Human Genetics Part II PDF

Human genetics- Part II Nermin Eissa, Ph.D. College of Health Sciences Abu Dhabi University Fall-2023  Learning Outcomes: Explain the purpose of mitosis. Explain the events that occur in each stage of mitosis. State the purpose of cytokinesis. L...

Human genetics- Part II Nermin Eissa, Ph.D. College of Health Sciences Abu Dhabi University Fall-2023  Learning Outcomes: Explain the purpose of mitosis. Explain the events that occur in each stage of mitosis. State the purpose of cytokinesis. List the stages of meiosis and describe what occurs in each stage. Explain how meiosis increases genetic variation. Differentiate between spermatogenesis and oogenesis with regard to occurrence and the number of functional gametes produced by each process. 2 ©2020 McGraw-Hill Education  Mitosis 2 Mitosis. Creates new cells in the developing embryo, fetus, and child. Responsible for replacement of cells in adults. During mitosis, the cell that divides is called the parent cell, and the new cells are called daughter cells. Referred to as duplication division since the two daughter cells are genetically identical to the parent cell. 3 ©2020 McGraw-Hill Education  The Importance of Mitosis (fresh wound): ©Scott Camazine/ Science Source; (healing wound): ©Edward Kinsman/Science Source 4 ©2020 McGraw-Hill Education  Overview of Mitosis DNA Replication and Division DNA is replicated during the S phase of interphase. At the end of the S phase, each chromosome contains two identical parts, called sister chromatids, held together at a centromere. Access the text alternative for these images 5 ©2020 McGraw-Hill Education  Overview of Mitosis 2 As mitosis begins, the chromosomes condense Following separation during mitosis, each chromatid is called a chromosome. Each daughter cell gets a complete set of chromosomes and is diploid (2n). The daughter cells are genetically identical to each other and to the parent cell. 6 ©2020 McGraw-Hill Education  The Mitotic Spindle 1 Centrosome—the microtubule organizing center of the cell. After they duplicate, they separate and form the poles of the mitotic spindle, where they assemble the microtubules that make up the spindle fibers. The chromosomes are attached to the spindle fibers at their centromeres. Aster—an array of microtubules. Each centrosome contains a pair of centrioles, which consist of short cylinders of microtubules. 7 ©2020 McGraw-Hill Education  Phases of Mitosis Mitosis is divided into phases: prophase, prometaphase, metaphase, anaphase, and telophase. The stages are continuous; one stage flows from the other with no noticeable interruption. 9 ©2020 McGraw-Hill Education  Prophase The centrosomes have duplicated, and move toward opposite ends of the nucleus. Spindle fibers appear. The nuclear envelope begins to fragment. The nucleolus disappears. The chromosomes condense (are now visible). Each is composed of two sister chromatids held together at a centromere. 10 ©2020 McGraw-Hill Education  Stages of Mitosis Access the text alternative for these images 39 ©2020 McGraw-Hill Education (photos) (early prophase, prophase, metaphase, anaphase, telophase): ©Ed Reschke; (prometaphase): ©Michael Abbey/Science Source  Prometaphase Prometaphase. The spindle fibers attach to the centromeres as the chromosomes continue to shorten and thicken. Chromosomes are randomly placed in the nucleus. Metaphase. The metaphase plate is a plane perpendicular to the axis of the spindle and equidistant from the poles. The chromosomes, attached to spindle fibers, line up at the metaphase plate. 12 ©2020 McGraw-Hill Education  Stages of Mitosis Access the text alternative for these images 42 ©2020 McGraw-Hill Education (photos) (early prophase, prophase, metaphase, anaphase, telophase): ©Ed Reschke; (prometaphase): ©Michael Abbey/Science Source  Anaphase Anaphase—centromeres divide. The sister chromatids separate and move toward opposite poles of the spindle. Sister chromatids are now called chromosomes. 14 ©2020 McGraw-Hill Education  Telophase Telophase. Begins when chromosomes arrive at the poles. Chromosomes become indistinct chromatin again. The spindle disappears. The nuclear envelope reappears. The nucleolus reappears. Characterized by the presence of two daughter nuclei. 15 ©2020 McGraw-Hill Education  Cytokinesis Cytokinesis—division of the cytoplasm and organelles. An indentation called a cleavage furrow passes around the circumference of the cell. Actin filaments form a contractile ring; as the ring becomes smaller, the cleavage furrow pinches the cell in half. 16 ©2020 McGraw-Hill Education  Check Your Progress Explain how the chromosome number of the daughter cell compares with the chromosome number of the parent cell following mitosis. List the phases of mitosis and explain what happens during each phase. Describe how the cytoplasm is divided between the daughter cells following mitosis. 17 ©2020 McGraw-Hill Education  Meiosis 2 Meiosis—reduction division. Has two consecutive cell divisions without an interphase in between. Results in four daughter cells, each of which has one of each type of chromosome. The parent cell is diploid (2n); the daughter cells are haploid (n). Introduces genetic variation; each of the daughter cells is genetically different from the parent cell. Possesses new combinations of the genetic material. 18 ©2020 McGraw-Hill Education  Overview of Meiosis At the start of meiosis, the parent cell is diploid (2n), and the chromosomes occur in pairs. The members of a pair are called homologous chromosomes. They look alike and carry genes for the same traits. 19 ©2020 McGraw-Hill Education  Meiosis I Meiosis I and meiosis II—the two cell divisions of meiosis. Prior to meiosis I, DNA has been replicated. Synapsis—homologous chromosomes come together and line up side by side during meiosis I. Keeps the four chromatids close during the first two phases of meiosis I. DNA does not replicate during interkinesis, the time between meiosis I and meiosis II. 20 ©2020 McGraw-Hill Education  Meiosis II During meiosis II, the centromeres divide. The sister chromatids separate and move toward opposite poles of the spindle. Sister chromatids are now called chromosomes. The daughter cells mature into gametes (sperm and egg). Fertilization restores the diploid number of chromosomes in the zygote. 21 ©2020 McGraw-Hill Education  Meiosis I Meiosis II Access the text alternative for these images 22 ©2020 McGraw-Hill Education  Meiosis and Genetic Variation Meiosis ensures that offspring will be diploid and have a combination of genetic characteristics different from that of either parent. Both meiosis I and meiosis II have the same four stages of nuclear division as mitosis. 23 ©2020 McGraw-Hill Education  Prophase I Synapsis occurs; homologous chromosomes line up side by side. Crossing-over—an exchange of genetic material between the nonsister chromatids of the homologous pair. Produces chromatids that are no longer identical. 24 ©2020 McGraw-Hill Education  Metaphase I During metaphase I, the homologous pairs align independently at the equator. 25 ©2020 McGraw-Hill Education  Spermatogenesis and Oogenesis Meiosis is a part of spermatogenesis, the production of sperm in males, and oogenesis, the production of eggs in females. Following meiosis, the daughter cells mature to become the gametes. Is continual in the testes starting at puberty. 300,000 sperm are made per minute; over 400 million per day. Primary spermatocytes—diploid (2n). Divide during meiosis I to form two secondary spermatocytes, which are haploid (n). 26 ©2020 McGraw-Hill Education  Spermatogenesis 2 Secondary spermatocytes divide during meiosis II to produce four spermatids. The chromosomes in secondary spermatocytes are duplicated and consist of two chromatids, whereas those in spermatids consist of only one. Spermatids mature into sperm (spermatozoa). All four daughter cells become sperm. 27 ©2020 McGraw-Hill Education  Oogenesis 1 Ovaries contain many immature follicles, which contain a primary oocyte arrested in prophase I. The primary oocyte, which is diploid (2n), divides during meiosis I into two haploid cells: Begins meiosis II but stops at metaphase II; doesn’t complete it unless a sperm fertilizes it. First polar body—holds discarded chromosomes. 28 ©2020 McGraw-Hill Education  Oogenesis 2 The secondary oocyte (egg) leaves the ovary during ovulation and enters a uterine tube. If it is fertilized, the oocyte is activated to complete the second meiotic division. Following meiosis II, there is one egg and two or possibly three polar bodies. The polar bodies disintegrate, which is a way to discard unnecessary chromosomes while keeping most of the cytoplasm in the egg. 29 ©2020 McGraw-Hill Education  Significance of Meiosis Significance of meiosis. One function is to keep the chromosome number constant from generation to generation. Another is that it results in genetic recombination. Genetic recombination ensures that offspring will be genetically different from each other and their parents. Results from crossing-over and independent alignment of chromosomes. Generates the diversity needed to survive in changing conditions. 30 ©2020 McGraw-Hill Education