General Science
Introduction to Biology
Meiosis
●
In this tutorial you will…
– Explain the Purpose of Meiosis
– Describe the Process of Meiosis
– Explain the Importance of Meiosis
Explain the Purpose
of Meiosis
● These new cells may be used …
– To grow larger.
– For repair of damaged tissue.
– To replace cells that have worn out.
● To function correctly, these cells must be genetically identical
to the original cells.
● These new cells are produced through a process called
Mitosis (cell division).
● Living organisms are constantly creating new cells 1 of 9
Explain the Purpose
of Meiosis
2 of 9
● We humans start life as a single celled zygote containing 46 chromosomes
● The process of mitosis, repeated thousands of
times, produced the billions of cells that make up your body today.
Explain the Purpose
of Meiosis
3 of 9
● The cells in your body originated from the zygote through mitosis. But…
Egg Sperm
Zygote
● Where did the zygote come from?
● A zygote results from the fusion of an egg and a sperm.
● But, how are Egg and Sperm cells different from the zygote or any other cell in your body?
Explain the Purpose
of Meiosis
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● Sexual reproduction blends the genetic information from the parents to create offspring.
Sperm Egg
23
23
Zygote
46
● The egg and the sperm each contain ½ the number of chromosomes of typical body cell.
● Human cells contain 46 chromosomes. This means the egg and sperm each contain 23 chromosomes.
Explain the Purpose
of Meiosis
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● A human zygote contains 46 chromosomes.
● 23 chromosomes came from the mother through the egg and 23 chromosomes came from the father through the sperm.
● Each chromosome from the sperm corresponds to a chromosome from the egg creating a pair.
● These pairs are called
homologous chromosomes
● 3 pairs of homologous chromosomes are shown.
Remember, humans have 46
Explain the Purpose
of Meiosis
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● Pairs of homologous chromosomes are numbered 1 – 23.
● The following represents a single homologous pair.
Chromosome #1 ♂
from the male
Chromosome #1 ♀
from the female
Explain the Purpose
of Meiosis
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● Your genetic information is contained on your chromosomes
Chromosome #1 ♂ from the male
Chromosome #1 ♀ from the female
● Chromosomes 1-23 carry
different information like the
chapters of a book have different parts of the story
● Homologous chromosomes carry similar genetic information.
Explain the Purpose
of Meiosis
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● The next question that we need to answer is; How do egg and sperm cells get 23 chromosomes?
● Egg and sperm are produced through a process called Meiosis (say my-O-sis).
● Meiosis is a type of cell division that occurs only in the reproductive organs.
Explain the Purpose
of Meiosis
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● An egg and a sperm fuse to form a __________.
● A typical human cell has ____ chromosomes while human egg and sperm cells have _____ chromosomes.
● What are homologous chromosomes?
zygote
46
23
Homologous chromosomes are pairs of
corresponding chromosomes one of which comes from the egg and one from the sperm.
Describe the Process
of Meiosis
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● In this section we will study how the process of meiosis works.
● Remember, the purpose of meiosis is to produce egg and sperm cells.
● A human cell has 46 chromosomes (23 homologous pairs). An egg or sperm cell has 23 chromosomes.
● Egg and sperm must have half the number of chromosomes of a typical body cell because…
● When they fuse to form a zygote the total number of chromosomes must return to the normal 46.
Describe the Process
of Meiosis
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● Complete the table below
Organism # Chromosomes in a normal cell
Pairs of Homologous Chromosomes
# Chromosomes in an egg or sperm cell
Camel 70
Alligator 16
Mosquito 3
35 35
32 16
Describe the Process
of Meiosis
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● Meiosis is the process of creating egg or sperm cells.
● Egg and sperm cells must have ½ the number of chromosomes of a normal cell.
● The process described in the following slides is a simplification of the actual process.
● In addition, we will study meiosis in a cell that
Describe the Process
of Meiosis
4 of 7
● The cell below has 6 chromosomes and is about to go through meiosis.
First the nuclear membrane breaks down and the chromosomes
coil and shorten
Notice there are 6 chromosomes. two # 1’s, two # 2’s, two # 3’s These are the 3 homologous pairs
One chromosome of each pair came from the organism’s father♂
and one came from its mother♀ .
#1♀
#1♂
#2♂ #2♀
#3♀
Describe the Process
of Meiosis
5 of 7
● In the next step, homologous chromosomes pair up
#2♂ #2♀ #1♀
#1♂
#3♀ #3♂
Notice there are 6 chromosomes. two # 1’s, two # 2’s, two # 3’s These are the 3 homologous pairs
One chromosome of each pair came from the organism’s father♂
and one came from its mother♀ . The way the homologous pairs line
up is totally random.
Pair #1 could line up or
Describe the Process
of Meiosis
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● Next the cell divides splitting the pairs of homologous chromosomes. #2♂ #2♀ #1♀ #1♂ #3♀ #3♂ #2♂ #2♀ #1♀ #1♂ #3♀ #3♂ Notice:
Two daughter cells are formed. Each cell has 3 chromosomes instead of 6. The total number of
chromosomes was cut in half. These daughter cells will
become egg or sperm. Notice:
Each daughter cell gets
a #1, a #2, and a #3 chromosome. But, it doesn’t matter whether they came from the organism’s
Describe the Process
of Meiosis
7 of 7
● What is the purpose of Meiosis?
● Where in the body do you think meiosis occurs?
Α To create egg or sperm cells with ½ the number
of chromosomes of a normal body cell.
Α Since the purpose of meiosis is to create egg or
sperm cells, it only occurs in the reproductive organs; ovaries and testes.
Explain the
Importance of Meiosis
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● There are three important features that you need to remember about meiosis.
● First, Meiosis is the process for making egg and sperm. Egg and sperm must have ½ the normal number of chromosomes.
● Second, it’s not enough to just halve the number of chromosomes. Each egg or sperm must have one of each homologous pair.
● Third, Meiosis helps to insure genetic variability of the offspring.
Explain the
Importance of Meiosis
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● Meiosis insures that when a sperm fertilizes an egg the resulting zygote will have a full set of
chromosomes.
● In the case of an organism with 6 chromosomes the egg and sperm would combine as follows.
Egg
Explain the
Importance of Meiosis
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● Note the role of meiosis in humans
♀ Parent 46 chromosomes
♂ Parent 46 chromosomes
Egg
23 Chromosomes
Sperm
23 Chromosomes
Offspring 46 chromosomes
Explain the
Importance of Meiosis
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● Next, the way the chromosome number is halved is very important
● Humans have 46 chromosomes in 23 pairs. It is essential that each daughter cell get one member of each pair. (a #1, a #2, a #3, etc.)
● This is why homologous chromosomes pair up
before dividing.
#2♂ #2♀ #1♀
#1♂
Explain the
Importance of Meiosis
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● Getting one chromosome of each pair in each daughter cell is important because…
● The genes on chromosome #1 control different things than the genes on chromosome #2.
● If a daughter cell doesn’t get one of each (#1-#23) then it doesn’t have all the information.
● In a book with 10 chapters, you need chapters 1 – 10 for the whole story.
● If you have two chapter 4’s but
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Explain the
Importance of Meiosis
● Finally, meiosis helps to insure genetic diversity.
● Remember homologous pairs line up at random.
● Pair #1 could line up #1♀ - #1♂ or #1♂ - #1♀
● This means the chromosome combinations in the resulting daughter cell could be very different.
● The cell at right shows
only one combination. #2♂
#2♀ #1♀
#1♂
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Explain the
Importance of Meiosis
● In a cell with 6 chromosomes (3 pairs), how many different ways could the chromosomes line up?
Combination One
Combination Two
Combination Three
Combination Four
1♂ 1♀ 1♀ 1♂ 1♀ 1♂ 1♀ 1♂
2♂ 2♀ 2♂ 2♀ 2♀ 2♂ 2♂ 2♀
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Explain the
Importance of Meiosis
● In a cell with 6 chromosomes, there are 4 ways the chromosomes could line up.
● Each combination would produce 2 different daughter cells.
● So, an organism with 3 pairs of chromosomes could make 8 different egg or sperm cells.
One Two Three Four
1♂ 1♀ 1♀ 1♂ 1♀ 1♂ 1♀ 1♂
2♂ 2♀ 2♂ 2♀ 2♀ 2♂ 2♂ 2♀
3♂ 3♀ 3♂ 3♀ 3♂ 3♀ 3♀ 3♂
2
3= 8
Number of ways one pair could line up
Pairs of homologous chromosomes
Explain the
Importance of Meiosis
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● How many different egg or sperm cells could be produced by a human with 23 pairs of
chromosomes?
2
23= 8,388,608
Number of ways one pair could line up Pairs of homologous
chromosomes
Explain the
Importance of Meiosis
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● This is how meiosis insures genetic variability.
● Because in meiosis the homologous pairs line up at random, a human female can produce over 8
million different egg cells. A human male can produce over 8 million different sperm cells.
● So, it is virtually impossible for children of the same parents to have the exactly the same genes.
● This genetic variability helps to make sure a
Explain the
Importance of Meiosis
● How is Meiosis different from Mitosis?
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Home
Produces identical copies of body cells Produces cells with ½
the normal number of chromosomes
Occurs in virtually all organs of the body Occurs only in
reproductive organs
Produces body cells for growth, repair, or
replacement Makes Egg or Sperm
cells
