reproduction

homologous chromosomes

The two members of a given pair of chromosomes are called homologous chromosomes. We get one of each homologous pair, or 23 chromosomes, from our father. We get the other one of each pair, or 23 chromosomes, from our mother. A gamete must have the haploid number of chromosomes. That way, when two gametes unite, the zygote will have the diploid number. How are haploid cells produced? The answer is meiosis.

sexual reproduction

Sexual reproduction is more complicated. It involves two parents. Special cells called gametes are produced by the parents. A gamete produced by a female parent is generally called an egg. A gamete produced by a male parent is usually called a sperm. An offspring forms when two gametes unite. The union of the two gametes is called fertilization. You can see a human sperm and egg uniting in Figure 5.12. The initial cell that forms when two gametes unite is called a zygote.

textbook_image

chromosome numbers

In species with sexual reproduction, each cell of the body has two copies of each chromosome. For example, human beings have 23 different chromosomes. Each body cell contains two of each chromosome, for a total of 46 chromosomes. You can see the 23 pairs of human chromosomes in Figure 5.13. The number of different types of chromosomes is called the haploid number. In humans, the haploid number is 23. The number of chromosomes in normal body cells is called the diploid number. The diploid number is twice the haploid number. In humans, the diploid number is two times 23, or 46.

asexual reproduction

Asexual reproduction is simpler than sexual reproduction. It involves just one parent. The offspring are genetically identical to each other and to the parent. All prokaryotes and some eukaryotes reproduce this way. There are several different methods of asexual reproduction. They include binary fission, fragmentation, and budding.

textbook_image

binary fission

Binary fission occurs when a parent cell simply splits into two daughter cells. This method is described in detail in the lesson "Cell Division." Bacteria reproduce this way. You can see a bacterial cell reproducing by binary fission in Figure 5.9.

textbook_image

fragmentation

Fragmentation occurs when a piece breaks off from a parent organism. Then the piece develops into a new organism. Sea stars, like the one in Figure 5.10, can reproduce this way. In fact, a new sea star can form from a single arm.

textbook_image

budding

Budding occurs when a parent cell forms a bubble-like bud. The bud stays attached to the parent while it grows and develops. It breaks away from the parent only after it is fully formed. Yeasts can reproduce this way. You can see two yeast cells budding in Figure 5.11.

textbook_image

advantage of sexual reproduction

Sexual reproduction is typically slower. However, it also has an advantage. Sexual reproduction results in offspring that are all genetically different. This can be a big plus for a species. The variation may help it adapt to changes in the environment. How does genetic variation arise during sexual reproduction? It happens in three ways: crossing over, independent assortment, and the random union of gametes. Crossing over occurs during meiosis I. It happens when homologous chromosomes pair up during prophase I. The paired chromosomes exchange bits of DNA. This recombines their genetic material. You can see where crossing over has occurred in Figures 5.15 and 5.16. Independent assortment occurs when chromosomes go to opposite poles of the cell in anaphase I. Which chromosomes end up together at each pole is a matter of chance. You can see this in Figures 5.15 and 5.16 as well. In sexual reproduction, two gametes unite to produce an offspring. Which two gametes is a matter of chance. The union of gametes occurs randomly. Due to these sources of variation, each human couple has the potential to produce more than 64 trillion unique offspring. No wonder we are all different!

advantage of asexual reproduction

Asexual reproduction can happen very quickly. It doesnt require two parents to meet and mate. Under ideal conditions, 100 bacteria can divide to produce millions of bacteria in just a few hours! Most bacteria dont live under ideal conditions. Even so, rapid reproduction may allow asexual organisms to be very successful. They may crowd out other species that reproduce more slowly.

advantages of sexual and asexual reproduction

Both types of reproduction have certain advantages.

meiosis ii

Meiosis II is just like mitosis. 1. Prophase II: Chromosomes form. The nuclear membrane breaks down. Centrioles move to opposite poles. Spindle fibers form. 2. Metaphase II: Spindle fibers attach to the centromeres of sister chromatids. Sister chromatids line up at the center of the cell. 3. Anaphase II: Spindle fibers shorten. They pull the sister chromatids to opposite poles. 4. Telophase II: The chromosomes uncoil. The spindle fibers break down. New nuclear membranes form. Meiosis II is also followed by cytokinesis. This time, four haploid daughter cells result. Thats because both daughter cells from meiosis I have gone through meiosis II. The four daughter cells must continue to develop before they become gametes. For example, in males, the cells must develop tails, among other changes, in order to become sperm.

meiosis i

After DNA replicates during interphase, the nucleus of the cell undergoes the four phases of meiosis I: 1. Prophase I: Chromosomes form, and the nuclear membrane breaks down. Centrioles move to opposite poles of the cell. Spindle fibers form between the centrioles. Heres whats special about meiosis: Homologous chromosomes pair up! You can see this in Figure below. 2. Metaphase I: Spindle fibers attach to the centromeres of the paired homologous chromosomes. The paired chromosomes line up at the center of the cell. 3. Anaphase I: Spindle fibers shorten, pulling apart the chromosome pairs. The chromosomes are pulled toward opposite poles of the cell. One of each pair goes to one pole. The other of each pair goes to the opposite pole. 4. Telophase I: The chromosomes uncoil, and the spindle fibers break down. New nuclear membranes form. Phases of meiosis I Meiosis I is followed by cytokinesis. Thats when the cytoplasm of the cell divides. Two haploid daughter cells result. Both of these cells go on to meiosis II.

textbook_image

meiosis

Meiosis is a special type of cell division. It produces haploid daughter cells. It occurs when an organism makes gametes. Meiosis is basically mitosis times two. The original diploid cell divides twice. The first time is called meiosis I. The second time is called meiosis II. However, the DNA replicates only once. It replicates before meiosis I but not before meiosis II. This results in four haploid daughter cells. Meiosis I and meiosis II occurs in the same four phases as mitosis. The phases are prophase, metaphase, anaphase, and telophase. However, meiosis I has an important difference. In meiosis I, homologous chromosomes pair up and then separate. As a result, each daughter cell has only one chromosome from each homologous pair. Figure 5.14 is a simple model of meiosis. It shows both meiosis I and II. You can read more about the stages below. You can also learn more about them by watching this video: . MEDIA Click image to the left or use the URL below. URL:

textbook_image

instructional diagrams

No diagram descriptions associated with this lesson

questions

The last phase of meiosis is

a. prophase II

b. anaphase II

c. metaphase II

-->  d. none of the above

The number of different types of chromosomes in the human species is

-->  a. 23

b. 44

c. 46

d. 32

During which phase of meiosis does independent assortment occur?

a. prophase I

b. prophase II

-->  c. anaphase I

d. anaphase II

Which of the following types of cells is a diploid cell?

a. sperm

b. gamete

-->  c. zygote

d. egg

The two copies of a given chromosome in a diploid cell are called

a. sister chromosomes

-->  b. homologous chromosomes

c. haploid chromosomes

d. crossover chromosomes

_type of asexual reproduction that occurs in yeasts

a. meiosis

b. fragmentation

c. diploid number

-->  d. budding

e. binary fission

f. fertilization

g. haploid number

_process in which two gametes unite to form a single cell

a. meiosis

b. fragmentation

c. diploid number

d. budding

e. binary fission

-->  f. fertilization

g. haploid number

_type of cell division that produces two identical daughter cells

a. meiosis

b. fragmentation

c. diploid number

d. budding

-->  e. binary fission

f. fertilization

g. haploid number

_type of cell division that produces four gametes

-->  a. meiosis

b. fragmentation

c. diploid number

d. budding

e. binary fission

f. fertilization

g. haploid number

Meiosis I is just like mitosis.

a. true

-->  b. false

Some organisms can produce both sexually and asexually.

-->  a. true

b. false

_number of chromosomes in a gamete

a. meiosis

b. fragmentation

c. diploid number

d. budding

e. binary fission

f. fertilization

-->  g. haploid number

Yeast cells can reproduce by budding.

-->  a. true

b. false

_type of asexual reproduction that occurs in sea stars

a. meiosis

-->  b. fragmentation

c. diploid number

d. budding

e. binary fission

f. fertilization

g. haploid number

_number of chromosomes in a normal body cell

a. meiosis

b. fragmentation

-->  c. diploid number

d. budding

e. binary fission

f. fertilization

g. haploid number

Binary fission results in four daughter cells.

a. true

-->  b. false

Sexual reproduction is simpler than asexual reproduction.

a. true

-->  b. false

Asexual reproduction

-->  a. is simpler than sexual reproduction.

b. occurs only in prokaryotes.

c. includes the union of gametes.

d. two of the above

The diploid number of chromosomes in a species is always

a. half the haploid number.

-->  b. twice the haploid number.

c. 46.

d. two of the above

How many chromosomes does a human individual normally inherit from each parent?

-->  a. 23

b. 46

c. the diploid number

d. two of the above

Methods of asexual reproduction include all of the following except

a. budding.

-->  b. fertilization.

c. fragmentation.

d. binary fission.

Which stage of meiosis occurs first?

a. anaphase I

-->  b. prophase I

c. telophase I

d. metaphase I

Which phase directly follows meiosis I?

a. interphase

-->  b. cytokinesis

c. prophase II

d. telophase II

How does genetic variation arise during sexual reproduction?

a. crossing over

b. random union of gametes

c. independent assortment

-->  d. all of the above

___Crossing over occurs during prophase I.

-->  a. true

b. false

___Meiosis II is followed by cytokinesis.

-->  a. true

b. false

___Bacteria produce gametes by binary fission.

a. true

-->  b. false

___Independent assortment occurs during meiosis II.

a. true

-->  b. false

___A new sea star can form from a single arm.

-->  a. true

b. false

___Sexual reproduction can occur more quickly than asexual reproduction.

a. true

-->  b. false

___Fertilization results in a haploid zygote.

a. true

-->  b. false

diagram questions

No diagram questions associated with this lesson