Asexual Reproduction is reproduction that requires only one parent and produces genetically identical offspring.

Transcription

1 4.2: Sexual Reproduction pg. 169 Asexual Reproduction is reproduction that requires only one parent and produces genetically identical offspring. Sexual Reproduction is reproduction that requires two parents and produces genetically distinct offspring. Figure 4.12 When gametes combine in fertilization, the resulting cell is diploid. Haploid ad Diploid Cells in Sexual Reproduction Gametes are the male or female reproductive cells (haploid). Zygote is a cell formed by the fusion of two gametes. Fertilization occurs in humans, the joining of male and female haploid gametes. Haploid is a cell that contains half the number of chromosomes as the parent cell. Diploid is a cell that contains pairs of homologous chromosomes.

2 During sexual reproduction the male sex gamete (sperm) fuses with the female sex gamete (ovum) to create a zygote. The process of the male and female sex cells fusing is called fertilization. Each of the sex gametes are haploid (23 chromosomes) cells, and when the zygote is formed the cell is now diploid (23 pairs of chromosomes). Sperm = n = 23 chromosomes Ovum = n = 23 chromosomes Human = 2n = 23 pairs of chromosomes (46) Meiosis Producing Haploid Gametes The process that produces gametes with a haploid number of chromosomes is called meiosis. Two key outcomes of Meiosis; 1. Genetic Reduction: Meiosis is a form of cell division that produces daughter cells with half the number of chromosomes of the parent cell. 2. Genetic Recombination: The products of meiosis have different combinations of alleles. Genetic recombination gives rise to offspring that are genetically different from one another and their parents. This greatly increases the genetic variation in a population.

3 Interphase During this stage the cell grows, functions and replicates its DNA. The content of genetic information is now 46 sister pairs of chromatids or 92 individual chromosomes. Phases of Meiosis Meiosis is divided into two stages, Meiosis I and Meiosis II, each having four phases. Each phase is identified by its name and than follow by a roman numeral to identify the stage it occurs in. Stage: Meiosis I: responsible for dividing the genetic material in half (haploid), but each cell will still contain 46 chromosomes. Prophase I: The nuclear membrane disappears, spindle fibres appear and centrosomes migrate to the poles. Homologous align themselves across from each other near the equator. Here a synapsis can occur, where genetic material maybe exchanged, this called Crossing Over. This leads to genetic diversity. Metaphase I: Here the homologous pairs are line up a long the equator. Spindle fibres appear to be attached to the centrosomes of the chromatids. Anaphase I: At this point the spindle fibres pull homologous to opposite poles from the equator. The sister chromatids do not separate at this point. The chromosome number is reduced from 2n (diploid) to 1n (haploid).

4 Telophase I: Homologous chromosomes begin to uncoil and the spindle fibres disappear. Cytokinesis starts and the two (haploid) cells are created, containing 46 chromosomes, and the genetic material is enclosed in a nuclear membrane. Stage: Meiosis II: is responsible for the reduction of the number of chromosomes from 46 chromosomes to 23 chromosomes. Prophase II: The nuclear membrane disappears, spindle fibres appear and centrosomes migrate to the poles. Each chromatid pair aligns themselves along the equator. Metaphase II: Here the chromatid pairs are lined up a long the equator. Spindle fibres appear to be attached to the centrosomes of each chromatid pairs. Anaphase II: At this point the spindle fibres pull chromatid pairs apart and chromosomes travel to opposite poles from the equator. The chromosome number is reduced from 46 to 23 chromosomes. Telophase II: the 23 chromosomes begin to uncoil and the spindle fibres disappear. Cytokinesis starts and the two (haploid) cells are created, and the genetic material is enclosed in a nuclear membrane.

5 Figure 4.13 Meiosis involves two complete cycles of four phases. Notice that each cell contains some chromosomes from the mother (yellow), some chromosomes from the father (blue), and some chromosomes with segments that have been exchanged (yellow and blue). A comparison of Mitosis and Meiosis Mitosis One stage/one division Produces two diploid cells Identical to the parent cell Meiosis Two stages/two divisions Produces four haploid cells Non-identical cells

6 Figure 4.14 Comparing mitosis and meiosis can help to understand key differences between the two processes. Learning Check, questions 7 12, page 172 Gamete Formation in Animals Spermatogenesis is the process of producing male gametes (sperm) in mammals. Spermatogenesis takes place in the testes of the male organism. Spermatogonia cells found in the testes are responsible for sperm production. The process of meiosis creates four haploid cells expressing genetic diversity. These immature sperms cells move to the epididymis of the testes where they mature into fully developed cells. The head region contains the genetic information; the midsection contains a large number of mitochondria to supply energy, and then the tail section, flagellum for locomotion.

7 Oogenesis is the process of producing female gametes (eggs) in mammals. Oogenesis occurs in the ovaries of the female organism. Oogonium cells are diploid, before birth they start meiosis, but only get as far as prophase I. Once the female reaches puberty, one cell once a month will start from prophase I to produce one mature ovum, which is haploid. This is an uneven division between four cells. The one cell that receives the most cytoplasm and nutrients will develop as the mature ovum. The other three cells form the polar body. Meiosis II will not finish unless the ovum is fertilized. Figure 4.15: In spermatogenesis, four haploid sperm cells form from one diploid cell. Multiple Births Multiple births can occur when more then one is release at time and these eggs are fertilized by different sperm. This creates fraternal twins. Identical twining occurs when only one egg is release and fertilized by one sperm, but divides into two separate zygotes. Identical twins are genetically identical to each other.

8 The Importance of Meiosis for Genetic Variation Meiosis is responsible for developing haploid cells that are genetically diverse from each other. Genetic variation is ensured in two ways: - By the creation of gametes that carry different combinations of maternal and paternal chromosomes. This is done by independent assortment. - By the exchange of genetic material between maternal and paternal chromosomes. This is done by crossing over. Independent Assortment Figure 4.18: The diploid cell for this organism has three chromosome pairs. The potential combinations of chromosomes produce eight genetically different gametes (2 3 = 8). In metaphase I the homologous chromosomes are align along the equator. At this point depending one the homologous have aligned different combinations are drawn to opposite poles. The number of possible genetically distinct gametes can be tabulated by using 2 n, where n represents the number of chromosomes pairs in a diploid cell. A human has 23chromosome in its diploid cell = combinations for a human gamete.

9 Crossing Over Crossing over occurs when homologous pairs align beside each other during prophase I. When non-sister chromatid cross over and switches genetic information with its homologue (maternal vs. paternal), new genetic variations are created. Figure 4.19: During synapsis in prophase I, non-sister chromatids cross over and exchange segments of DNA to produce a new combination of genes on a chromosome. Creating Genetic Variation 1. Synapsis (Crossing Over) 2. Independent Assortment 3. Fertilization Errors during Meiosis The two processes that lead to genetic variation during meiosis can also lead to errors. Most gametes do not survive, but in some situations the gamete does survive. If these situation if fertilization occurs, a zygote is produced with a genetic error. There are two situations that can occur that leads to errors; a) change in chromosome structure, and b) change in chromosome number.

10 Errors Caused by Changes in Chromosomes Structure 1) Deletion: a piece of chromosome is deleted. 2) Duplication: a section of a chromosome appears two or three times in a row. 3) Inversion: a section of a chromosome is inverted. 4) Translocation: a segment of one chromosome becomes attached to a different chromosome. Table 4.1: Chromosome Structural Errors Errors Caused by Changes in Chromosome Number This type of error occurs because of non-disjunction, where the homologous pairs fail to separate during prophase I or prophase II. Figure 4.20: Non-disjunction results in gametes with too many or too few chromosomes. Non-disjunction may take place during anaphase I (A) or anaphase II (B).

11 Genetic Disorders Associated with Chromosome Number The have been a number of genetic disorders identified, which have caused by an incorrect number of chromosomes present. Down syndrome is an example of having 3 copies of chromosome number 23. The individual has 47 chromosomes instead of 46. This is a Trisomic situation caused by non-disjunction of chromosome 21 in one of the gametes. This can occur in 1 in 1490 births, at a young age (20 24), to 1 in 106 births at age 40, and 1 in 11 births for woman around 49. Figure 4.21: Many cases of Down syndrome are due to the individual having an extra chromosome 21. Trisomies and Monosomies (see Table 4.2 Chromosomal Abnormalities) Monosomy is the loss of a chromosome as a result of nondisjunction. Turner syndrome involves a missing X chromosome. Trisomy is the gain of an extra chromosome as a result of nondisjunction. This is common for chromosomes 13, 18, and 21, and sex chromosomes. There are not therapies or cures for these disorders.

12 Prenatal Genetic Testing Prenatal testing is testing performed on the fetus, looking for genetic abnormalities. These tests are only performed on pregnant women in high-risk situations, such as; nondisjunction disorders in women over the age of 35, or women with a family history of genetic disorders. Decisions that are made after the test results are received are very difficult and complicated. This results decisions, such as; pregnancy termination and potential discrimination against persons with disabilities. Prenatal Testing Procedures Figure 4.22: In amniocentesis and chorionic villus sampling, chromosome abnormalities, genetic disorders, and certain malformations of the spine and brain are monitored. Section 4.2 Review, questions 1 16, page 181