The Human Genome and Meiosis

Transcription

1 The Human Genome and Meiosis The evolution of sexual reproduction was very important to eukaryote biology. For the first time, organisms produced offspring that were significantly different from themselves. This increased the genetic variation in sexual species, resulting in organisms that were better able to respond to evolutionary pressure. Succinctly, parents could now have offspring with a better chance of surviving than they do. Sexual reproduction introduced two unique situations. First, since offspring inherit a set of genetic information from both parents, sexual organisms have two copies of each chromosome (a maternal and paternal copy). Second, since two parents combine to produce one offspring, the parents must use a specialized form of nuclear reproduction that cuts their amount of genetic material in half (so the maternal and paternal parents each contribute half a genome, both combining into the offspring s full genome). The human genome At the individual level, a genome is all of the genetic information found in a cell. This is mainly the DNA found in the chromosomes in the nucleus, though mitochondrial DNA does contribute. With the completion of the Human Genome Project in 2003, scientists now know the sequence of all 3 billion nitrogenous bases (A, T, C, and G) from the first position of chromosome 1 to the last base on the Y chromosome. Decades of genetic and molecular biology research, combined with computer analysis, have allowed scientists to begin a detailed analysis of the human genome. The haploid human chromosomal genome is composed of 2.85 billion base pairs of DNA. i While this is a lot of DNA (the DNA in each of your cells is over two meters long), much of the DNA appears to be non-coding. Only about 1% of the DNA encode products like enzymes or proteins, or is used directly as RNA (like rrna, trna, or ribozymes). Another 0.5% of DNA is involved in controlling the expression of these genes (such as serving as promoters, terminators, and other control mechanisms). We know that 3.5% more DNA is evolutionarily constrained, or cannot withstand severe mutations we just do not know what it does. ii The remaining 95% is sometimes referred to as junk DNA. It is important to understand that this DNA is not useless. Examples of vital junk DNA that we do know play important roles in the cell would be the ori, the telomeres, and the centromere. Humans contain only 20,000-25,000 genes. This number can be misleading. Remember that the alternate splicing theory says that one gene can actually make a number of different proteins, so the actual number of proteins encoded by these genes is greater than the number of genes. Chromosome Duplicated Chromosome Figure 1. Chromosome structure The human genome is distributed over 46 chromosomes, in 23 homologous pairs. Recall that chromosomes are long strands of DNA tightly wrapped around proteins (e.g., the spool-like histones). Because we inherit a complete set of DNA from each parent, we actually have two copies of each chromosome. The maternal and paternal chromosomes make up a Figure 2. Homologous pairs

2 homologous pair. It is important to understand that although the chromosomes in a homologous pair contain the same genes in the same order, the maternal and paternal copies of each chromosome are not identical. The chromosome from each parent has a unique evolutionary history, and their own unique combination of mutations. Forty-four of the chromosomes are autosomes, chromosomes with true homologous pairs. The final pair is the sex chromosomes, the X and the Y chromosome. The X chromosome is incorrectly referred to as the female chromosome. It is actually one of the largest chromosomes (with 1669 genes) iii, and contains genes involved in such vital systems as blood clotting, eyesight, and bone formation. You cannot survive without at least one X chromosome. Healthy human females are XX. The Y chromosome is one of the smallest chromosomes (with only 426 genes) iv and is not essential for survival that should be obvious, as half the human population lacks a Y chromosome all together. This chromosome converts a female human fetus into a male fetus. Healthy males are XY. As seen above, males have only one X chromosome and females have two. Females do not use both X chromosomes, however. Females randomly compact and inactivate one of their X chromosomes. They only express the genes on the other, active X chromosome. The compacted, useless X is called a Barr Body, and is visible as a dark dot in the nucleus of female cells. Barr bodies form early in the development of the embryo, about 4 days after fertilization v. At this time, each cell in the developing female randomly selects one of the two X chromosomes, and disregards the other. This leads to the unique coat patterns in American calico and European tortoiseshell cats. In these cats, one X contains a gene for black hair pigment, the other X a mutant form of the gene that encodes orange hair. Each cell randomly selects one of these chromosomes, and then continues to divide to grow into the adult cat. The result is a cat with splotches of orange and black fur, and each cat is unique because in different cats different combinations of cells select either the black or the orange allele. (Note: in American calicos, a third mutation in different gene introduced splotches of white fur as well.) Figure 3. X inactivation and tortoiseshell cats All of the chromosomes in an organism can be visualized through a karyotype. This test is most often performed on fetuses developing in the womb. A human fetus floats in a sac of amniotic fluid, which cushions it. As it is bathed in the amniotic fluid, fetal cells are washed off and become suspended in the fluid. Some of the amniotic fluid is drawn out in a syringe and the fetal cells are collected and grown in culture. The fetal cells are given a drug (colchicin) which freezes the cells in metaphase of mitosis, with the chromosomes clearly visible. The cells are then blown open, spraying out the chromosomes. The chromosomes are photographed and analyzed. To analyze a karyotype: (1) check the gender of the child by looking at the sex chromosomes, (2) check that each chromosome is in a pair (no singles or triples), and (3) check that each chromosome in the pair is the same length (no missing or extra pieces). Figure 4. Karyotype of a healthy human male

3 Meiosis Eukaryotes that reproduce sexually have two copies of each chromosome (homologous pairs). These organisms are called diploid (di = two, for two chromosomes), which is abbreviated 2n. Humans are diploid, and since they have 46 chromosomes they have a diploid number of 2n=46. All somatic (body) cells in a human are diploid. To reproduce, organisms must create gametes or germ cells, special reproductive cells that have only one copy of each chromosome. Gametes are haploid, and are abbreviated n. Human egg and sperm cells are haploid, with only one copy of each chromosome, so n=23. Two haploid germ cells fuse together to make one diploid organism (n+n=2n, in humans 23+23=46 chromosomes). In order to make reproductive cells, organisms go through meiosis. Meiosis is a specialized form of nuclear reproduction where the homologous pairs of chromosomes are separated, creating haploid cells. Meiosis evolved from mitosis: it uses the same machinery and stages as mitosis, though to split the homologous pairs the nucleus must divide twice. Before the homologous pairs are separated and the chromosome number cut in half, something very significant happens in meiosis. The maternal and paternal DNA is recombined into new combinations. This ensures that the daughter cells of meiosis are genetically very different from the parent. To make meiosis truly annoying, the parent cell proceeds through the cell cycle and duplicates its DNA in S phase. Each chromosome is copied, forming two sister chromatids (exactly as you learned in mitosis). As a result, since each chromosome in the homologous pair is copied, we start out dealing with tetrads, or groups of four: two copies of the maternal chromosome and two copies of the paternal chromosome. Meiosis consists of two rounds of cell division: prophase I, metaphase I, anaphase I, telophase I; then prophase II, metaphase II, anaphase II, and telophase II. In meiosis I, the cells become haploid the homologous pairs are separated. In Meiosis II, the duplicate sister chromatids are separated. Note that in humans meiosis occurs only at certain times and in certain cells. Only the gonads (testes or ovaries) can perform meiosis (all other cells in the body reproduce through mitosis). Females go though meiosis I as fetuses and then meiosis II after puberty. Males can only undergo meiosis after puberty. Stages of Meiosis Prophase I As in mitosis, the nuclear membrane disappears and the centrioles begin moving to opposite sides of the cell as the growing spindle fibers push them apart. Specific to meiosis, the tetrads adhere together tightly, a process called synapsis. This leads to crossing over: a very important event in meiosis. In crossing over, the maternal and paternal chromosomes swap pieces. This leads to completely new chromosomes that are genetically unique mixtures of the original pair. Figure 5. Prophase I

4 Metaphase I The homologous chromosome pairs are moved to the middle of the cell. Recall that in mitosis, the chromosomes were NOT in their homologous pairs: each chromosome operated independently. Anaphase I The homologous pairs separate and begin moving to opposite sides of the cell. The cell is becoming haploid: there will be only one version of the chromosome in each cell. Remember that the chromosomes were duplicated during S phase, which is why the chromosomes are still composed of the X-shaped sister chromatids. Note that due to crossing over, the chromosomes are now mixtures of the original maternal and paternal chromosomes. Telophase I and cytokinesis The nuclei reform and the cell splits via the contractile ring in cytokinesis. What happens next depends on the gender of the human. Males will immediately proceed into meiosis II to form four haploid sperm cells. For the production of egg cells, females proceed to this point as fetuses, and then stop in a special state called interkinesis for about a decade. They will not proceed to meiosis II until puberty. Meiosis II Meiosis II proceeds exactly as mitosis. Note that both of the cells from meiosis I will divide, yielding four daughter cells. Note that when the sister chromatids separate, the resulting cells are haploid with half the number of chromosomes as the original cell. Just as important, these cells are all genetically different. This is due to both crossing over and a concept called independent assortment, which you will learn more about in the genetics unit. Figure 6. Metaphase I Figure 7. Anaphase I Figure 8. Telophase I & cytokinesis Figure 9. Stages of meiosis II

5 Comparing Mitosis and Meiosis It is important that you be able to compare and contrast mitosis and meiosis, and be able to tell from diagrams of dividing cells which process is occurring. You will see questions like this on every major test, such as the MCAS and (more importantly) the SAT subject tests. In mitosis, the duplicate chromosomes line up singly in the middle of the cell during metaphase. There is only one round of division, resulting in two daughter cells. The cells are identical. In meiosis, there are two rounds of cellular division. During prophase I, synapsis leads to crossing over. In metaphase I, homologous pairs line up. Cells in meiosis II have half the number of duplicated chromosomes as the original cell. The results are four genetically unique cells. Problems with Meiosis: Nondisjunction & Chromosomal Anomalies Sometimes errors can occur in meiosis. In nondisjunction, either the homologous chromosomes pair or the sister chromatids fail to separate. This results in a gamete with one extra chromosome, and a gamete with one less. When these fertilize a normal gamete, it leads to an embryo with extra or missing chromosomes. Some examples: Trisomy 21 (three copies of chromosome 21) leads to Down syndrome, a condition characterized by a number of signs, such as mental retardation and changes in facial structure. Turners Syndrome (X-) is seen in females with only one X chromosome. They never reach puberty, and are infertile. Klinefelters Syndrome (XXY) is seen in males with more than one X. They are infertile, and sometimes show female traits. Metafemales (XXX+) are seen in females with more than two X chromosomes. They are healthy and fertile. XYY males are a controversial group. While healthy and fertile, some groups argue they are prone to violence. There is scant research to support this. Figure 10. Nondisjunction You should be able to identify these conditions from a karyotype. Figure 11. Karyotype showing trisomy 21 Figure 12. Karyotype showing Turners syndrome Figure 13. Karyotype showing Klinefelters syndrome i International Human Genome Sequencing Consortium*, Nature 431, 932 (2004) ii Blaxter, M. Science 330, 1758 (2010). iii NCBI website, verified January 2011 iv NCBI website, verified January 2011 v Molecular Cell Biology 6 th, Lodish et al.