria data to answer
questions 1-3.