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sexual
reproduction
and meiosis
what is sexual reproduction?

• Sexual reproduction
is a type of reproduction
in which the genetic
materials from two
different cells combine,
producing an offspring.

• The cells that combine
are called sex cells.
what is sexual reproduction?
• Sex cells form in

reproductive organs.
There are two types of
sex cells—eggs and
sperm.

• An egg is the female sex
cell, which forms in an
ovary.

• A sperm is the male

sex cell, which forms in a
testis.
what is sexual reproduction?
• Fertilization occurs
when an egg cell and a
sperm cell join
together. When an egg
and a sperm join
together, a new cell is
formed.

• The new cell that

forms from fertilization
is called a zygote.
diploid cells

• After fertilization, a

zygote goes through
mitosis and cell division.

• Mitosis and cell division

produce nearly all of the
cells in a multicellular
organism.
diploid cells
• Organisms that

reproduce sexually form
two kinds of cells—body
cells and sex cells.

• In the body cells of

most organisms,
chromosomes occur in
pairs. Cells that have
pairs of chromosomes
are called diploid
cells.
diploid cells
• Organisms that reproduce sexually form two kinds of
cells—body cells and sex cells.

• In the body cells of most organisms, chromosomes

occur in pairs. Cells that have pairs of chromosomes
are called diploid cells.
diploid cells
• Organisms that reproduce sexually form two kinds of
cells—body cells and sex cells.

• In the body cells of most organisms, chromosomes

occur in pairs. Cells that have pairs of chromosomes
are called diploid cells.
diploid cells

chromosomes
• Pairs of chromosomes that have genes for the same
traits arranged in the same order are called
homologous chromosomes.
diploid cells

chromosomes
• Because one chromosome is inherited from each

parent, the chromosomes are not always identical.
diploid cells

chromosomes
• For example, a kittens can inherited a gene for orange
fur color from their mother. They can also inherit a
gene for gray fur color from their father. Some kittens
might be orange, and some kittens might be gray. No
matter what the color of a kitten’s fur, both genes for
fur color are found at the same place on homologous
chromosomes. In this case, each gene codes for a
different color.
diploid cells

chromosomes
• Different organisms have different numbers of
chromosomes.

• Human diploid cells have 23 pairs of chromosomes.

This means that human diploid cells have a total of 46
chromosomes.
chromosomes
How many chromosomes does a cat have?

38
chromosomes
How many chromosomes does an ant have?

2
chromosomes
How many chromosomes does a cow have?

60
chromosomes
How many chromosomes does a chicken have?

78
chromosomes
How many chromosomes does a turkey have?

82
chromosomes
How many chromosomes does a kangaroo have?

12
chromosomes
How many chromosomes does a mosquito have?

6
chromosomes
How many chromosomes does a hermit crab have?

254
chromosomes
How many chromosomes does a goldfish have?

94
chromosomes
How many chromosomes does a rabbit have?

44
chromosomes
What species has the most chromosomes?
ophioglossum reticulatum

1200

chromosomes
chromosomes
• It is important to have the correct number of
chromosomes.

• If a zygote has too many or too few chromosomes, it
will not develop properly.

• The process of meiosis helps maintain the correct
number of chromosomes.
chromosomes
• If a mistake occurs in animal cells, sometimes the
zygote dies. If the zygote lives, every cell in that
organism will have the wrong amount of
chromosomes and may not grow normally.

• A person with Down Syndrome has one extra
Chromosome 21.
chromosomes

donkey:
62 chromosomes

zebra:
46 chromosomes
chromosomes
Zonkey: ____
54
chromosomes
donkey: 62 chromosomes

zebra: 46 chromosomes
haploid cells
• Organisms that reproduce sexually also form egg and
sperm cells, or sex cells.

• Sex cells have only one chromosome from each pair
of chromosomes.

• Haploid cells are cells that have only one
chromosome from each pair.
haploid cells
• Organisms produce sex cells using a special type of
cell division called meiosis.

• In meiosis, one diploid cell divides and makes four
haploid sex cells.

• Meiosis occurs only during the formation of sex cells.
the phases of meiosis
• Recall that mitosis and cytokinesis involve one
division of the nucleus and cytoplasm.

• Meiosis involves two divisions of the nucleus and the
cytoplasm.

• These two divisions are phases called meiosis I and
meiosis II.

• Meiosis results in four haploid cells, each with half
the number of chromosomes as the original cell.
the phases of meiosis
the phases of meiosis
• A reproductive cell goes through interphase before
beginning meiosis I.

• During interphase, the reproductive cell grows and
copies, or duplicates, its chromosomes.

• Each duplicated

chromosome
consists of two
sister chromatids
joined by a
centromere.
meiosis i

prophase I
• During prophase I,

duplicated chromosomes
condense, or shorten, and
thicken.

• Homologous

chromosomes come
together and form pairs.

• The membrane around the
nucleus breaks apart and
the nucleolus disappears.
meiosis i

metaphase I
• During metaphase I,

homologous chromosome
pairs line up along the
middle of the cell.

• A spindle fiber attaches to
each chromosome.
meiosis i

anaphase I
• During anaphase I,

chromosome pairs
separate and are pulled
toward opposite ends of
the cell.

• The sister chromatids stay
together.
meiosis i

telophase I
• During telophase I, a

membrane forms around
each group of duplicated
chromosomes.

• The cytoplasm divides

through cytokinesis, and
two daughter cells form.

• Sister chromatids remain
together.
meiosis i
• After meiosis I, the two cells formed during this stage
go through a second division of the nucleus and the
cytoplasm.

• This process is called meiosis I.
meiosis i
meiosis ii

prophase II
• Unlike prophase I,

chromosomes are not copied
again before prophase II. They
remain short and thick sister
chromatids.

• During prophase II, the

membrane around the
nucleus breaks apart, and the
nucleolus disappears in each
cell.
meiosis ii

metaphase II

• During metaphase II, the pairs
of sister chromatids line up
along the middle of the cell in
single file.
meiosis ii

anaphase II

• During anaphase II, the sister

chromatids of each duplicated
chromosome are pulled apart.

• They then move toward

opposite ends of the cells.
meiosis ii

telophase II
• The final phase of meiosis is
telophase II.

• During telophase II, a nuclear

membrane forms around each
set of chromatids.

• The chromatids are again
called chromosomes.

• The cytoplasm divides

through cytokinesis, and four
haploid cells form.
meiosis ii
meiosis ii
the phases of meiosis
why is meiosis important?
• Meiosis is important to sexual reproduction.
• It forms the correct haploid number of chromosomes.
• This maintains the correct diploid number of
chromosomes in organisms when sex cells join.
why is meiosis important?
why is meiosis important?
• Meiosis also leads to genetic variation.
why is meiosis important?

maintaining diploid cells

• Meiosis helps maintain diploid cells in offspring by
making haploid sex cells.

• When haploid sex cells join together during

fertilization, they make a diploid zygote, or fertilized
egg.

• The zygote divides by mitosis and cell division and
creates a diploid organism.
why is meiosis important?

creating haploid cells

• The result of meiosis is
haploid sex cells.

• The formation of haploid

cells is important because it
results in genetic variation.

• Sex cells can have different

sets of chromosomes,
depending on how
chromosomes line up during
metaphase I.
how do mitosis & meiosis differ?
advantages of sexual reproduction

• The main advantage of sexual reproduction is that it
results in genetic variation among offspring.

• Offspring inherit half their DNA from each parent.
• Inheriting different DNA means that each offspring has
a different set of traits.
advantages of sexual reproduction

genetic variation
• Genetic variation exists among humans.You can look
at your friends to see genetic variations.

• Some people have blue eyes; others have brown eyes.
Some people have blonde hair; others have red hair.

• Genetic variation

occurs in all organisms
that reproduce sexually.
advantages of sexual reproduction

genetic variation
• Because of genetic variation, individuals within a

population have slight differences. These differences
might be an advantage if the environment changes.

• Some individuals might have traits that make them
able to survive harsh conditions.

• For example, some plants within a population might be
able to survive long periods of dry weather.

• Sometimes the traits might help keep an organism
from getting infected by a disease.
advantages of sexual reproduction

selective breeding
• Selective breeding is a process that involves breeding

certain individuals within a population because of the
traits they have.

• For example, a farmer might choose plants with the
biggest flowers and stems. These plants would be
allowed to reproduce and grow.

• Over time, the offspring of the plants would all have
big flowers and stems.

• Selective breeding has been used to produce many
types of plants and animals with certain traits
disadvantages of
sexual reproduction

• Sexual reproduction takes time and energy.
• Organisms have to grow and develop until they are
mature enough to produce sex cells.

• Before they can reproduce, organisms have to find

mates. Searching for a mate takes time and energy.

• The search might also expose individuals to predators,
diseases, or harsh environmental conditions.

• Sexual reproduction can be limited by certain factors.
• For example, fertilization cannot take place during

pregnancy, which can last as long as two years in some
mammals.

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Sexual Reproduction & Meiosis

  • 2. what is sexual reproduction? • Sexual reproduction is a type of reproduction in which the genetic materials from two different cells combine, producing an offspring. • The cells that combine are called sex cells.
  • 3. what is sexual reproduction? • Sex cells form in reproductive organs. There are two types of sex cells—eggs and sperm. • An egg is the female sex cell, which forms in an ovary. • A sperm is the male sex cell, which forms in a testis.
  • 4. what is sexual reproduction? • Fertilization occurs when an egg cell and a sperm cell join together. When an egg and a sperm join together, a new cell is formed. • The new cell that forms from fertilization is called a zygote.
  • 5. diploid cells • After fertilization, a zygote goes through mitosis and cell division. • Mitosis and cell division produce nearly all of the cells in a multicellular organism.
  • 6. diploid cells • Organisms that reproduce sexually form two kinds of cells—body cells and sex cells. • In the body cells of most organisms, chromosomes occur in pairs. Cells that have pairs of chromosomes are called diploid cells.
  • 7. diploid cells • Organisms that reproduce sexually form two kinds of cells—body cells and sex cells. • In the body cells of most organisms, chromosomes occur in pairs. Cells that have pairs of chromosomes are called diploid cells.
  • 8. diploid cells • Organisms that reproduce sexually form two kinds of cells—body cells and sex cells. • In the body cells of most organisms, chromosomes occur in pairs. Cells that have pairs of chromosomes are called diploid cells.
  • 9. diploid cells chromosomes • Pairs of chromosomes that have genes for the same traits arranged in the same order are called homologous chromosomes.
  • 10. diploid cells chromosomes • Because one chromosome is inherited from each parent, the chromosomes are not always identical.
  • 11. diploid cells chromosomes • For example, a kittens can inherited a gene for orange fur color from their mother. They can also inherit a gene for gray fur color from their father. Some kittens might be orange, and some kittens might be gray. No matter what the color of a kitten’s fur, both genes for fur color are found at the same place on homologous chromosomes. In this case, each gene codes for a different color.
  • 12. diploid cells chromosomes • Different organisms have different numbers of chromosomes. • Human diploid cells have 23 pairs of chromosomes. This means that human diploid cells have a total of 46 chromosomes.
  • 13. chromosomes How many chromosomes does a cat have? 38
  • 14. chromosomes How many chromosomes does an ant have? 2
  • 15. chromosomes How many chromosomes does a cow have? 60
  • 16. chromosomes How many chromosomes does a chicken have? 78
  • 17. chromosomes How many chromosomes does a turkey have? 82
  • 18. chromosomes How many chromosomes does a kangaroo have? 12
  • 19. chromosomes How many chromosomes does a mosquito have? 6
  • 20. chromosomes How many chromosomes does a hermit crab have? 254
  • 21. chromosomes How many chromosomes does a goldfish have? 94
  • 22. chromosomes How many chromosomes does a rabbit have? 44
  • 23. chromosomes What species has the most chromosomes? ophioglossum reticulatum 1200 chromosomes
  • 24. chromosomes • It is important to have the correct number of chromosomes. • If a zygote has too many or too few chromosomes, it will not develop properly. • The process of meiosis helps maintain the correct number of chromosomes.
  • 25. chromosomes • If a mistake occurs in animal cells, sometimes the zygote dies. If the zygote lives, every cell in that organism will have the wrong amount of chromosomes and may not grow normally. • A person with Down Syndrome has one extra Chromosome 21.
  • 27. chromosomes Zonkey: ____ 54 chromosomes donkey: 62 chromosomes zebra: 46 chromosomes
  • 28. haploid cells • Organisms that reproduce sexually also form egg and sperm cells, or sex cells. • Sex cells have only one chromosome from each pair of chromosomes. • Haploid cells are cells that have only one chromosome from each pair.
  • 29. haploid cells • Organisms produce sex cells using a special type of cell division called meiosis. • In meiosis, one diploid cell divides and makes four haploid sex cells. • Meiosis occurs only during the formation of sex cells.
  • 30. the phases of meiosis • Recall that mitosis and cytokinesis involve one division of the nucleus and cytoplasm. • Meiosis involves two divisions of the nucleus and the cytoplasm. • These two divisions are phases called meiosis I and meiosis II. • Meiosis results in four haploid cells, each with half the number of chromosomes as the original cell.
  • 31. the phases of meiosis
  • 32. the phases of meiosis • A reproductive cell goes through interphase before beginning meiosis I. • During interphase, the reproductive cell grows and copies, or duplicates, its chromosomes. • Each duplicated chromosome consists of two sister chromatids joined by a centromere.
  • 33. meiosis i prophase I • During prophase I, duplicated chromosomes condense, or shorten, and thicken. • Homologous chromosomes come together and form pairs. • The membrane around the nucleus breaks apart and the nucleolus disappears.
  • 34. meiosis i metaphase I • During metaphase I, homologous chromosome pairs line up along the middle of the cell. • A spindle fiber attaches to each chromosome.
  • 35. meiosis i anaphase I • During anaphase I, chromosome pairs separate and are pulled toward opposite ends of the cell. • The sister chromatids stay together.
  • 36. meiosis i telophase I • During telophase I, a membrane forms around each group of duplicated chromosomes. • The cytoplasm divides through cytokinesis, and two daughter cells form. • Sister chromatids remain together.
  • 37. meiosis i • After meiosis I, the two cells formed during this stage go through a second division of the nucleus and the cytoplasm. • This process is called meiosis I.
  • 39. meiosis ii prophase II • Unlike prophase I, chromosomes are not copied again before prophase II. They remain short and thick sister chromatids. • During prophase II, the membrane around the nucleus breaks apart, and the nucleolus disappears in each cell.
  • 40. meiosis ii metaphase II • During metaphase II, the pairs of sister chromatids line up along the middle of the cell in single file.
  • 41. meiosis ii anaphase II • During anaphase II, the sister chromatids of each duplicated chromosome are pulled apart. • They then move toward opposite ends of the cells.
  • 42. meiosis ii telophase II • The final phase of meiosis is telophase II. • During telophase II, a nuclear membrane forms around each set of chromatids. • The chromatids are again called chromosomes. • The cytoplasm divides through cytokinesis, and four haploid cells form.
  • 45. the phases of meiosis
  • 46. why is meiosis important? • Meiosis is important to sexual reproduction. • It forms the correct haploid number of chromosomes. • This maintains the correct diploid number of chromosomes in organisms when sex cells join.
  • 47. why is meiosis important?
  • 48. why is meiosis important? • Meiosis also leads to genetic variation.
  • 49. why is meiosis important? maintaining diploid cells • Meiosis helps maintain diploid cells in offspring by making haploid sex cells. • When haploid sex cells join together during fertilization, they make a diploid zygote, or fertilized egg. • The zygote divides by mitosis and cell division and creates a diploid organism.
  • 50. why is meiosis important? creating haploid cells • The result of meiosis is haploid sex cells. • The formation of haploid cells is important because it results in genetic variation. • Sex cells can have different sets of chromosomes, depending on how chromosomes line up during metaphase I.
  • 51. how do mitosis & meiosis differ?
  • 52. advantages of sexual reproduction • The main advantage of sexual reproduction is that it results in genetic variation among offspring. • Offspring inherit half their DNA from each parent. • Inheriting different DNA means that each offspring has a different set of traits.
  • 53. advantages of sexual reproduction genetic variation • Genetic variation exists among humans.You can look at your friends to see genetic variations. • Some people have blue eyes; others have brown eyes. Some people have blonde hair; others have red hair. • Genetic variation occurs in all organisms that reproduce sexually.
  • 54. advantages of sexual reproduction genetic variation • Because of genetic variation, individuals within a population have slight differences. These differences might be an advantage if the environment changes. • Some individuals might have traits that make them able to survive harsh conditions. • For example, some plants within a population might be able to survive long periods of dry weather. • Sometimes the traits might help keep an organism from getting infected by a disease.
  • 55. advantages of sexual reproduction selective breeding • Selective breeding is a process that involves breeding certain individuals within a population because of the traits they have. • For example, a farmer might choose plants with the biggest flowers and stems. These plants would be allowed to reproduce and grow. • Over time, the offspring of the plants would all have big flowers and stems. • Selective breeding has been used to produce many types of plants and animals with certain traits
  • 56. disadvantages of sexual reproduction • Sexual reproduction takes time and energy. • Organisms have to grow and develop until they are mature enough to produce sex cells. • Before they can reproduce, organisms have to find mates. Searching for a mate takes time and energy. • The search might also expose individuals to predators, diseases, or harsh environmental conditions. • Sexual reproduction can be limited by certain factors. • For example, fertilization cannot take place during pregnancy, which can last as long as two years in some mammals.