SEXUAL REPRODUCTION (Recovered from corrupted file)
Learning Outcomes
Describe the difference in outcomes between mitosis and meiosis
Indicate which type of cells undergo mitosis and meiosis
Explain what haploid and diploid numbers represent
Describe how haploid gametes are produced in sexual reproduction (plants and
animals)
Mitosis
_You have seen that mitosis is responsible for cell division that all plants
and animals require for:
Asexual reproduction
Growth
Repair of body cells
The characteristics that identify mitosis are:
There is always one parent.
The daughter cells are identical to the parent.
Sexual Reproduction
_Sexual reproduction usually requires two separate parents where specialized
cells called gametes from each parent combine in a process called
fertilization to form a new cell called a zygote. Note that two gametes
combine to form one zygote. The zygote is the offspring of the two parents.
_You know that your parents, brothers, sisters, and grandparents look
different than you do, but a family portrait will likely demonstrate that
there are strong similarities among the family. Sexual reproduction is
responsible for these differences between the parents and their offspring.
Meiosis
_In complex plants and animals where sexual reproduction occurs, there are
two types of cells.
Somatic cells are the cells that make up our body. These cells include
muscles, blood, the skeleton, and so on. These cells reproduce by mitosis.
Reproductive cells are sex cells. These cells are specialized for reproducing
offspring through sexual reproduction. There are two types of reproduction
cells: the male reproduction cell is called sperm and the female reproduction
cell is called the egg.
All our body cells contain 23 pairs of chromosomes for 46 chromosomes per
cell. The two chromosomes in each pair match each other in size and shape and
are called homologous pairs. Human body cells are called diploid and are
written as 2n. The human body cells are called diploid since they have 2 x 23
chromosomes (“di” means double). When body cells reproduce through mitosis,
they replicate the diploid number of cells and an exact reproduction occurs.
_
_In complex plants and animals, there are reproductive cells that have a
haploid number of chromosomes. The reproductive cells produce sex cells. The
male produces a gamete called the sperm and the female produces a gamete
called the egg.
In order that sexual reproduction occurs, the sperm gamete must unite with
the egg gamete. In order that the proper number of chromosomes remains
constant, each gamete can contribute only half the total number of
chromosomes when they combine to form the zygote. The chromosomes in each
gamete are haploid or “n” in number; that is, they contain 23 chromosomes,
not 23 pairs of chromosomes each. The chromosomes from each gamete are
combined at fertilization to produce the diploid number (23 pairs of
homologous chromosomes) in the offspring or zygote. The process by which the
gametes are formed is called meiosis, which results in haploid cells.
Stages of Meiosis
The first stage of meiosis is called meiosis I. This stage is the reduction
part of meiosis. Each of the sex cells has its chromosomes reduced from a
diploid number to a haploid number.
Early Prophase – The chromosomes do not appear to be double stranded, but
they are because the homologous chromosomes have paired.
Late Prophase – The nuclear membrane begins to disappear. Homologous
chromosomes begin to thicken and become visible. The spindle becomes visible.
Metaphase – The chromosome pairs line along the equator of the cell. Spindle
fibres attach to the centromere region of each homologous pair.
Anaphase – The chromosome pairs are drawn to the opposite poles by the
spindle fibers.
Telophase – The chromosome pairs move to the opposite poles and the cell
divides to form two new cells, each having a haploid number of duplicated
chromosomes.
The second stage of meiosis is called meiosis II and is known as the division
stage.
The phases in the second stage are very much like the phases in mitosis. The
difference lies in the number of chromosomes in the cells from meiosis I. The
two cells produced from meiosis I have a haploid number of paired
chromosomes. As meiosis II takes place, the paired chromosomes are separated,
leaving a haploid number of chromosomes in the four sex cells at the
interphase.
Summary
There are two types of cells. Somatic or body cells use mitosis to reproduce
cells for growth and repair. These cells have a diploid number of chromosomes
and are identical to the parent cell. Reproductive cells produce two types of
gametes called sperm and egg. These cells have a haploid number of
chromosomes so that when the sperm combines with an egg the offspring will
have a diploid number of chromosomes.
There are two steps to complete cell reproduction by meiosis. The first step
is a reduction step where the chromosomes are reduced from a diploid number
to a haploid number. This step is called meiosis I. The next step is a
division step where the two daughter cells divide to produce four daughter
cells. This step is called meiosis II.
In addition to providing haploid cells, meiosis allows for “crossing over,” a
process by which chromosomes exchange genetic information. Crossing over
provides the opportunity for offspring to differ from their parents.