Chapter 3 - Meiosis, Development, and Aging PDF

Chapter 3 Meiosis, Development, and Aging CHAPTER OVERVIEW This chapter covers human development from meiosis to aging, in the context of gene activity, anatomy and physiology, and environmental influences. Genetic conditions may affect individuals at any st...

Chapter 3 Meiosis, Development, and Aging CHAPTER OVERVIEW This chapter covers human development from meiosis to aging, in the context of gene activity, anatomy and physiology, and environmental influences. Genetic conditions may affect individuals at any stage of existence. Sexual reproduction (meiosis and fertilization) maintains the diploid chromosome number and recombines alleles from generation to generation, protecting against environmental change at the population and species levels. The developing human is vulnerable to teratogens and detrimental environmental agents, which can lead to birth defects. Genome studies are revealing the genetic basis of longevity. CHAPTER OUTLINE Sperm and oocytes are the male and female sex cells, or gametes, respectively. 3.1 The Reproductive System 1. Genes govern growth and development from fertilization. Mutations can impact health at any stage. 2. The male and female reproductive systems have paired gonads that produce the gametes, tubes to transport these cells, and glands whose secretions enable the cells to function. The Male 1. Sperm develop in the seminiferous tubules, which wind inside the testes. 2. Sperm mature and collect in each epididymis, which lead from each testis into the ductus deferentia. These tubes join at the urethra in the penis. 3. The prostate gland, seminal vesicles, and bulbourethral glands contribute secretions to semen. 4. About 200 million to 600 million mature sperm are discharged per ejaculation. The Female 1. Oocytes develop in the ovaries. 2. Each month, one oocyte is released from an ovarian follicle and is captured by fingerlike projections of a uterine tube. 3. Each tube leads to the uterus, which nurtures a fertilized ovum, called a zygote. 4. The lower end of the uterus narrows to form the cervix, which opens into the vagina. 5. Hormones control oocyte development and release, as well as uterine preparation. 3.2 Meiosis 1. Meiosis is a form of cell division that produces haploid (1n) gametes from diploid (2n) germline cells. Copyright 2021 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 2. Meiosis conserves chromosome number and generates genetic variability through two stages: Meiosis I and meiosis II. 3. Each meiotic division proceeds through prophase, metaphase, anaphase, and telophase. 4. Reduction division (meiosis I) halves the chromosome number. 5. Equational division (meiosis II) mitotically divides each of the two cells from meiosis I, yielding four haploid cells. 6. Chromosome number is halved in two cell divisions but only one DNA replication. 7. Crossing over (during prophase I) and independent assortment (due to random alignment of homologous pairs of chromosomes on the equator during metaphase I) generate genetic diversity. 8. In an individual, more than 8 million combinations of the 23 chromosome pairs are possible. 9. More than 70 trillion combinations are possible when a sperm fertilizes an oocyte. 3.3 Gametes Mature Sperm Form 1. In spermatogenesis, a diploid spermatogonium divides mitotically, yielding a stem cell and a primary spermatocyte. 2. In meiosis I, each primary spermatocyte halves its genetic material to form two haploid secondary spermatocytes. 3. In meiosis II, each secondary spermatocyte divides, yielding two equal-sized spermatids. 4. Spermatids mature into spermatozoa that have the characteristic sperm tail. 5. A mature sperm has a tail, body or midpiece, and head region with an acrosome on the front that contains enzymes that digest the protective layers around an oocyte. 6. Many sperm that carry mutations or are damaged do not swim well and have a disadvantage in fertilizing an oocyte. Oocytes Form 1. In oogenesis, a diploid oogonium accumulates cytoplasm and replicates its chromosomes, becoming a primary oocyte. 2. In meiosis I, the primary oocyte divides, forming a small polar body and a large, haploid secondary oocyte. 3. In meiosis II, the secondary oocyte divides, forming another small polar body and a mature ovum. 4. The million or more oocytes that females are born with arrest at prophase I. At ovulation, meiosis continues and is completed after the secondary oocyte is fertilized. If fertilization does not occur, the secondary oocyte degenerates and leaves the body in the menstrual flow. 5. Only about 400,000 oocytes survive past puberty. Of these only about 400 oocytes will be ovulated during the reproductive life of the woman. Meiosis and Mutations 1. The fact that gametes of older people are more likely to harbor new mutations reflects events of meiosis. Copyright 2021 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 2. New mutations in females are likely to affect entire chromosomes because meiosis is arrested between stages. 3. New mutations in males are more likely to affect single genes because errors occur during DNA replication. The meiotic timetable for males is much faster. 3.4 Prenatal Development A prenatal human is an embryo for the first 8 weeks, when beginnings of all organs form. It is a fetus for the remainder of development, when organs grow and elaborate. Sperm and Oocyte Meet at Fertilization 1. Intercourse deposits sperm in the vagina. A sperm cell can survive there up to 6 days, but the oocyte can be fertilized within 12 to 24 hours of ovulation. 2. In a woman's body, sperm are capacitated and chemically attracted to the oocyte. 3. When a sperm cell meets an oocyte, its acrosome bursts and releases enzymes that cut through the oocyte’s protective layer. 4. The sperm's penetration of the oocyte triggers chemical and electrical changes in the oocyte's surface that block entry of other sperm. 5. The two sets of chromosomes (pronuclei) meet and merge, forming a zygote. The Embryo Cleaves and Implants 1. The zygote divides mitotically a day after fertilization, beginning cleavage. A solid morula forms after several cell divisions, which hollows to form a blastocyst. 2. The outermost cells (trophoblast) secrete human chorionic gonadotropin (hCG), which prevents menstruation. A collection of cells on the inside face, the inner cell mass, will develop into the embryo. The blastocyst implants in the uterine lining. 3. hCG in a woman’s blood or urine indicates pregnancy. The Embryo Forms 1. During week 2 of pregnancy the amniotic cavity forms. 2. Then, the primary germ layers (ectoderm, endoderm, and mesoderm) form. 3. The growing structure is now termed a gastrula or primordial embryo. 4. Cells of the primary germ layers are fated to develop into specific body parts. 5. Epigenetic changes include methyl groups binding to DNA at certain points, guiding the gene expression that lies behind development. 6. Homeotic genes control how the embryo develops parts in the correct places. Copyright 2021 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. Supportive Structures Form 1. Chorionic villi develop during week 3 and extend toward the woman's bloodstream, facilitating diffusion of nutrients and oxygen to the embryo and removal of its wastes. 2. The placenta forms by 10 weeks and connects the woman to the fetus. It secretes hormones that alter the woman's metabolism to send nutrients to the fetus. 3. The yolk sac and allantois manufacture blood cells and the umbilical cord forms. The amniotic sac expands with fluid that cushions the embryo. 4. Amniocentesis and chorionic villus sampling can check fetal chromosomes early in development. 5. The umbilical cord yields pluripotent stem cells for research and medical therapy. Multiples 1. Monozygotic twins result from splitting of one fertilized ovum. 2. Dizygotic twins result from two fertilized ova. 3. Conjoined twins arise when two individuals share tissues or organs. The Embryo Develops 1. Organogenesis occurs as cells of the germ layers develop into distinct organs. 2. During week 3, the primitive streak appears, which gives rise to the notochord, which induces formation of the neural tube in neighboring ectoderm. If the neural tube does not completely close, a neural tube defect develops. 3. The heart forms and beats, limbs extend, blood forms, and lungs and kidneys begin to develop. The central nervous system develops from the notochord. 4. By week 8, all organs have begun to develop. The prenatal human is now a fetus. The Fetus Grows 1. The fetus begins to resemble a newborn as structures grow, specialize, and interact. 2. Bone replaces cartilage in the skeleton and sex organs become distinct. 3. During the first trimester, the fetus displays neuromuscular activity such as sucking its thumb, breathing, and kicking. 4. The second trimester fetus curls into a head-to-knee position and moves. 5. In the final trimester, the fetus grows rapidly. Fat fills out the skin. The digestive and respiratory systems mature last. 6. The baby is ready to be born at around 266 days after fertilization. 3.5 Birth Defects A birth defect that results from a mutation can be passed to future generations, but an environmentally caused birth defect cannot. The Critical Period 1. The critical period is when a prenatal structure is sensitive to damage by a faulty gene or environmental insult. 2. Most birth defects originate in the embryo stage, and are generally more severe than problems that arise later in pregnancy. Copyright 2021 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 3. Teratogens are chemicals or agents that cause birth defects, such as alcohol, nicotine, excesses of certain nutrients, malnutrition, occupational hazards, and infectious agents. Teratogens 1. Gene variants can affect whether or not an environmental exposure is teratogenic. 2. Thalidomide affects the development of limb buds in the early embryo similar to the inherited condition phocomelia. 3. Exposure to chemicals in cigarettes can cause growth deficiencies and miscarriage. 4. Alcohol exposure can result in poor growth and intellectual disability. 5. Excess of nutrients such as vitamin A can affect development. 6. Occupational hazards such as radiation and toxins can cause birth defects. 7. Viruses can harm a fetus or newborn. 3.6 Maturation and Aging 1. Aging is progression through the life cycle. 2. Cellular and bodily functions decline with age. 3. Diseases that begin later in life tend to be complex (caused by genes and environmental factors) because the role of the environment becomes increasingly important. Adult-Onset Inherited Disorders 1. Aging is genetically controlled and occurs throughout life as cells die. 2. Intrauterine growth retardation predisposes the individual for specific health problems in adulthood. 3. Single gene recessive disorders generally strike early in life. 4. Dominantly inherited disorders may not begin until early to middle adulthood. Disorders That Resemble Accelerated Aging 1. The rare rapid aging syndromes help to understand the genetics of normal aging. 2. The progeroid syndromes result from faulty DNA repair, which enables mutations to persist. 3. Werner syndrome has an adult onset usually apparent by age 20. Death usually occurs before 50. 4. Hutchinson-Gilford progeria syndrome is caused by a single DNA base change in the gene for lamin A, which generates a form of the protein progerin that touches chromatin through the nuclear membrane. The contact stimulates changes that accelerate aging. Genes and Longevity 1. About 20% of centenarians do not get the diseases that kill most people; 40% get common disorders at much older ages; and 40% live with and survive the common disorders. 2. Environmental causes of death predominate from ages 60 to 85, but after that genetic effects predominate. Copyright 2021 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC. 3. Blood relatives of long-lived individuals tend to live longer too. 4. Centenarians inherit directly protective gene variants and wild type alleles of genes that cause disease when mutant. 5. Researchers have identified more than 130 genes that have variants that affect length of life. These affect immunity, glucose metabolism, stress response, the cell cycle, DNA repair, metabolism, and response to oxidative damage. 6. An isolated population in Ecuador has many individuals with Laron syndrome. They are very long-lived and do not develop conditions typical of aging. 7. Several projects are sequencing and analyzing the genomes of long-lived individuals. Gene expression studies reveal that the cells of centenarians are more adept at autophagy—cleaning up debris. Copyright 2021 © McGraw Hill LLC. All rights reserved. No reproduction or distribution without the prior written consent of McGraw Hill LLC.