Interactive Quizzes
Interactive quizzes are self-grading quizzes which provide immediate feedback on answer selections. Most of the quizzes are multiple-choice but other formats are represented. Length and difficulty will vary with each quiz but all of the quizzes provided here are appropriate for high school biology.
Want to make your own online quizzes? The sites below will help you get started!
-Discovery School's Quiz Center
-Quia (free required, trial available)
Quiz Category List
Nucleic Acids & Protein Synthesis
Showing posts with label BIOLOGY. Show all posts
Showing posts with label BIOLOGY. Show all posts
Friday, September 4, 2009
Hot Biology QUIZZES!
Wednesday, July 15, 2009
Wednesday, June 24, 2009
Biology ++Protein++
Proteins play an important role in the formation of cellular components, cells and tissues in the human body.
They are also needed for growth and development. Without them, we will not grow or be able to repair damaged tissues in our body.
Among the substances made up of proteins in the human body are haemoglobins, antibodies, enzymes, certain hormones, muscle tissues and part of the cell membrane.
- The basic building block of proteins is the amino acid.
- There are 20 types of amino acids.
- Of these, the essential amino acids are the ones that cannot be synthesised by the body and must be obtained from the food we eat.
- Five example of amino acids are leucine, tryptophan, glutamic acid, valine and serine.
- If amino acids are known as monomers, proteins are polymers.
- When two amino acids are joined together, a dipeptide is formed.
- The bond between two amino acids is called a peptide bond.
- The process in which polypeptides are formed from amino acids is known as condensation.
- Protein can be broken down to amino acids in a process called hydrolysis.
- A molecule that is a protein in nature can be used to speed up the process mentioned above. This molecule is an enzyme.
Resonance structures of the peptide bond that linksindividual amino acids to form a protein polymer.
Chemical structure of the peptide bond
A peptide bond between leucine and threonine
Saturday, June 13, 2009
Heart.
Blood is circulated around the body through blood vessels by the pumping action of the heart. In humans, blood is pumped from the strong left ventricle of the heart through arteries to peripheral tissues and returns to the right atrium of the heart through veins. It then enters the right ventricle and is pumped through the pulmonary artery to the lungs and returns to the left atrium through the pulmonary veins.
Blood then enters the left ventricle to be circulated again. Arterial blood carries oxygen from inhaled air to all of the cells of the body, a waste product of metabolism by cells, carbon dioxide, to the lungs to be exhaled.
However, one exception includes pulmonary arteries which contains the most deoxygenated blood in the body, while the pulmonary veins contain oxygenated blood.
Additional return flow may be generated by the movement of skeletal muscles which can compress veins and push blood through the valves in veins towards the right atrium.
Sunday, May 31, 2009
[Biology form 4] Enzyme Extraordinaire
By teacher Naza
Many reactions take place within the cells of living organisms. If these reactions were to happen at a normal pace, they would take forever!
So, cells depend on specific protein molecules to speed up or catalyse the rate of the biochemical reactions in them. These catalysts are called enzymes.
Since enzymes are proteins in nature, they are easily affected by factors such as temperature and pH. Hence, conditions within the cell must be optimal for enzymes to function effectively.
The best way to master the topic on enzymes is to use the comprehensive S.M.A.R.T. guide.
S.M.A.R.T. guide to learning biology
Structure function relationship - knowing the function of every structure.
1. Structure: What chemical compound is an enzyme made up of?
Proteins.
2. Function: What is the role played by enzymes in living organisms?
Enzymes speed up the rate of chemical reactions in living organisms.
Mastery of facts - by asking the right questions and learning from the answer.
A simple way to accomplish this is by using the alphabetical ABCDEFG sub-guide. ABCDEFG means "Always (A) Begin (B) with CDEFG".
C is for characteristics
3. State the characteristics of enzymes:
a) Enzymes are made up of proteins.
b) They are catalysts that speed up the rate of a reaction.
c) They are not destroyed / altered by the reactions they catalyse.
d) Enzymes have specific active sites where they bind with substrates.
e) Enzymes show substrate specificity - they can only bind with specific substrates.
f) Enzymes are needed only in small amounts.
g) Enzyme-catalysed reactions are reversible - they catalyse reactions in either direction.
h) The activity of enzymes can be slowed down by inhibitors.
i) Enzymes work more effectively with the help of co-factors.
j) Enzymes are affected by factors such as temperature, pH, substrate concentration and enzyme concentration.
Many reactions take place within the cells of living organisms. If these reactions were to happen at a normal pace, they would take forever!
So, cells depend on specific protein molecules to speed up or catalyse the rate of the biochemical reactions in them. These catalysts are called enzymes.
Since enzymes are proteins in nature, they are easily affected by factors such as temperature and pH. Hence, conditions within the cell must be optimal for enzymes to function effectively.
The best way to master the topic on enzymes is to use the comprehensive S.M.A.R.T. guide.
S.M.A.R.T. guide to learning biology
Structure function relationship - knowing the function of every structure.
1. Structure: What chemical compound is an enzyme made up of?
Proteins.
2. Function: What is the role played by enzymes in living organisms?
Enzymes speed up the rate of chemical reactions in living organisms.
Mastery of facts - by asking the right questions and learning from the answer.
A simple way to accomplish this is by using the alphabetical ABCDEFG sub-guide. ABCDEFG means "Always (A) Begin (B) with CDEFG".
C is for characteristics
3. State the characteristics of enzymes:
a) Enzymes are made up of proteins.
b) They are catalysts that speed up the rate of a reaction.
c) They are not destroyed / altered by the reactions they catalyse.
d) Enzymes have specific active sites where they bind with substrates.
e) Enzymes show substrate specificity - they can only bind with specific substrates.
f) Enzymes are needed only in small amounts.
g) Enzyme-catalysed reactions are reversible - they catalyse reactions in either direction.
h) The activity of enzymes can be slowed down by inhibitors.
i) Enzymes work more effectively with the help of co-factors.
j) Enzymes are affected by factors such as temperature, pH, substrate concentration and enzyme concentration.
Thursday, April 9, 2009
DNA Structure
During the 1950s, a tremendous explosion in biological research occurred, and the methods of gene expression were elucidated. The knowledge generated during this period helped explain how genes function in microorganisms and gave rise to the science of molecular genetics. This science is concerned with the activity of deoxyribonucleic acid (DNA) and how that activity brings about the production of proteins in microbial and other cells.
As proposed originally in 1953 by Watson and Crick, deoxyribonucleic acid (DNA) consists of two long chains of nucleotides. The two nucleotide chains twist around one another to form a double helix, which resembles a spiral staircase. The two chains of nucleotides are held to one another by weak hydrogen bonds between bases of the chains.
A nucleotide in the DNA chain consists of three parts: a nitrogenous base, a phosphate group, and a molecule of deoxyribose. The nitrogenous bases of each nucleotide chain are of two major types: purines and pyrimidines. Purines have two fused rings of carbon and nitrogen atoms, while pyrimidines have only one ring. The two purine bases in DNA are adenine (A) and guanine (G). The pyrimidine bases in DNA are cytosine (C) and thymine (T). Purine and pyrimidine bases are found in both strands of the double helix.
The phosphate group of DNA is derived from a molecule of phosphoric acid and connects the deoxyribose molecules to one another in the nucleotide chain. Deoxyribose is a five-carbon carbohydrate. The purine and pyrimidine bases are attached to the deoxyribose molecules and stand opposite one another on the two nucleotide chains. Adenine always stands opposite and binds to thymine. Guanine always stands opposite and binds to cytosine. Adenine and thymine are said to be complementary, as are guanine and cytosine. This is known as the principle of complementary base pairing.
DNA replication. Before a cell enters the process of binary fission or mitosis, the DNA replicates itself to ensure that the daughter cells can function independently. In the process of DNA replication, specialized enzymes pull apart, or “unzip,” the DNA double helix.
As the two strands separate, the purine and pyrimidine bases on each strand are exposed. The exposed bases then attract their complementary bases and induce the complementary bases to stand opposite. Deoxyribose molecules and phosphate groups are brought into the environment, and the enzyme DNA polymerase unites all the nucleotide components to one another and forms a long strand of nucleotides. Thus, the old strand of DNA directs the synthesis of a new strand of DNA through complementary base pairing.
After the synthesis has occurred, one old strand of DNA unites with a new strand to reform a double helix. This process is called semiconservative replication because one of the old strands is conserved in the new DNA double helix.
As proposed originally in 1953 by Watson and Crick, deoxyribonucleic acid (DNA) consists of two long chains of nucleotides. The two nucleotide chains twist around one another to form a double helix, which resembles a spiral staircase. The two chains of nucleotides are held to one another by weak hydrogen bonds between bases of the chains.
A nucleotide in the DNA chain consists of three parts: a nitrogenous base, a phosphate group, and a molecule of deoxyribose. The nitrogenous bases of each nucleotide chain are of two major types: purines and pyrimidines. Purines have two fused rings of carbon and nitrogen atoms, while pyrimidines have only one ring. The two purine bases in DNA are adenine (A) and guanine (G). The pyrimidine bases in DNA are cytosine (C) and thymine (T). Purine and pyrimidine bases are found in both strands of the double helix.
The phosphate group of DNA is derived from a molecule of phosphoric acid and connects the deoxyribose molecules to one another in the nucleotide chain. Deoxyribose is a five-carbon carbohydrate. The purine and pyrimidine bases are attached to the deoxyribose molecules and stand opposite one another on the two nucleotide chains. Adenine always stands opposite and binds to thymine. Guanine always stands opposite and binds to cytosine. Adenine and thymine are said to be complementary, as are guanine and cytosine. This is known as the principle of complementary base pairing.
DNA replication. Before a cell enters the process of binary fission or mitosis, the DNA replicates itself to ensure that the daughter cells can function independently. In the process of DNA replication, specialized enzymes pull apart, or “unzip,” the DNA double helix.
As the two strands separate, the purine and pyrimidine bases on each strand are exposed. The exposed bases then attract their complementary bases and induce the complementary bases to stand opposite. Deoxyribose molecules and phosphate groups are brought into the environment, and the enzyme DNA polymerase unites all the nucleotide components to one another and forms a long strand of nucleotides. Thus, the old strand of DNA directs the synthesis of a new strand of DNA through complementary base pairing.
After the synthesis has occurred, one old strand of DNA unites with a new strand to reform a double helix. This process is called semiconservative replication because one of the old strands is conserved in the new DNA double helix.
Female Reproductive System
The organs of female reproduction include the ovaries, two oval organs lying within the pelvic cavity, and adjacent to them, two Fallopian tubes. Also known as oviducts, the Fallopian tubes are the passageways that egg cells enter after release from the ovaries. The Fallopian tubes lead to the uterus (womb), a muscular organ in the pelvic cavity. The inner lining, called the endometrium, thickens with blood and tissue in anticipation of a fertilized egg cell. If fertilization fails to occur, the endometrium degenerates and is shed in the process of menstruation.
The opening at the lower end of the uterus is a constricted area called the cervix. The tube leading from the cervix to the exterior is a muscular organ called the vagina. During periods of sexual arousal, the vagina receives the penis and the semen. The sperm cells in the semen pass through the cervix and uterus into the Fallopian tubes, where fertilization takes place.
In the human female, egg cell production begins before birth, when about 2 million primitive cells known as oogonia accumulate in the ovaries. These oogonia are formed in the early stages of meiosis. After the age of puberty, the oogonia develop into primary oocytes and then into egg cells at a rate of one per month. Egg cell production occurs by the process of meiosis.
The egg cells develop within the ovary in a cluster of cells called the Graafian follicle, which secretes female hormones called estrogens that regulate the development of secondary female characteristics. Egg cell development within the follicle requires approximately 14 days. The development is controlled by two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both hormones are secreted by the anterior lobe of the pituitary gland.
During the 14 days of egg cell development, the endometrium increases its supply of blood and nutrients in anticipation of a fertilized egg cell. On about the 14th day, the release of the egg cell from the follicle takes place. This process is called ovulation. The egg cell is swept into the Fallopian tube and begins to move toward the uterus. Meanwhile, the follicle is changed into a mass of cells known as the corpus luteum. The LH stimulates the conversion. The corpus luteum then secretes the hormone progesterone, which together with estrogens continues to regulate the buildup of tissue in the endometrium and inhibit contractions of the uterus.
The egg cell remains alive in the Fallopian tubes for 24 to 72 hours. If fertilization by a sperm cell fails to occur, the egg cell moves toward the uterus, and the corpus luteum begins to degenerate. This degeneration causes the level of progesterone and estrogen to drop off. Within two weeks, the hormone level declines to a point where it cannot inhibit contractions of the uterus. Uterine contractions then occur, and the endometrium is released in the process of menstruation. Follicle development begins again, but in the opposite ovary.
The opening at the lower end of the uterus is a constricted area called the cervix. The tube leading from the cervix to the exterior is a muscular organ called the vagina. During periods of sexual arousal, the vagina receives the penis and the semen. The sperm cells in the semen pass through the cervix and uterus into the Fallopian tubes, where fertilization takes place.
In the human female, egg cell production begins before birth, when about 2 million primitive cells known as oogonia accumulate in the ovaries. These oogonia are formed in the early stages of meiosis. After the age of puberty, the oogonia develop into primary oocytes and then into egg cells at a rate of one per month. Egg cell production occurs by the process of meiosis.
The egg cells develop within the ovary in a cluster of cells called the Graafian follicle, which secretes female hormones called estrogens that regulate the development of secondary female characteristics. Egg cell development within the follicle requires approximately 14 days. The development is controlled by two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). Both hormones are secreted by the anterior lobe of the pituitary gland.
During the 14 days of egg cell development, the endometrium increases its supply of blood and nutrients in anticipation of a fertilized egg cell. On about the 14th day, the release of the egg cell from the follicle takes place. This process is called ovulation. The egg cell is swept into the Fallopian tube and begins to move toward the uterus. Meanwhile, the follicle is changed into a mass of cells known as the corpus luteum. The LH stimulates the conversion. The corpus luteum then secretes the hormone progesterone, which together with estrogens continues to regulate the buildup of tissue in the endometrium and inhibit contractions of the uterus.
The egg cell remains alive in the Fallopian tubes for 24 to 72 hours. If fertilization by a sperm cell fails to occur, the egg cell moves toward the uterus, and the corpus luteum begins to degenerate. This degeneration causes the level of progesterone and estrogen to drop off. Within two weeks, the hormone level declines to a point where it cannot inhibit contractions of the uterus. Uterine contractions then occur, and the endometrium is released in the process of menstruation. Follicle development begins again, but in the opposite ovary.
Miosis
Most plant and animal cells are diploid. The term diploid is derived from the Greek diplos, meaning “double” or “two”; the term implies that the cells of plants and animals have two sets of chromosomes. In human cells, for example, 46 chromosomes are organized in 23 pairs. Hence, human cells are diploid in that they have two sets of 23 chromosomes per set.
During sexual reproduction, the sex cells of parent organisms unite with one another and form a fertilized egg cell. In this situation, each sex cell is a gamete. The gametes of human cells are haploid, from the Greek haplos, meaning “single.” This term implies that each gamete contains a single set of chromosomes—23 chromosomes in humans. When the human gametes unite with one another, the original diploid condition of 46 chromosomes is reestablished. Mitosis then brings about the development of the diploid cell into an organism.
The process by which the chromosome number is halved during gamete formation is meiosis. In meiosis, a cell containing the diploid number of chromosomes is converted into four cells, each having the haploid number of chromosomes. In human cells, for instance, a reproductive cell containing 46 chromosomes yields four cells, each with 23 chromosomes.
Meiosis occurs by a series of steps that resemble the steps of mitosis. Two major phases of meiosis occur: meiosis I and meiosis II. During meiosis I, a single cell divides into two. During meiosis II, those two cells each divide again. The same demarcating phases of mitosis take place in meiosis I and meiosis II.
As shown in Figure 1 , first, the chromosomes of a cell duplicate and pass into two cells. The chromosomes of the two cells then separate and pass into four daughter cells. The parent cell has two sets of chromosomes and is diploid, while the daughter cells have a single set of chromosomes each and are haploid. Synapsis and crossing over occur in the Prophase I stage.
Figure 1
The process of meiosis, in which four haploid cells are formed.
The members of each chromosome pair within a cell are called homologous chromosomes. Homologous chromosomes are similar but not identical. They may carry different versions of the same genetic information. For instance, one homologous chromosome may carry the information for blond hair while the other homologous chromosome may carry the information for black hair.
As a cell prepares to enter meiosis, each of its chromosomes has duplicated, as in mitosis. Each chromosome thus consists of two chromatids.
Meiosis I
At the beginning of meiosis 1, a human cell contains 46 chromosomes, or 92 chromatids (the same number as during mitosis). Meiosis I proceeds through the following phases:
*
Prophase I: Prophase I is similar in some ways to prophase in mitosis. The chromatids shorten and thicken and become visible under a microscope. An important difference, however, is that a process called synapsis occurs. A second process called crossing over also takes place during prophase 1.
During prophase 1, the two homologous chromosomes come near each other. Because each homologous chromosome consists of two chromatids, there are actually four chromatids aligned next to one another. This combination of four chromatids is called a tetrad, and the coming together is the process called synapsis.
After synapsis has taken place, the process of crossing over occurs. In this process, segments of DNA from one chromatid in the tetrad pass to another chromatid in the tetrad. These exchanges of chromosomal segments occur in a complex and poorly understood manner. They result in a genetically new chromatid. Crossing over is an important driving force of evolution. After crossing over has taken place, the four chromatids of the tetrad are genetically different from the original four chromatids.
*
Metaphase I: In metaphase I of meiosis, the tetrads align on the equatorial plate (as in mitosis). The centromeres attach to spindle fibers, which extend from the poles of the cell. One centromere attaches per spindle fiber.
*
Anaphase I: In anaphase 1, the homologous chromosomes separate. One homologous chromosome (consisting of two chromatids) moves to one side of the cell, while the other homologous chromosome (consisting of two chromatids) moves to the other side of the cell. The result is that 23 chromosomes (each consisting of two chromatids) move to one pole, and 23 chromosomes (each consisting of two chromatids) move to the other pole. Essentially, the chromosome number of the cell is halved. For this reason the process is a reduction-division.
*
Telophase I: In telophase I of meiosis, the nucleus reorganizes, the chromosomes become chromatin, and a cytoplasmic division into two cells takes place. This process occurs differently in plant and animal cells, just as in mitosis. Each daughter cell (with 23 chromosomes each consisting of two chromatids) then enters interphase, during which there is no duplication of the DNA. The interphase period may be brief or very long, depending on the species of organism.
Meiosis II
Meiosis II is the second major subdivision of meiosis. It occurs in essentially the same way as mitosis. In meiosis II, a cell containing 46 chromatids undergoes division into two cells, each with 23 chromosomes. Meiosis II proceeds through the following phases:
*
Prophase II: Prophase II is similar to the prophase of mitosis. The chromatin material condenses, and each chromosome contains two chromatids attached by the centromere. The 23 chromatid pairs, a total of 46 chromatids, then move to the equatorial plate.
*
Metaphase II: In metaphase II of meiosis, the 23 chromatid pairs gather at the center of the cell prior to separation. This process is identical to metaphase in mitosis.
*
Anaphase II: During anaphase II of meiosis, the centromeres divide, and the 46 chromatids become known as 46 chromosomes. Then the 46 chromosomes separate from one another. Spindle fibers move one chromosome from each pair to one pole of the cell and the other member of the pair to the other pole. In all, 23 chromosomes move to each pole. The forces and attachments that operate in mitosis also operate in anaphase 11.
*
Telophase II: During telophase II, the chromosomes gather at the poles of the cells and become indistinct. Again, they form a mass of chromatin. The nuclear envelope develops, the nucleoli reappear, and the cells undergo cytokinesis as in mitosis.
During meiosis II, each cell containing 46 chromatids yields two cells, each with 23 chromosomes. Originally, there were two cells that underwent meiosis II; therefore, the result of meiosis II is four cells, each with 23 chromosomes. Each of the four cells is haploid; that is, each cell contains a single set of chromosomes.
The 23 chromosomes in the four cells from meiosis are not identical because crossing over has taken place in prophase 1. The crossing over yields variation so that each of the four resulting cells from meiosis differs from the other three. Thus, meiosis provides a mechanism for producing variations in the chromosomes. Also, it accounts for the formation of four haploid cells from a single diploid cell.
During sexual reproduction, the sex cells of parent organisms unite with one another and form a fertilized egg cell. In this situation, each sex cell is a gamete. The gametes of human cells are haploid, from the Greek haplos, meaning “single.” This term implies that each gamete contains a single set of chromosomes—23 chromosomes in humans. When the human gametes unite with one another, the original diploid condition of 46 chromosomes is reestablished. Mitosis then brings about the development of the diploid cell into an organism.
The process by which the chromosome number is halved during gamete formation is meiosis. In meiosis, a cell containing the diploid number of chromosomes is converted into four cells, each having the haploid number of chromosomes. In human cells, for instance, a reproductive cell containing 46 chromosomes yields four cells, each with 23 chromosomes.
Meiosis occurs by a series of steps that resemble the steps of mitosis. Two major phases of meiosis occur: meiosis I and meiosis II. During meiosis I, a single cell divides into two. During meiosis II, those two cells each divide again. The same demarcating phases of mitosis take place in meiosis I and meiosis II.
As shown in Figure 1 , first, the chromosomes of a cell duplicate and pass into two cells. The chromosomes of the two cells then separate and pass into four daughter cells. The parent cell has two sets of chromosomes and is diploid, while the daughter cells have a single set of chromosomes each and are haploid. Synapsis and crossing over occur in the Prophase I stage.
Figure 1
The process of meiosis, in which four haploid cells are formed.
The members of each chromosome pair within a cell are called homologous chromosomes. Homologous chromosomes are similar but not identical. They may carry different versions of the same genetic information. For instance, one homologous chromosome may carry the information for blond hair while the other homologous chromosome may carry the information for black hair.
As a cell prepares to enter meiosis, each of its chromosomes has duplicated, as in mitosis. Each chromosome thus consists of two chromatids.
Meiosis I
At the beginning of meiosis 1, a human cell contains 46 chromosomes, or 92 chromatids (the same number as during mitosis). Meiosis I proceeds through the following phases:
*
Prophase I: Prophase I is similar in some ways to prophase in mitosis. The chromatids shorten and thicken and become visible under a microscope. An important difference, however, is that a process called synapsis occurs. A second process called crossing over also takes place during prophase 1.
During prophase 1, the two homologous chromosomes come near each other. Because each homologous chromosome consists of two chromatids, there are actually four chromatids aligned next to one another. This combination of four chromatids is called a tetrad, and the coming together is the process called synapsis.
After synapsis has taken place, the process of crossing over occurs. In this process, segments of DNA from one chromatid in the tetrad pass to another chromatid in the tetrad. These exchanges of chromosomal segments occur in a complex and poorly understood manner. They result in a genetically new chromatid. Crossing over is an important driving force of evolution. After crossing over has taken place, the four chromatids of the tetrad are genetically different from the original four chromatids.
*
Metaphase I: In metaphase I of meiosis, the tetrads align on the equatorial plate (as in mitosis). The centromeres attach to spindle fibers, which extend from the poles of the cell. One centromere attaches per spindle fiber.
*
Anaphase I: In anaphase 1, the homologous chromosomes separate. One homologous chromosome (consisting of two chromatids) moves to one side of the cell, while the other homologous chromosome (consisting of two chromatids) moves to the other side of the cell. The result is that 23 chromosomes (each consisting of two chromatids) move to one pole, and 23 chromosomes (each consisting of two chromatids) move to the other pole. Essentially, the chromosome number of the cell is halved. For this reason the process is a reduction-division.
*
Telophase I: In telophase I of meiosis, the nucleus reorganizes, the chromosomes become chromatin, and a cytoplasmic division into two cells takes place. This process occurs differently in plant and animal cells, just as in mitosis. Each daughter cell (with 23 chromosomes each consisting of two chromatids) then enters interphase, during which there is no duplication of the DNA. The interphase period may be brief or very long, depending on the species of organism.
Meiosis II
Meiosis II is the second major subdivision of meiosis. It occurs in essentially the same way as mitosis. In meiosis II, a cell containing 46 chromatids undergoes division into two cells, each with 23 chromosomes. Meiosis II proceeds through the following phases:
*
Prophase II: Prophase II is similar to the prophase of mitosis. The chromatin material condenses, and each chromosome contains two chromatids attached by the centromere. The 23 chromatid pairs, a total of 46 chromatids, then move to the equatorial plate.
*
Metaphase II: In metaphase II of meiosis, the 23 chromatid pairs gather at the center of the cell prior to separation. This process is identical to metaphase in mitosis.
*
Anaphase II: During anaphase II of meiosis, the centromeres divide, and the 46 chromatids become known as 46 chromosomes. Then the 46 chromosomes separate from one another. Spindle fibers move one chromosome from each pair to one pole of the cell and the other member of the pair to the other pole. In all, 23 chromosomes move to each pole. The forces and attachments that operate in mitosis also operate in anaphase 11.
*
Telophase II: During telophase II, the chromosomes gather at the poles of the cells and become indistinct. Again, they form a mass of chromatin. The nuclear envelope develops, the nucleoli reappear, and the cells undergo cytokinesis as in mitosis.
During meiosis II, each cell containing 46 chromatids yields two cells, each with 23 chromosomes. Originally, there were two cells that underwent meiosis II; therefore, the result of meiosis II is four cells, each with 23 chromosomes. Each of the four cells is haploid; that is, each cell contains a single set of chromosomes.
The 23 chromosomes in the four cells from meiosis are not identical because crossing over has taken place in prophase 1. The crossing over yields variation so that each of the four resulting cells from meiosis differs from the other three. Thus, meiosis provides a mechanism for producing variations in the chromosomes. Also, it accounts for the formation of four haploid cells from a single diploid cell.
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