Figure 5.2
The cell cycle is the repeated process of growth and division. Notice that most of the cell cycle is spent in interphase (G1, S, and G2) (I).
During the mitotic phase, nuclear division occurs, which is known as mitosis. Also cytokinesis, the division of the cytoplasm, occurs. After cytokinesis, cell division is complete and two genetically identical daughter cells have been produced from one parent cell. The term "genetically identical" refers to the fact that each resulting cell has an identical set of DNA, and this DNA is also identical to that of the parent cell.
Mitosis and Chromosomes
During cell division, two nuclei must form during the process of mitosis, so that one nucleus can be given to each of cells that form from cytokinesis. In the nucleus, the genetic information of the cell, DNA, is stored. The copied DNA needs to be moved into a new nucleus for the new cell to have a correct set of genetic instructions.
The DNA in the nucleus is condensed into chromosomes, structures composed of DNA wrapped around proteins. Each organism has a unique number of chromosomes; in human cells our DNA is divided up into 23 pairs of chromosomes. When a cell is not undergoing division, such as during interphase, the complex of DNA and proteins is a tangled mass of threads known as chromatin. As mitosis begins, however, the DNA becomes tightly coiled into the chromosomes which become visible under a microscope.
Figure 5.3
The DNA double helix wraps around histone proteins (2) and tightly coils a number of times to form a condensed chromosome (5). The chromosomes contains millions of nucleotide bases. This figure illustrates the complexity of the coiling process. The red dot shows the location of the centromere, where the microtubules attach during mitosis and meiosis.
As mentioned previously, the DNA is replicated during the S stage of interphase. Each chromosome now has two identical molecules of DNA, called sister chromatids, forming the "X" shaped molecule depicted in Figure above. During mitosis, the two sister chromatids must be split apart to give rise to two identical chromosomes (in essence, each resulting chromosome is made of 1/2 of the "X"). Through this process, each daughter cell receives one copy of each chromosome.
Mitosis is divided into four phases: prophase, metaphase, anaphase, and telophase. During prophase, the chromosomes become tightly wound and become visible under the microscope. Also, the nuclear envelop dissolves, and the spindle begins to form. The spindle is a structure containing many fibers that helps to move the chromosomes. By late prophase, the chromosomes are attached to the spindle fibers. The spindle fibers will later pull the chromosomes into alignment.
During metaphase, the chromosomes line up across the center of the cell. The chromosomes line up in a row, one on top of the next. During anaphase, the two sister chromatids of each chromosome separate, resulting in two sets of identical chromosomes. During telophase, the spindle dissolves and nuclear envelopes form around the chromosomes. The drawings of Figure below show this process. This is further shown in Figure below. Each new nucleus contains the exact same number and types of chromosomes as the original cell. The cell is now ready for cytokinesis, producing two genetically identical cells, each with its own nucleus.
Figure 5.4
An overview of mitosis: during prophase (I and II) the chromosomes condense, during metaphase the chromosomes line up (III and IV), during anaphase the sister chromatids are pulled to opposite sides of the cell (V and VI), during telophase the nuclear envelope forms ( VII and VIII).
Figure 5.5
This is a picture of dividing plant cells. Cell division in plant cells differs slightly from animal cells as a cell wall must form. Note that most of the cells are in interphase. Can you find examples of the different stages of mitosis?
Lesson Summary
Cells divide for growth, development, reproduction and replacement of injured or worn-out cells.
The cell cycle is a series of regulated steps by which a cell divides.
During mitosis, the newly duplicated chromosomes are divided into two daughter nuclei.
Review Questions
In what phase of mitosis are chromosomes moving toward opposite sides of the cell?
In what phase of mitosis do the duplicated chromosomes condense?
What step of the cell cycle is the longest?
What is the term for the division of the cytoplasm?
What happens during the S stage of interphase?
Interphase used to be considered the “resting” stage of the cell cycle. Why is this not correct?
What are some reasons that cells divide?
During what stage of the cell cycle does the cell double in size?
Why must cell division be tightly regulated?
What is the purpose of mitosis?
Further Reading / Supplemental Links
http://en.wikipedia.org/wiki/Mitosis
http://www.biology.arizona.edu/Cell_bio/tutorials/cell_cycle/cells3.html
http://biology.clc.uc.edu/courses/bio104/mitosis.htm
http://en.wikipedia.org/wiki/Cell_cycle
http://www.cellsalive.com/mitosis.htm
http://www.wisc-online.com/objects/index_tj.asp?objID=AP13604
Vocabulary
anaphase
Third phase of mitosis where sister chromatids separate and move to opposite sides of the cell.
cell cycle
Sequence of steps in eukaryotic cells that leads to cell division.
chromatin
Complex of DNA and proteins that is visible when a cell is not dividing.
chromosomes
DNA wound around proteins; forms during prophase of mitosis and meiosis.
cytokinesis
Division of the cytoplasm after mitosis or meiosis.
interphase
Stage of the cell cycle when DNA is synthesized and the cell grows; composed of the first three phases of the cell cycle.
metaphase
Second phase of meiosis where the chromosomes are aligned in the center of the cell.
mitosis
Sequence of steps in which a nucleus is divided into two daughter nuclei, each with an identical set of chromosomes.
prophase
Initial phase of mitosis where chromosomes condense, the nuclear envelope dissolves and the spindle begins to form.
spindle
Fibers that move chromosomes during mitosis and meiosis.
telophase
Final phase of mitosis where a nuclear envelop forms around each of the two sets of chromosomes.
Points to Consider
How might a cell without a nucleus divide?
How are new cells made that incorporate the DNA of two parents?
Lesson 5.2: Reproduction
Lesson Objectives
Name the types of asexual reproduction.
Explain the advantage of sexual reproduction.
List the stages of meiosis and explain what happens in each stage.
Check Your Understanding
Can something that does not reproduce still be considered living?
What stores the genetic information that is passed on to offspring?
How many chromosomes are in the human nucleus?
Introduction
Can an organism be considered alive if it cannot make the next generation? For a species to survive, reproduction, the ability to make the next generation, is absolutely necessary. For a species to be successful, it not only needs to be well adapted to its environment, but also needs to be successful at reproduction. Reproduction allows a population of organisms to pass on their genetic information to the next generation. There are many different ways that organisms reproduce, and these methods are categorized as either sexual or asexual reproduction. There are advantages and disadvantages to each method, but the result is always the same: a new life begins.
Asexual Reproduction
Some organisms can reproduce asexually, meaning that the offspring have a single parent and share the exact same genetic materia
l as the parent. The advantage of asexual reproduction is that it can be very quick and does not require the meeting of two individuals of the opposite sexes. The disadvantage of asexual reproduction is that it does not involve genetic recombination, a process that can result in an adaptive new set of traits. For example, you might inherit one advantageous trait from your maternal grandmother, another adaptive trait from your paternal grandmother, and other adaptive traits from your paternal grandfather. You have the benefit of the many genes from two lineages combining in a new way. An organism that is born through asexual reproduction, however, only has the DNA from one parent, and it is the exact copy of that parent. Therefore, no new combinations of traits can happen.
Prokaryotic organisms, which as you might recall are single-celled, reproduce asexually. Bacteria reproduce through binary fission, where they basically divide in half (Figure below). First, their chromosome replicates and the cell enlarges. After cell division, the two new cells each have one identical chromosome. Mitosis is not necessary as there is no nucleus. Then new membranes form to separate the two cells. This simple process is beneficial to the bacteria, allowing very rapid reproduction.
Figure 5.6
Bacteria reproduce by binary fission. Shown is one bacterium reproducing and becoming two bacteria.
There are also several animals that can reproduce asexually. Flatworms can divide in two, then each half regenerates into a new flatworm identical to the original. Many types of insects, fish, and lizards (Figure below)can reproduce asexually through parthenogenesis. Parthenogenesis is a process by which an unfertilized egg cell grows into a new organism. The resulting organism has half the amount of genetic material of the parent, as the starting egg cell has half the amount of DNA compared to the parent. Parthenogenesis is common in honeybees. The fertilized eggs in a hive become workers, while the unfertilized eggs become drones.
Egg cells (and also sperm cells) are produced through a cell division mechanism in which the amount of DNA is halved. This process is called meiosis and will be discussed shortly.
Figure 5.7
This Komodo dragon was born by parthenogenesis.
Sexual Reproduction
During sexual reproduction, two parents are involved. Most animals are dioecious, meaning there is a separate male and female sex, with the male producing sperm and the female producing eggs. When a sperm and egg meet, a zygote, the first cell of a new organism, is formed (Figure below). The zygote will divide and grow into the embryo.
Figure 5.8
During sexual reproduction, a sperm fertilizes an egg.
Animals often have gonads, specialized organs that produce eggs or sperm. The male gonads are the testes, which produce the sperm, and the female gonads are the ovaries, which produce the eggs. Sperm and egg, the two sex cells, are known as gametes, and unite through a variety of methods. Fish and other aquatic animals release their gametes in the water, which is called external fertilization (Figure below). Animals that live on land, however, usually practice internal fertilization. Typically males have a penis that deposits sperm into the vagina of the female. Other anatomical features can accomplish the same goal; birds, for example, have a chamber called the cloaca that they place close to another bird’s cloaca to deposit sperm. Whatever method of fertilization is used, the net result is the same: a zygote that contains DNA from both the male and female.
Figure 5.9
This fish guards her eggs, which will be fertilized externally.
Plants also can reproduce sexually, but their reproductive organs are somewhat different than animals’ gonads. Most plants are flowering plants, meaning their reproductive parts are contained in a flower. The sperm is contained in the pollen, while the egg is contained in the ovary deep within the flower. The sperm can reach the egg through several methods. In self-pollination, the egg is fertilized by the pollen of same flower. In cross-pollination, the sperm from the pollen from one flower fertilizes the egg of another flower. Cross-pollination increases the genetic diversity of the population. Like other types of sexual reproduction, cross-pollination allows new combinations of traits. Cross-pollination can be achieved when pollen is carried by the wind to another flower, or many flowers rely on animal pollinators, like honeybees, or butterflies (Figure below) to carry the pollen from flower to flower.
Figure 5.10
Butterflies receive nectar when they deposit pollen into flowers, resulting in cross-pollination.
Fungi can also reproduce sexually, but instead of female and male sexes, they have (+) and (-) strains. When the filaments of a (+) and (-) fungi meet, the zygote is formed. As with the sexual reproduction in plants and animals, each zygote receives DNA from two parent strains.
Meiosis and Gametes
The formation of gametes, the reproductive cells such as sperm and egg, is necessary for sexual reproduction. As gametes are produced, the number of chromosomes must be reduced to half. In humans, our cells have 23 pairs of chromosomes, and each chromosome within a pair is called a homologous chromosome. For each of the 23 chromosome pairs, you received one chromosome from your father and one chromosome from your mother. The homologous chromosomes have the same genes, although there might be alternate forms of each gene, called alleles, which vary between the chromosomes. These homologous chromosomes are separated during gamete formation, therefore gametes have only 23 chromosomes, not 23 pairs. This separation of chromosomes is random. The probability or chance that a particular allele will be in a gamete is 1 in 2. The gamete may receive either the paternal allele (inherited from the father) or the maternal allele (inherited from the mother). This random separation of chromosomes (and therefore alleles) occurs for each chromosome, resulting in an widely varied combination of chromosomes in each gamete. With 23 pairs of chromosomes, this results in over 8 million different combinations of chromosomes a gamete.
Haploid vs. Diploid
A cell with two sets of chromosomes is diploid, referred to as 2n, where n is the number of sets of chromosomes. A cell with one set of chromosomes, such as a gamete, is haploid, referred to as n. So when a haploid sperm and a haploid egg combine, a diploid zygote will be formed; in essence, when a zygote is formed, half of the DNA in the diploid zygote comes from each parent. The process of cell division that reduces the chromosome number by half is called meiosis.
Meiosis
Prior to meiosis, DNA replication occurs, so each chromosome contains two sister chromatids that are identical to the original chromosome. Meiosis is divided into two nuclear divisions: meiosis I and meiosis II. Each of these nuclear divisions shares many aspects of mitosis and can be divided into the same phases: prophase, metaphase, anaphase, and telophase; however, between the two divisions, DNA replication does not occur. Through this process, one diploid cell will divide into four haploid cells.
Meiosis I
During meiosis I, the pairs of homologous chromosomes are separated from each other. During prophase I, the homologous chromosomes line up together. During this time, crossing-over can occur (Figure below), the exchange of DNA between homologous chromosomes. Crossing-over increases the new allele combinations in the gametes. Without crossing-over, the offspring would always inherit all of the many alleles on one of the homologous chromosomes. Because of crossing over, the alleles on the homologous chromosomes can be scrambled to pass on unique combinations of alleles on the chromosome. Also during prophase I, the spindle forms and the chromosomes condense as they coil up tightly. The spindle has the same function as in mitosis.
Figure 5.11
During crossing-over, segments of DNA are exchanged between sister chromatids. Notice how this can result in an allele (M) on one sister chromatid being moved onto the other sister chromatid.
During metaphase I, the homologous chromosomes line up in pairs in the middle of the cell; that is, both chromosome of a pair will line up together. The maternal chromosomes or paternal chromosomes can each attach to either side of the spindle. The assignment of which side is random
, so all the maternal or paternal chromosomes do not end up in one gamete. The gamete will contain some chromosomes from the mother and some chromosomes from the father. Note this is different than during metaphase of mitosis; although chromosomes still line up during mitosis, the sister chromatids are separated, and each cell obtains both the maternal and paternal chromosome of each pair.
During anaphase I, the homologous chromosomes separate. In telophase I, the spindle dissolves, but a new nuclear envelop does not need to form. That’s because after a brief resting stage, the nucleus will divide again. No DNA replication happens between meiosis I and meiosis II as the chromosomes are already duplicated, carrying sister chromatids.
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