Meiosis & Sexual Reproduction

Meiosis and Sexual

Reproduction

A few quick reminders…

• Chromosomes carry the genetic information for

an organism. DNA makes up chromosomes.

• Chromosomes are made of sister chromatids.

• Gametes are the reproductive cells in an

organism.

• Female gametes are called eggs.
• Male gametes are called sperm.

Sexual Reproduction

Male Human =

46 chromosomes

Female Human =
46 chromosomes

Sexual Reproduction

•Male and female
each produce a
GAMETE

•Those GAMETES
combine during
FERTILIZATION.

Sexual Reproduction

•If those GAMETES
carried with them
their entire
CHROMOSOME
number, then the
ZYGOTE (offspring)
would have double
the number of
CHROMOSOMES of
its parents.

46

chromosomes

46

chromosomes

92

chromosomes!!!

Sexual Reproduction

•THIS WON’T
WORK!!

•Each
generation
would have a
different
number of
chromosomes
than the next.

46

chromosomes

46

chromosomes

92

chromosomes

Sexual Reproduction

• GAMETES contain ½
the number of
chromosomes as a
regular SOMATIC
(body) cell.

23

chromosomes

23

chromosomes

46

chromosomes

• That way, when they
combine during
FERTILIZATION, the
number in the ZYGOTE
is the same as the
number in each parent.

Open Ended

In humans, the diploid (2n) chromosome number is 46. The haploid (n) number is 23. Using this information, work with your neighbor to come up with definitions for DIPLOID and HAPLOID.

Dropdown

Reproductive cells are known as ​

Select answer

. These cells have ​

Select answer

the number of chromosomes as all the other cells. Body cells are called ​

Select answer

cells and the cells have a ​

Select answer

set of chromosomes.

WAIT A MINUTE MS. WALLS!!

•If a parent only gives HALFof his/her
chromosomes to the offspring, isn’t
some information lost?

•No….that is where HOMOLOGOUS
CHROMOSOMES start to matter.

Homologous Chromosomes

• Pair of chromosomes (maternal and paternal) that are

similar in shape and size.

• Homologous pairs (tetrads) carry genes controlling

the same inherited traits.

• Humans have 23 pairs of homologous chromosomes.

22 pairs of autosomes

1 pair of sex chromosomes

Homologous chromosomes

Homologous chromosomes

sister chromatids

sister chromatids

​Tetrad

Meiosis: The process of creating
gametes (egg and sperm cells).

Gametes (haploid) are different from somatic
(diploid) cells (skin, muscle, brain or any other
cell in the body) because they have half the
number of chromosomes that somatic cells have.
Ex. In humans, diploid cells like skin cells have 46
chromosomes. Egg and sperm cells have 23.

Multiple Choice

If the haploid number of chromosomes for a horse is 32, how many chromosomes do horses have?

• 64

• 16

• 32

• 8

Match

Match the following

• Homologous chromosomes

• Gametes

• Haploid

• Diploid

• Sister chromatids

pairs of chromosomes that contain the same genetic information

reproductive cells

1 set of chromosomes

2 sets of chromosomes

Identical copies of DNA formed during replication that make up replicated chromosomes

• pairs of chromosomes that contain the same genetic information

• reproductive cells

• 1 set of chromosomes

• 2 sets of chromosomes

• Identical copies of DNA formed during replication that make up replicated chromosomes

Meiosis

Meiosis is Two cell divisions

(called meiosis I and meiosis II)

Meiosis I: The Separation of homologous

chromosomes

Meiosis I (four phases)

a. prophase I
b. metaphase I
c. anaphase I
d. telophase I

Prophase I

• Starts with a diploid cell (In humans, 46

chromosomes)

• Tetrads form. Tetrads are homologous chromosomes.

This process is called synapsis.

• Crossing over occurs. This creates genetic variation.

centrioles

​spindle
fiber

Crossing Over

creates variation (diversity) in the offspring’s traits.

variation

Tetrad

Open Ended

What does "genetic variation" mean?

Open Ended

Explain how crossing over increases the genetic variation in the cells formed during meiosis.

Metaphase I

metaphase
plate

O
R

metaphase
plate

Homologous chromosomes line up in the center of

the cell.

Anaphase I

• Spindle fibers pull apart the chromosome pairs.

Telophase I

• Chromosomes move to opposite ends of the

cell

• Forms a cleavage furrow

Cytokinesis

• The parent cell splits into 2 haploid daughter

cells.

Telophase I

Reorder

Reorder the following

• Prophase

• Metaphase

• Anaphase

• Telophase

1

2

3

4

• 1
• 2
• 3
• 4

Dropdown

X shows meiosis I. The first cell is ​

Select answer

and has 4 chromosomes (or 2 pairs of ​

Select answer

chromosomes). At the end of meiosis I, there are only 2 chromosomes in each cell produced, making them ​

Select answer

cells.

Open Ended

X shows meiosis I. Looking at the two cells produced in meiosis I, at the cells genetically identical or different? Is this the only possible combination of chromosomes that could have happened

Meiosis II:

The Separation of Sister

Chromatids

Prophase II

• same as mitosis: spindle fibers reform

and attach to the centromere between the
sister chromatids

Metaphase II

• same as mitosis: chromosomes line up

single file in the middle of the cell

metaphase
plate

metaphase
plate

Anaphase II

• same as mitosis: the attached spindle

fibers pull the sister chromatids apart.

Telophase II

• same as mitosis: The chromatids move to

opposite sides of the cells they are in. The
nucleus begins to form around each set of
chromatids.

Cytokinesis

• The two daughter cells split into 4 haploid

daughter cells.

• These cells will become egg and sperm cells.
• Because crossing over, each cell is genetically

different. The offspring made from these cells will
be different from their parents

Telophase II

Reorder

Reorder the following

1

2

3

4

• 1
• 2
• 3
• 4

Multiple Select

Which 2 descriptions accurately explain what happens during meiosis I and meiosis II?

• Meiosis I is the process of separating the homologous chromosomes.

• Meiosis II is the process of separating the sister chromatids.

• Meiosis II is the process of separating the homologous chromosomes.

• Meiosis I is the process of separating the sister chromatids.

Fertilization

• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg

Nondisjunction

• Nondisjunction occurs when the homologous

chromosomes or sister chromatids do not

separate correctly. This results in two or more of

our daughter cells having too many or not

enough chromosomes.

• These daughter cells can lead to trisomy (a
chromosome set having three chromosomes) or
monosomy (a chromosome set having only one

chromosome)

Draw

Use the picture below to show what would happen if each egg were fertilized with a normal sperm with 23 chromosomes. How many chromosomes would the zygotes have?

Common Nondisjunction Disorders

• Edward’s Syndrome – Trisomy 18
• Down’s Syndrome – Trisomy 21
• Turner’s Syndrome – Monosomy 23 (X)
• Klinefelter's Syndrome – Trisomy 23 (XXY)

Karyotype

(picture of an individual’s chromosomes)

One of the ways to

analyze the

amniocentesis is

to make a
Karyotype

What genetic

disorder does this
karyotype show?

Autosomes

(chromosomes that code for specific traits)

In Humans the
“Autosomes”
are sets 1 – 22

This means that our

hair color, eye
color, and other

things about us are
determined by our

autosomes.

Sex Chromosomes

• Sex chromosomes determine an organism’s

genetic sex. Females will carry XX
chromosomes while males carry one X and one
Y chromosome.

• A woman’s sex cells (eggs) will only carry X

chromosomes while a man’s sex cells (sperm)
can carry an X or a Y. Because of this, males
will determine the gender of their baby.

Sex Chromosomes

“Sex Chromosomes”

…….the 23rd set

23

This person has 2

“X” chromosomes…

Draw

Does this karyotype show a male or female and does is show any genetic abnormalities? Circle your evidence.

Draw

Does this karyotype show a male or female and does is show any genetic abnormalities? Circle your evidence.