1.Explain the structure of chromosomes

DNA (deoxyribonucleic acid) is a long, thin molecule that contains the information needed to direct a cell’s activities and to determine a cell’s characteristics. This vast amount of vital information encoded in DNA is organized into genes. A gene is a segment of DNA that transmits information from parent to offspring. A single molecule of DNA has thousands of genes, which are linked up like the railroad cars of a train. When genes are being used, the strand of DNA is extended, enabling other molecules to retrieve its information. However, when a cell prepares to divide, the DNA molecule coils and twists into a dense structure called a chromosome. A chromosome is a rod-shaped structure that forms when a single DNA molecule and its associated proteins coil tightly before cell division. If DNA did not coil and form a chromosome, a single DNA strand would be about 5 centimeters (approximately 2 inches) long. This is too long to fit inside a cell. But when the thread of DNA is coiled around a protein scaffold, it can be compacted into a smaller, more manageable structure. Your chromosomes are approximately 40% DNA and 60% protein. Chromosomes become visible through a microscope only after they have condensed prior to cell division. By this time, each chromosome are called chromatids. Chromatids form prior to cell division when the DNA molecule duplicates itself, ensuring that each new cell will have the same genetic information as the old cell. The two chromatids are attached by a protein disk at a point called a centromere. Human cells have 23 different chromosomes. Your body cells (also called somatic cells) contain two copies of each chromosome, for a total of 46 chromosomes. The two copies of each chromosome are called homologous chromosomes, or homologues. Homologous chromosomes are similar in shape and size and have similar genetic information. You received one homologue of each chromosome from your mother and the other homologue from your father. When a cell contains two homologues of each chromosome, it is termed diploid. Biologists use the symbol 2n to represent the diploid number of chromosomes in a cell. For humans, 2n=46. Not all cells are diploid. In the life cycle of animals, including humans, gametes—egg cells and sperm cells – are haploid. A haploid cell contains only one homologue of each chromosome. The fusion of two haploid gametes forms a diploid zygote. A zygote is a fertilized egg cell, the first cell of a new individual. Being haploid ensures that when an egg and a sperm fuse, the resulting zygote will contain the characteristic diploid number of chromosomes for that organism. Biologists use the symbol n to represent the haploid number of chromosomes. For humans, n=23.

2. Explain what consequences occur to humans when chromosomes structure or numbers are changed. Use at least five real examples.

Each of the 46 human characteristics has thousands of genes that play important roles in determining how a person’s body develops and functions. All these genes must be present in an individual’s cells for the same reason that a car must have an engine, a transmission, and wheels—to function properly. Therefore, a person must have the characteristic number of chromosomes in his or her cells. In most cases, humans who are missing even one chromosome do not survive embryonic development. The condition in which a diploid cell is missing a chromosome is called monosomy. And just as a car will not function correctly with two engines stuffed under the hood, a human embryo will not develop properly with more that two copied of most chromosomes. The condition in which a diploid cell has an extra chromosome is called trisomy. Deviations in chromosome number can be detected by analyzing a karyotype, the collection of chromosomes found in an individual’s cells. The traits produced by having an extra copy of chromosome 21 were first described in 1866 by the British physician J. Langdon Down and are collectively celled Down syndrome, or trisomy 21 syndrome. The features of that characterize Down syndrome include a short stature, a round face with upper eyelids that cover the inner corners of the eyes, and, most significantly varying degrees of mental retardation. Down syndrome occurs in all racial groups with the same frequency, approximately 1 in 1,000 children. It is much younger than 30 years old, the incidence is only about 1 in 1,500 births, while in mothers 30 to 35 years old, the incidence doubles to 1 in 750 births. In mothers older than 45, the risk is as high as 1 in 16 births. The reason that more babies with Down syndrome are born to older mothers is that all the eggs a female will ever produce are present in her ovaries when she is born. As the female ages, the eggs can accumulate an increasing amount of damage; males, in contrast, produce new sperm throughout adult life. What events cause an individual to have an extra copy of a chromosome? When a cell divides normally, each chromosome and its homologue separate, and event called disjunction. When normal disjunction does not occur, one or more chromosomes may fail to separate properly. This accident in chromosome separation is called nondisjunction, which results in one new cell receiving both chromosomes and the other new cell receiving none. Trisomics arise as a result of nondisjunction. In the case of Down syndrome, nondisjunction occurs with chromosome 21.

3.Explain the following:

A .Polyploidy

B. Nondisjunction

C. Translocation

D. Point Mutations

1. Deletions
2. Duplications
3. Inversions
4. Frame Shift Mutations

E. Crossing-over

1. Polyploidy, an increase in the number of chromosome beyond the typical diploid (2n) complement, is a well-documented mechanism by which new species are produced. Polyploidy may arise as a result of nondisjunction during mitosis or meiosis, or may be generated when the chromosomes divide properly during mitosis or meiosis but cytokinesis does not subsequently occur. Polyploidy individuals can be produces deliberately in the laboratory by the use of the drug colchicine, which prevents separation of chromosomes during mitosis. Polyploidy leading to the formation of new species sometimes occurs as a result of doubling of the chromosome number within a species, a process known as autopolyploidy. More frequently, however, new species are generated by a doubling of the chromosome number in hybrid organisms, a process known as allopolyploidy.
2. What events cause an individual to have an extra copy of a chromosome? When a cell divides normally, each chromosome and its homologue separate, an event called disjunction. When normal disjunction does not occur, one or more chromosomes may fail to separate properly. This accident in chromosome separation is called nondisjunction, which results in one new cell receiving both chromosome and the other new cell receiving none. Trisomics arise as a result of nondisjunction. In the case of Down syndrome, nondisjunction occurs with chromosome 21.
3. A translocation occurs when a deleted portion of one chromosome is transferred to and becomes part of another, non-homologous chromosome.
4. Although rare, changes in an organism’s chromosome structure do occur. Some alternations cause mutations, changes in an organism’s genetic material.

1. When a fragment of a chromosome breaks off, it can be lost when a cell divides, causing a mutation called a deletion. As a result of deletion, a new cell will lack a certain set of genes.
2. In a mutation called a duplication, the chromosome fragment attaches to its homologous chromosome, which will then carry two copies of a certain set of genes.
3. Sometimes the fragment reattaches to the original chromosome in the reverse orientation, producing a mutation called an inversion.
4. In a Frame-Shift mutation, a nucleotide is knocked out of the nucleotide sequence. Every nucleotide sequence and every nucleotide after where the mutation occurs if affected. A frame-shift mutation sometimes results in a miscarriage. These can be deadly if they are at the beginning of the sequence in which case you have big trouble.

E. In the beginning of meiosis I, homologous chromosomes pair up next to each other. While paired, the arms of the chromosomes exchange reciprocal segments of DNA in a process called crossing-over. Crossing-over is an efficient way to produce genetic recombination, the formation of new combinations of genes. As a result of crossing-over, the two chromatids of a chromosome no longer contain identical genetic material. Crossing-over thus provides a source of genetic variation. Since the speed at which a species can change is often limited by the amount of genetic variation available, crossing-over has an enormous impact on how rapidly organisms evolve.

4. How is sex determined in most animals? Are there any exceptions?

Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are the same in males and females. These chromosomes are called autosomes. The chromosomes that differ between males and females are called the sex chromosomes because they carry the genes that determine an individual’s sex. Sex chromosomes exists in either of two forms – as an X chromosome or as a sorter Y chromosome. In humans and many other organisms, the genes that cause a fertilized egg into a male are located on the Y chromosome. Thus, any individual with a Y chromosome is a male, and any individual without a Y chromosome is a female. In these cases, females are designated XX because they have two X chromosomes, and males are designated XY because they have one X chromosome and one Y chromosome. Because a female can donate only an X chromosome to her offspring, the sex of an offspring is determined by the male, who can donate either an X or a Y.

In some insects, such as grasshoppers, there is no Y chromosome. In such cases, the females are characterized as XX and the males as XO ( the O indicates the absence of a chromosome). In birds, moths, and butterflies, the male has two X chromosomes and the female only one.

The primary difference between Mitosis and Meiosis is that Mitosis is caused by asexual reproduction and thus the offspring or the two new cells formed will be identical, however, Meiosis is caused by sexual reproduction and the offspring receives half of the genetic information of one of the parents and half of the mother, thus none of the organisms are identical. Meiosis is in the ovaries and testes and mitosis is in every cell. Meiosis II occurs in each cell formed during meiosis 1 and is not preceded by DNA replication. The phases of mitosis and meiosis are described below:

Before a cell can begin mitosis and actually divide, it must replicate its DNA, synthesize more of the histones and other proteins associated with the DNA in the chromosomes, produce a supply of organelles adequate for two daughter cells, and assemble the structures needed to carry out mitosis and cytokinesis. These preparatory processes occur during the G₁, S, and G₂ phases of the cell cycle, which are known collectively as interphase. During interphase preceding meiosis, the chromosomes are replicated, so that by the beginning of meiosis each chromosome consists of two identical sister chromatids held together at the centromere region. The first of the two nuclear divisions in meiosis then proceeds through the stages of prophase, metaphase, anaphase, and telophase (all of these are given the designation I to indicate that they are sub-stages of meiosis I). Meiosis II resembles mitosis except that it is not preceded by replication of the chromosomal material. A short interphase may occur, during which the chromosomes partially unfold, but meiosis in many species proceeds from telophase directly to prophase II. In mitosis you have to replicate DNA, synthesize histones and proteins associated with DNA, provide a supply of organelles for two daughter cells, and assemble the structures to carry out mitosis and cytokinesis, in meiosis you only need to replicate the chromosomes partly because meiosis does not have cytokinesis. A third interphase occurs at the end of meiosis. Mitosis has only two interphase's whereas meiosis has three.

In meiosis, DNA strands coil, shorten and thicken and are referred to as chromosomes. As in the prophase of Mitosis, spindle fibers appear. Then the nuclear membrane and the nucleus disappear. These are all the same as in Mitosis. A second step which does not take place during mitosis occurs: every chromosome lines up to its homologue. This pairing of chromosomes is called synapses. These homologous chromosomes twist around each other to form a tetrad, which is a group of two chromosomes. The word "tetrad" means four and refers to the four chromatids that compose the two chromosomes. As tetrads from, portions of chromatids may be exchanged , either between the two homologues or between sister chromatids. This phenomenon is called crossing over and results in exchange of genes. Crossing over is important in Prophase of Meiosis and happens and is a lot more likely to happen. As in Meiosis in Prophase during Mitosis the nucleus and nuclear membrane disappear and centrioles move away from one another going to the opposite sides of the cell to extend spindle fibers from centriole to centriole and from centromeres to centrioles. These control the chromosome movement. Asters are also formed during this process which is very similar to that of Meiosis. Mitosis doesn’t have much crossing over. Prophase II of Meiosis, the chromosomes of some organisms, especially plants, coil again having uncoiled between Telophase 1 and Prophase II. New spindle fibers form. Prophase II shares many commonalties with Prophase of mitosis except that, especially in animals, chromosomes are already coiled.

In Meiosis, the tetrads are moved by the spindle fibers to the equator of the cell. Where homologous pairs of chromosomes remain together. In mitosis, the chromosomes, instead of tetrads, line up on the equator of the cell held by the kinetochore fibers. Metaphase can be characterized as the arrangement of all chromosomes along the equator of the cell. Metaphase II of Meiosis ,in both phases (Metaphase of Mitosis and Metaphase II) the chromosomes are moved to the equator, however, in metaphase II the chromosomes are in sister pairs and joined by a centromere while in the Mitosis chromosomes are not in sister pairs.

In meiosis, the homologous pairs of chromosomes separate, as in the anaphase phase in mitosis , one chromosome of each pair is pulled by action of the spindle fibers to one pole of the cell, and the other is pulled to the opposite pole. Each chromosome is still composed of two chromatids joined by a centromere. Anaphase is the third stage of Mitosis. Unlike the Anaphase 1 in Meiosis the centromeres of each pairs of chromatids divide. The chromosomes then move to the opposite sides of the cell very rapidly. In Anaphase II of Meiosis, the centromeres divide the sister chromatids and move them to the opposite poles of the cell. Same division of centromeres and movement of chromatids happens in Anaphase of Mitosis.

In mitosis, the cytoplasm divides forming two daughter cells. The parent cell has produced two daughter cells now, each with one member of each pair of homologous chromosomes. Chromosome number has been halved but each chromosome, having been replicated earlier has twice the original amount of DNA. This explains why there are two phases in Meiosis 1 and 2 the process is almost complete, but there is twice the original DNA, so the cell must divide again to half the original amount or eventually after many generations the cell would burst because of too much DNA. Telophase I of Meiosis does not share anything in common with the Telophase of Mitosis. Telophase in Mitosis, unlike in Meiosis where the opposite sister chromatids go to each cell in Mitosis during telophase two identical sets of chromatids are clustered at the opposite sides of the cell. As in Meiosis the spindle fibers dissolve and a nuclear membrane and nucleus appear. Unlike in Meiosis there are only two cells formed from the parent cell and their genetic make up is identical. In Telophase II, in Meiosis, the spindle fibers dissolve and a nuclear membrane forms around the chromosomes in each daughter cell. Meiosis II is now complete and cytokinesis occurs. The two nuclear divisions result in four daughter cells from a single parent cell, each with half the number of chromosomes of each parent.

In mitosis, during cytokinesis the cytoplasm of the two cells divides and forms two new cells. Each of the cells houses nuclei formed during Mitosis. Other structures are separated between the two cells, such as ribosome’s, golgi bodies and mitochondria, The two new cells are generally equal in size. Meiosis does not contain cytokinesis unlike mitosis, but instead has Meiosis II.

6. Explain the relationship of all of the above has to evolution and YOUR life.

Entire basis of evolutionary theory lies in reproduction. Crossing-over mixes up the genes and makes a new combination. Recombination is the mixing up of genes and mutations are the basis of change. Mutations go on every second. Every fruit is made good by genetics and mutations. Fruit is mutated to be bigger and better. Evolution causes change and diversity. Grafting is a type of cloning by which fruit is grown. Genetic Engineering is on the rise. In theory cloning humans would be like cloning plants but there are many unsolved processes and legal and ethical issues involving cloning. Crossing over has a great significance in heredity. Some paired chromosomes don’t carry identical information because of crossing over. In a female, 400 eggs are produced per lifetime. Selective breeding improves crops and is a process which is ongoing to feed the population. If you had Mayan corn, it only has about 7-8 grains on it and is tiny, modern corn is made by genetic mutations as are other fruits which you eat. Everybody has mutated and evolved and is still mutating at a very slow rate. If you didn’t have selective breeding you would have tiny fruit which could be bad, rotten, etc.