Cells 9:
Body cells are called somatic cells to distinguish them from reproductive (sex) cells. Of course most of the cells in a multicellular organism, like those of humans, are somatic cells. These cells reproduce through cell division (involving the process of mitosis) and result in daughter cells identical in the number of chromosomes and the genetic codes for traits. There are 23 pairs of chromosomes, for a total of 46 in each human somatic cell. Reproduction cells on the other hand, have half as many chromosomes as the parent cell. Reproduction cells are also called gametes. Male gametes are called sperm, and female gametes are called eggs or ova.
Chromosome Numbers (See also: Forming the Zygote on page 3)
Since all humans have 46 chromosomes and our offspring result from the fusing of genetic material from two individual parents, it would make sense to assume that our offspring should have 92 chromosomes (46 from each parent). To ensure that the number of chromosomes in our species is not doubling from generation to generation, gametes (reproduction cells) must have their chromosomes halved. Meiosis is the name of the process which ensures that gametes have half the number of chromosomes as the parent.
The reproductive cells, or gametes, are the only cells that undergo meiosis. Since meiosis cuts the number of chromosomes in half, the process is sometimes called “reduction division”. Each human gamete contains 23 chromosomes – half the genetic material of the parent. Cells containing half the number of chromosomes as the parent are called haploid, and are symbolized with a single n. On the other hand, somatic cells with the full set of chromosomes are called diploid and are symbolized as 2n.
Homologous Chromosomes
Each diploid somatic cell has a corresponding pair of chromosomes which share in common a similar set of gene trait codes. What this means is that chromosomes are paired up so that partneredchromosomes have the same gene trait variations, on the same gene segments, as their partner. This does not mean that the chromosome pairs, called homologous chromosomes, are identical. Homologous chromosomes are similar. For example, a homologous pair may have your mother’s gene variation of eye color (lets say she has blue eyes) on one chromosome, and your dad’s eye color gene code (lets say he has brown eyes) on the other partnered chromosome. When chromosomes are reduced to half of the parent (n), the split initially will occur between the partnered homologous chromosomes. This means, in our example, that some of the resulting gametes (in fact half of them) will have the gene for blue eyes, while the rest will have the gene for brown eyes.
Mitosis vs. Meiosis
Before we cover the stages of meiosis, it is important to realize that many stages of mitosis are similar. In both mitosis and meiosis, sister chromatids (recall that these are identical), must join together (resembling an X). The key difference between the two processes is that when the sister chromatids split in mitosis, the homologous pairs divide in meiosis. The split between chromatids in meiosis does not occur until later.
Review of Mitosis(For more details refer to Cell Division on page 8)
Recall: For most of its life, the cell is in interphase and exists as a working cell. Late in interphase the chromosomes are replicated.
Prior to mitosis the chromosomes have been copied or replicated, and have begun to shorten and thicken. When mitosis begins the sister chromatids join up (each sister with identical genetics) and begin to be pulled to the cells middle by spindle fibers. After lining up in the middle of the cell, the fully formed spindle pulls the sister chromatids apart and towards the poles*. (This marks the first significant difference between mitosis and meiosis).This ensures that identical genetic information is heading towards each opposite end of the cell. After this the cell begins to pinch in and two new nuclear membranes begin to form. (Recall: the process of dividing the cytoplasm into two identical daughter cells is called cytokinesis). When the cell divides, two genetically identical, diploid (2n) daughter cells are formed. (Meiosis on the other hand, will form four haploid (n) daughter cells).
Meiosis
There are two phases of meiosis, meiosis I and meiosis II. In meiosis I, the homologous chromosomes are divided, and then the cell divides for the first time. Since the homologous chromosomes are corresponding rather than identical the daughter cells in meiosis I are not the same. (Some of your gene trait variations are in one cell and some are in the other, but all the information necessary to retain the human genome are in both). In meiosis II the homologous chromosomes (which are still attached to their sister chromatids at the end of meiosis I) are split between the sister chromatids and the final cell division results in four haploid daughter cells (gametes).
The Stages of Meiosis
During interphase, chromosomesof reproductive cells, replicate to form joined sister chromatids (resembling an X). These will shorten, thicken and begin to move to the center of the cell in prophase. In metaphase the sister chromatids will arrange themselves in the center, opposite of their homologous pair (with similar genes), ready to be pulled in the opposite direction (but do not split apart). (*Recall, in mitosis the sister chromatids are all in a line in the center, readyingto be split apart). In anaphase, the homologous chromosomes (each still joined to their sister chromatid) will be pulled by the spindle to the opposite poles of the cell. Telophase will produce a split which results in two daughter cells with corresponding (but not identical) genes. Each of these cell will contain joined sister chromatids. These will need to be split in meiosis II.
Meiosis II is very similar to mitosis in that the joined sister chromatids (Xs) will line up in the center and then separate. The key differences are that meiosis II starts with two cells, each with half the number of sister chromatids, and the sister chromatids, from both cells split, the cells divide into four haploid daughter cells (gametes).
Forming the Zygote: From Haploid back to DiploidFertilization refers to the joining or fusing of two gametes. Recall that offspring result from the fusing of genetic material from two parents. Meiosis halves the number of chromosomes so that each gamete is haploid. When two haploid daughter cells (n) combine during fertilization, the resulting diploid cell contains a complete set of chromosomes (2n).
In humans, 23 chromosomes from the male gamete, called a sperm, combines with 23 chromosomes from the female gamete, called an egg, to form a diploid cell with the complete set of 46 chromosomes. The first cell of fertilization is called a zygote. In other words, a haploid sperm cell and a haploid egg cell combine to form the first diploid cell called a zygote. Once the zygote forms, the new organism will undergo cell division to increase in size.
Variation
You do not look like the person next to you because of variation amongst gene traits. Some noticeable human gene trait variations include, eye color, hair color, hairlines (which can be straight or widow’s peak), earlobes (attached or unattached), little finger shape (bent toward ring finger or straight), thumb shape (curved or straight), and nose shape (straight and small or totally huge and crooked – joke). Scientists refer to the different forms or variations of the same genes as alleles.
The dividing of homologous chromosomes in meiosis results in species variation in the fist place.This is because different gene variations end up in the daughter cells which result from the first (meiosis) cell division. Consider that the division of homologous chromosomes, producing different gametes, occursin each parent. Combine this with the huge variation of genes that exists in our species -from knobby knees to double chins - and it is not hard to see how the the mixing and matching of two parent gametes results in widely different looking offspring, from family to family, throughout the globe.
Dominance
Somewhere on one of your chromosomes there is a gene for hairline, on its paired chromosome there is another gene for hairline. It is possible that one gene in the chromosome pair will have an allele for widow’s peak, while the gene on the other chromosome will have an allele for a straight hairline. In this case one allele will overwrite the other. The victorious allele is called the dominant allele. The allele that is overwritten is called the recessive allele. In other words, the dominant allele will express the trait (and show up in your appearance) while the other will remain hidden (possibly to show up in a later generation). In hairline the dominant allele is for widow’s peak, so if you have a straight line allele on the homologous chromosome, you will not know it, because it will have been overwritten.
Human Karyotype
During metaphase scientists have been able to take magnified picture of individual chromosomes. By matching up pictures of the homologous pairs (they are similar in shape and size as well as gene variations), they have created what is known as the Karyotype – the arrangement of homologous chromosomes from a body cell. The human Karyotype contains 23 pairs of or 46 chromosomes.
Upon close examination of the Karyotype scientists were able to determine that only 22 pairs of chromosomes were homologous – the 23rd pair was of different sizes. These two chromosomes are the gender or sex chromosomes because they determine whether the individual is a male or a female. The two types of sex chromosomes are the X chromosome and the smaller Y chromosome.
Males have an X chromosome and a Y chromosome, and females have two X chromosomes. In other words the 23rd pair of chromosomes for males is XY and for females it is XX. Recall, in meiosis, chromosomes are halved. For males this means that one gamete will have and X chromosomes and the other gamete will have Y chromosome. For females the pair that is divided is XX, so that both gametes have an X chromosome. In other words male gametes (sperm) can contain X chromosomes or Y chromosomes, while female gametes (eggs) always contain X chromosomes.
Zygotes and X and Y Chromosomes
Since female gametes only contain X chromosomes, male gametes always determine gender. If a sperm with Y chromosome fertilizes the egg, the zygote will have and XY chromosome pair and the offspring will be a male. If a sperm with an X chromosome fertilizes the egg, the zygote will have an XX chromosome pair and the offspring will be female.
Unlike females which have 23 pairs of (matching) homologous chromosomes, males only have 22 pairs that are homologous. This is because and XX pair of chromosomes match up, but theXY pair (male pair) does not. Since X and Y chromosomes do not entirely carry matching genes (some of the genetic material on the Y chromosome is missing), males have what is known as sex-linked characteristics. The missing information on the Y chromosome of the XY male determining pair, leads to characteristics in males that are rarely seen in females. These characteristics include baldness, color blindness and the blood clotting disease hemophilia (which can lead to excessive bleeding).
Atypical Meiosis(Not on Final Exam)
Meiosis is a complicated process that happens correctly most of the time, but things do go wrong on occasion. One of the most common errors occurs when the homologous chromosomes do not separate properly. This is called non-disjunction and results in gametes with the wrong number of chromosomes. Instead of a 23 to 23 split, one gamete only receives 22 chromosomes while the other receives 24 chromosomes.
Chromosome Disorders(Not on Final Exam)
If a nondisjunction 22 chromosome gamete fuses with a normal 23 chromosome gamete the resulting zygote will only have 45 chromosomes (instead of 46). If a nondisjunction 24 chromosome gamete, joins with a normal 23 chromosome gamete a 47 chromosome zygote will form. Down syndrome is a fairly common disorder which results from a 47 chromosome zygote.
The 22 chromosome, 24 chromosome split can get quite complicated for male gametes, because sometimes the nondisjunction occurs with the XY chromosome. Instead of dividing between the two gametes, one sperm receives both the XY together (24 chromosomes) while the other receives neither (22 chromosomes). If the nondisjunction 24 chromosome gamete with an XY fertilizes and normal egg, containing a single X chromosome, the resulting zygote will have an XXY chromosome (an extra X). A zygote with XXY will have the disorder known as Klinefelter syndrome. If the gamete with 22 chromosomes (with neither and X nor Y) fertilizes a normal egg, with a single X chromosome, a zygote with a single X chromosome will form (in other words the egg received neither an X nor Y from the sperm). A zygote with a single X chromosome will exhibit Turner syndrome.
A 22, 24 chromosome split can also happen in such a way that one set of female gametes has no X chromosomes (22 total) while the other set has a both X chromosomes (24 total with both XX). In other words, something can go wrong in such a way that the XX does not split up; hence, one set of gametes receives both chromosomes (XX) and the other set has none. If an XX egg is fertilized by X containing sperm, a zygote containing XXX will form. A zygote with an extra X chromosome (XXX) will develop into what is known as a trisomic female.
Some Final Exam Review
DNA to Protein(For more, refer to Review of Protein Synthesis on p.11)
The DNA molecules which make up a chromosome resemble a twisted ladder. Untwisted the base pairs make up the rungs of the ladder. A gene can be thought as a segment of the ladder. In other words, a gene is a segment of DNA.
Most people have a generalized sense of “genes”. For example, you may be tall and lanky (built for speed) while your friend may be stocky and more powerfully built. In other words you and your friend have different traits. The answer to why you and your friend are so different is commonly answered, “It’s in the genes”.
While it is true to state that the difference in characteristics between you and your friend is in the genes, it is also true to state that it is in the proteins as well. Proteins form your body’s structure and perform nearly all the work involved in being alive. So you see; it is how proteins are put together that determines whether you have fast long muscles or more powerful stout muscles. In fact proteins determine all of your traits including the most observable ones like eye color, hair texture, and skin tone.
So, what do genes have to do with proteins? A gene is a code built into DNA molecules which provides instructions for your cells to produce a specific protein.
Building a protein from a gene segment of DNA is a multistep process. Key to this process is the formation of a molecule very similar to DNA called RNA. In fact RNA looks exactly like half a segment of DNA (or half of the ladder). This is because RNA is a copy of one half or one side of the gene segment of DNA.
In order to construct the RNA molecule, the gene segment of DNA must “unzip” (split down the middle). Once split, one half of the DNA ladder is used to construct a copy of the other half. Once constructed, the copied half, now called RNA, must travel out of the nucleus into the cytoplasm.
In the cytoplasm, ribosomes (which are housed in the rough endoplasmic reticulum) have the job of “reading” the RNA code and constructing the protein. The instructions provide ribosomses with the correct sequence of amino acid links (the building blocks of proteins) necessary to assemble the correct protein molecule.
Cell Division
Eventually all cells must divide. Cell division involves two processes; first, mitosis divides up two sets of identical chromosomes, and then cytokinesis splits one cell into two identical daughter cells. But, before this the cell is in interphase.
Most of a cells life is spent as a “working cell”; performing such routines as converting glucose and oxygen into energy, building proteins and ridding itself of waste. This stage is called interphase.
Near the end of interphase, chromosomes will replicate (clone) and join together in preparation for cell division. This marks the beginning of mitosis.
The Stages of Mitosis
Replicated chromosomes shorten and thicken at the end of interphase. These are now called chromatids. At the start of mitosis the each pair of identical chromatids will join at the middle forming an “X” like structure called a sister chromatids.