Sexual Reproduction
Sexual Reproduction
We know all about asexual reproduction
1. Only one parent required.
2. Offspring are identical to parents.
3. The cells that produce the offspring are not usually specialized
for reproduction
Sexual Reproduction is different in almost every
way:
1. Sexual reproduction almost always requires 2 parents
2. Offspring are not genetically identical to parents
3. Always requires the formation of specialized cells
Before we begin. . .
How many chromosomes does a normal human
cell have?
46!!
If
a man and woman mate, each having 46
chromosomes, how many chromosomes will the
offspring have?
46!!
How?
By
meiosis (gamete production) and fertilization
Sexual Reproduction
The specialized cells that are required for
sexual reproduction are known as
GAMETES
And come from the process of:
Meiosis is another form of cell division and
is the basis of sexual reproduction
Meiosis refers to the production of
gametes, or sex cells
Gametes contain half the number of
chromosomes found in body cells (skin
cells, muscle cells, etc.)
Gametes carry genetic info from one
generation to another
Meiosis ensures variation within a
species
-more
on this later
Before we get to meiosis. . .
To the right is a set of
human chromosomes
Each chromosome is
paired with another
chromosome.
Each normal human cell
has 46 chromosomes,
each of these is in a pair
with another similar
chromosome
Before we get to meiosis. . .(cont’d)
In humans, body cells contain 46 chromosomes
Half are from your mother, half from your father
These cells are said to be diploid (2n), which
means there are 2 sets of chromosomes
The diploid number can be expressed as 2n =
46
All normal cells in our bodies are diploid
Sex chromosomes
Of the 23 pairs of human chromosomes,
22 have a matching chromosome
A pair of matching chromosomes is called
a pair of homologous chromosomes
The 2 chromosomes that form the 23rd
pair are called sex chromosomes
Sex chromosomes
For females, the sex chromosomes are an
identical “X” shape
This pair is referred to as XX
For the male, the chromosomes are not
homologous and don’t look identical
One chromosome has an X shape, and the other
smaller chromosome a Y shape
This pair is referred to as XY
Quick Check – Answer the
following as True or False:
There are 2 sets of chromosomes in
humans
One set of human chromosomes has 46
chromosomes
Two sets of human chromosomes has 46
chromosomes
Humans have 46 pairs of homologous
chromosomes
For females, the homologous pair of sex
chromosomes is XY
Gametes
Male and female gametes contain half the
number of chromosomes that other body
cells have
Therefore, these cells are said to be
haploid (n)
Gametes
The haploid number in a human sex cell is
n=23
In males, the sex cell is the sperm cell
In females, the sex cell is the egg cell
Both of these specialized cells are
produced through the process of meiosis
As mentioned above 2 parents are needed in
sexual reproduction,
If
each parents reproductive cell had 46
chromosomes the offspring would have 92
chromosomes
These extra chromosomes would disturb the
cells functioning
To avoid extra chromosomes MEIOSIS occurs
What have we learned so far?
Meiosis reduces the number of chromosomes in
a cell by 1/2 (46 to 23)
The cells that meiosis produces are known as
“gametes”
Gametes have 1/2 the number of chromosomes
as a regular cell and are known as “HAPLOID”
Meiosis changes cells from diploid to haploid
Let’s get to meiosis already!
Meiosis has 2 main phases: meiosis I and
meiosis II (Mr. Heinrich is a goof)
The phase before meiosis begins is called
interphase
Each
homologous pair of chromosomes is replicated
In meiosis I, the homologous chromosomes
separate into 2 cells
In meiosis II, sister chromatids separate into 4
cells
Outcome = 4 cells with one haploid set of
chromosomes in each cell
Easy Breezy!
Meiosis can be thought of as two cell
divisions in series (Meiosis I and II)
Have similar sub-phases just like mitosis
(Prophase, Metaphase, etc.)
If you can remember what happens during
mitosis, then meiosis should pose no
problem at all!
MEIOSIS I
Prophase I
•Chromosomes coil
•Nuclear membrane disappears
•Chromosome pairs move
toward each other
Metaphase I
•Pairs of chromosomes move to
the midline of cell
MEIOSIS I
Anaphase I
•Paired chromosomes move
apart, one to each end of cell
(cont’d)
Telophase I
•Paired chromatids uncoil
•Paired chromatids remain
attached
•Nuclear membrane reforms
•Each end of the cell has half
the number of chromosomes the
parent cells had
•Cytokinesis occurs
•Spindles disappear
Meiosis I
Prophase I
Metaphase I Anaphase I
Telophase I
MEIOSIS II
Prophase II
•Paired chromatids coil
•Spindle forms
•Nuclear membrane
disappears
Metaphase II
•Paired chromatids line up in
middle of cell
Anaphase II
•Each pair of chromosomes
split to form 2 independent
chromosomes
•New chromosomes move
to opposite ends of cell
MEIOSIS II
Telophase II
Chromosomes uncoil
Nuclear membranes form
around 4 new nuclei
Spindles disappear
(cont’d)
Meiosis II
Prophase II
Metaphase II Anaphase II Telophase II
Meiosis animation
Result of Meiosis
Meiosis results in four cells with ½ the full
number of chromosomes (haploid cells)
Cells are genetically unique in comparison
to the parents
23 chromosomes in each new cell
So, when fertilization occurs 23
chromosomes (sperm) + 23 chromosomes
(egg) = 46 chromosomes (embryo)
Where does Meiosis Occur?
Mitosis is needed for growth and repair
and occurs in normal cells
Meiosis only occurs in reproductive cells
(i.e. in the testes and ovaries)
Organism
Number of
chromosomes
in daughter
cell
Diploid
number
Haploid
number
# of
homologous
pairs of
chromosomes
Number of
chromosomes present
in the following
stages
Start
Meiosis
I
Chimpanzee
Fruit Fly
48
48
8
Black Bear
Peanut
Start
Meiosis
II
38
10
Comparison of meiosis and mitosis
Meiosis
Mitosis
Keeps the number of
chromosomes from doubling
each generation by producing
haploid sperm or egg cells
Ensures variation in a species
because chromosomes from each
parent are combined
Responsible for growth, tissue
repair, and some forms of
asexual reproduction
Has 2 cell divisions
Has 1 cell division
Makes the diploid sex cell
become a haploid egg or sperm
cell
Makes 2 exact copies
(daughter cells) of a parent
cell
Ensures that all cells produced
are identical
Quick Check
Haploid is the diploid as n is to ________.
Mitosis is to meiosis as identical daughter cell
is to _________________.
Meiosis is to mitosis as n is to ___________.
Male is to female as sperm cell is to
________________.
A pair of shoes is to diploid as one shoe is to
______________.
Comparison of asexual and sexual
reproduction
Asexual reproduction
Sexual reproduction
Requires 1 parent
Requires 2 parents
Produces offspring identical
to parent
Can produce many offspring
quickly
Produces offspring different
from parent
Requires more time and
energy to produce offspring
Results in limited variability Results in extensive
within a species
variation within a species
end
Review of Reproduction
1.
Asexual Reproduction
•
•
•
2.
one parent
no special reproductive cells are involved
offspring identical to parents and each
other
Sexual Reproduction
•
•
•
two parents
involves special reproductive cells
offspring not identical to parents or each
other
Sexual Reproduction
two parents are required
there is an exchange of DNA between the
parents
there are two kinds of sexual
reproduction:
1.
2.
conjugation – union of similar cells
fertilization – union of different cells
(gametes)
Three Basic Stages of Sexual
Reproduction
1.
2.
3.
Gamete Production (Meiosis)
Fertilization
Embryo Development
Fertilization
Female gamete
the
egg (or ovum)
usually larger than
the male gamete
Male gamete
–
–
the sperm (or spermatozoon)
usually smaller than the female
gamete
Stages in fertilization
1. Sperm meets the egg!!!
2. The sperm breaks
through the outer
covering of the egg
3. Nuclei of the sperm
and egg combine, to form
a diploid zygote
Kinds of sexual reproduction
1.
Self-fertilization
2.
Internal fertilization
3.
one animal produces both gametes
fertilization occurs within the body
External fertilization
fertilization occurs outside the body
1.Self-fertilization
one parent produces
both gametes
common in plants,
also possible for
animals
Not very good due to
inbreeding of DNA
2.Internal-fertilization
Sperm is placed in
the female body
Sperm joins with the
egg(s)
The zygote is
nourished
immediately by the
body
3. External-fertilization
Eggs are laid
unfertilized
Sperm are placed on
afterwards.
If conditions are not
ideal many eggs are
not fertilized
Sexual Reproduction: Good or
Bad?
Advantages:
Recombination
of DNA in the gametes results
in genetic variation among the offspring.
In an environment which changes, this allows
the process of natural selection to occur.
Disadvantages:
Two
parents are required.
Last Question of the Day…
If a new species of fish has 84
chromosomes in their skin cells, how
many chromosomes will their sperm have?
84/2 = 42!!
Now What?
1.
2.
Sexual Reproduction
Review 16.3 (3, 4, 5,
6, 7, 8)
Worksheet
comparing mitosis
and meiosis
This should be
you, busy like a
beaver.
Twins?
Identical twins are also called monozygotic
meaning one egg. Their genes are identical.
They are always of the same sex.
Fraternal twins are called dizygotic or two egg.
They share approximately 25 percent of their
genes, as do any two brothers or sisters. Onehalf of fraternal twin pairs are boy-girl, onequarter are boy-boy, and one-quarter are girlgirl.