Sexual Cell Reproduction Meiosis

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Sexual Cell Reproduction


Sexual Cell Reproduction

• All living cells arise from pre-

existing cells, tracing the lineage of every living things back to some primordial ancestor. If mitosis

were the only process involved the production of new cells, then all

cells would be exactly the same.

• Instead, there exists an incredible

variety among organisms


• The life cycle of a multicellular organism is the sequence of

stages leading from the adults of one

generation to the adults of the next

Gametes and the Life Cycle of a Sexual Organism

Figure 8.13

Diploid zygote (2n = 46) Multicellular

diploid adults (2n = 46)

Mitosis and development Egg cell

Sperm cell

Meiosis Fertilization


DNA Sequence

• It is the sequence of the bases in a gene that holds the genetic code

• And therefore holds the code for all cellular

processes and the variety of inherited traits we

see in organisms



The cellular bases for sexual reproduction

-the mixing of genetic traits produces offspring different from each parent

- Fertilization requires 2

gametes (egg and sperm)

- These gametes contain ½ the genetic information

- Each sperm or egg produced carries 1 of over 8 million possible

combinations of parental chromosomes

n 23

n 23 n

23 2n (46) 2n


2n 46 chrom




Meiosis versus Mitosis

3 Important Differences:

1. Takes place in 2 stages involving 2 successive divisions

3. Therefore, the four daughter cells are not necessarily identical and have only ½ the number of chromosomes present in the original parent cell.

2. The chromosomes arrange themselves in homologous pairs

(pair up with another chromosome of the same size and shape)



• The production of gametes with half the number of

chromosomes as the original “parent” cell

• During this process

specialized cells in the gonads (ovary & testes), produce sex cells that contain only one set of chromosomes.

A human germ cell with 46 chromosomes will

undergo meiosis and

produce gametes that

have 23 chromosomes



• The 46 chromosomes number is

referred to as diploid and is written as 2n

• The 23 chromosomes number is

referred to as haploid and is written as n

• Fertilization occurs when 2 gametes

(sperm & egg) fuse, forming a diploid zygote (46 chromosomes) Sperm 23

Chromosomes (haploid)


23 chromosomes (haploid)

Zygote 46


s (diploid)


Each chromosome has a partner

• Keep in mind that the 23 chromosomes are not just any 23, but one member from each pair.

• Each of the 23

chromosomes that you receive from your father is matched by 23

chromosomes from your mother

• Example: Your father

gives you a chromosome

with genes that code for

eye colour and so does

your mother


Homologous Chromosomes

• The paired chromosomes are called homologous chromosomes

• Homologus Pairs = a pair of

chromosomes that have similar lengths, shapes and carry genes controlling the same traits

• However each chromosome does not necessarily carry the same genetic information

For example: both chromosomes in a homologous pair may carry a gene for eye colour but one may code for blue and the other for brown eyes

• Each cell produced during meiosis

contains one member from each pair

of homologous chromosomes After DNA



Phases of Meiosis

Meiosis I

• Interphase

• Prophase I

• Metaphase I

• Anaphase I

• Telophase I

Meiosis II

• Prophase II

• Metaphase II

• Anaphase II

• Telophase II



• Meiosis involves 2 cell divisions that produces 4 haploid cells

• To keep things simple, in our example we will use cells that contain 2n=4 chromosomes.

• Therefore, the cell will contain two pairs of

homologous chromosomes.



• As in mitosis, interphase (cell growth and

DNA replication), must occur before cell can replicate

Very important = interphase occurs before prophase I but not before prophase II

Centriol es




Interphase: DNA

replication of homologous pairs




Prophase 1

1. Chromatin coils tightly to form chromosomes - DNA has already


2. Homologous

chromosomes pair up side by side in a

process called synapsis.

- When 2 homologous chromosomes are

paired, the structure is called a bivalent ( = 2 chromosomes)

Maternal homolog Paternal




Prophase 1

3. Chromosomes shorten & thicken - the homologous chromosomes

(bivalents) pair up to form tetrads.

Each tetrad contains 2 homologous

chromosomes and 4 chromatids




Crossing Over

• As the homologous chromosomes come close together, they often intertwine

• Sometimes chromatids break &

exchange segments. This process is called crossing over

The area(s) where the chromatids

overlap is called a chiasma

Here homologous chromosomes

exchange genetic



Crossing Over

This leads to enormous genetic variation even between siblings

• There are over 8 million possible

combinations of parental chromosomes 5. The chiasma begins to disappear

The nuclear membrane disintegrates

**In females this is where meiosis stops until puberty

(Prophase I)


Prophase I : 2n = 6 or n=3 • Draw a germ

cell in prophase I

• Label:

1. Homologous

chromosomes &

tetrad 2. Nuclear

membrane 3. Centrioles,

astral rays

Homologous pair



Metaphase I

• The tetrads line up on the equatorial plate

• The homologous pairs line up randomly

• There is a 50-50 chance for the daughter cells to get either pair from each chromosome.

You could get this one or that one

Metaphas e plate



Draw a diagram

• Spindle fibres attach to the centromeres of the chromosomes

• Each pair of sister chromatids from each homologous chromosome is ready to

move to opposite poles of the cell


Anaphase I

• Homologous chromosomes separate and move to opposite poles

This process is known as segregation – the separation of paired genes

• At this point, reduction division has occurred!

• Each chromosome remains double



Draw a diagram: 2n = 6,



Telophase I

• Cytoplasm divides = Cytokinesis

• A nuclear membrane begins to form

• Chromosomes unwind into chromatin

• The two new daughter cells

contain 2 chromosomes each.

Two haploid cells (cells

with half the chromosome number) start to form.

Chromosomes are half the

number, but still double



Draw a diagram

• Label the following:

1. Cleavage furrow 2. Daughter cells

3. Nuclear membrane

4. Chromatin


Meiosis I versus Mitosis

• The first division is different from mitosis because the daughter cells are not identical

• Each daughter cell contains 1

member of the chromosome pair

• Remember homologous pairs are

similar but they are not identical


Meiosis II

****The short phase

between Meiosis I and II is referred to as


There may or may not be an interphase II

depending on species

• The next set of cell

divisions will separate the

chromatids • Begin with the 2 daughter cells from meiosis I

DNA replication does NOT occur

• Nuclear membrane dissolves, spindle fibres form

Prophase II


Draw a diagram


Metaphase II

• The chromosomes , each with two sister

chromatids, align on the equatorial plate

• The centromeres attach

to spindle fibres


Anaphase II

• The centromeres separate and

chromatids move towards opposite poles


Telophase II

• 4 new cells will be formed

Each of the new cells will contain only one member from each homologous pair =

haploid (n)

• The parent cell had 6 chromosomes; the

daughter cells have 3 chromosomes each


Meiosis Summary

• The haploid cells complete the meiotic cycle and differentiate to produce

gametes (egg & sperm)

Important to remember:

-Meiosis I is referred to as reductional

division because the chromosome number is reduced by half

- Meiosis II is called equational division

and is similar to mitosis as centromeres on

sister chromatids separate and chromosome

number remains unchanged


MEIOSIS I: Homologous chromosomes separate


Centrosomes (with

centriole pairs)


envelope Chromatin

Sites of crossing over Spindle


chromatids Tetrad


plate Sister chromatids

remain attached


chromosomes separate

Meiosis I


MEIOSIS II: Sister chromatids separate



Sister chromatids separate




daughter cells forming

Meiosis II





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