Chapter 3: Meiosis, Development & Aging Cells

Human Genetics Chapter 3: Meiosis, Development, and Aging Cells

• Human development is orchestrated by genes from conception to adulthood.
• Gametes (sperm and oocytes) are formed when a male sperm and female oocyte join.
• Reproductive systems have paired gonads, tubular structures for transport, and hormones regulating reproduction.
• Sperm develop in the seminiferous tubules of the testes.
• Sperm mature and are stored in the epididymis.
• Secretions from the prostate gland, seminal vesicles, and bulbourethral glands form the seminal fluid.
• Sperm exit through the urethra and out of the penis.
• Oocytes mature in the ovaries.
• Each month, an ovary releases an oocyte into a uterine tube.
• If fertilized, the oocyte travels to the uterus, divides, and develops.
• If not fertilized, the oocyte and uterine lining are expelled.
• Hormones control oocyte maturation and uterine preparation for fertilization.

Learning Outcomes

• Describe male and female reproductive structures.
• Explain the necessity of meiosis for reproduction.
• Summarize the events of meiosis.
• Detail sperm and oocyte formation steps.
• Describe early prenatal development.
• Compare and contrast embryo and fetus.
• Define "critical period".
• Identify examples of teratogens.
• Describe adult-onset diseases.
• Explain rapid aging syndromes.

Meiosis (1)

• Meiosis is a cell division halving the chromosome number.
• Homologous pairs have the same genes but different alleles.
• Gametes are haploid, somatic cells are diploid.
• Meiosis is vital for genetic diversity and protection from genetically overloaded cells.
• Improper meiosis can lead to genetically overloaded cells.

Meiosis (2)

• Meiosis involves two divisions: Meiosis 1 (reduction division) and Meiosis 2 (equational division).
• Meiosis 1 reduces the chromosome number from 46 to 23.
• Meiosis 2 produces four cells from the two cells produced in meiosis 1.
• Meiosis, similar to mitosis, follows an interphase period where DNA duplication (replication) occurs.
• Each division consists of four phases (prophase, metaphase, anaphase, telophase)

Mitosis vs Meiosis

• Mitosis produces two identical daughter cells, while meiosis produces four genetically different daughter cells.
• Mitosis occurs in somatic cells throughout the life cycle for growth, repair, and asexual reproduction.
• Meiosis occurs in germline cells after sexual maturity, crucial for sexual reproduction and generating gene combinations.

Overview of Meiosis

• Meiosis I is a reduction division, halving the chromosome number.
• Meiosis II is an equational division, maintaining the halved chromosome number in the resulting cells.

Prophase 1

• Spindle formation occurs.
• Homologous chromosomes pair up and cross over.
• Chromosomes condense.
• Synapsed chromosomes separate but remain attached at a few points.

Crossing Over

• Exchange of genetic material between homologous chromosomes occurs.
• Results in unique combinations of alleles.

Metaphase 1

• Homologous pairs align along the cell's equator.
• Random alignment leads to independent assortment of genes.

Anaphase and Telophase 1

• Homologous chromosomes separate to opposite poles.
• Chromosomes decondense.
• Nuclear membranes partially assemble around chromosomes.
• Cytokinesis divides the cell into two cells, each with half the original chromosome number.

Meiosis 2

• Sister chromatids separate and move to opposite poles.

Prophase 2 and Metaphase 2

• Chromosomes condense again and are visible.
• Chromosomes align along the equator of the cell.

Anaphase 2 and Telophase 2

• Centromeres divide.
• Newly formed unreplicated chromosomes move to opposite poles.
• Nuclear membranes reform.
• Cytokinesis divides the cell into four haploid daughter cells.

Results of Meiosis

• Four nonidentical haploid daughter cells.
• Each carries a new gene and chromosome and one copy of the genome.

Gametes Mature

• Meiosis in both sexes has differences in cell component distributions impacting sperm and oocyte.
• Male produces sperm constantly from puberty onward.
• Female has oocytes that mature after puberty but only if fertilized.

Spermatogenesis

• A diploid spermatogonium undergoes mitosis producing a stem cell and another cell specializing into a mature sperm.
• In meiosis 1, primary spermatocytes produce two haploid secondary spermatocytes
• In meiosis 2, secondary spermatocytes produce two equal-sized spermatids, which then mature into spermatozoa (sperm).

Oogenesis

• Begins with a diploid oogonium.
• Meiosis 1 produces a large secondary oocyte and a small polar body.
• After puberty, one oocyte each month continues meiosis 1, but arrest at metaphase 2 until fertilization.
• Completion of meiosis 2 occurs only with fertilization, forming an ovum and additional polar bodies.
• A female ovulates about 400 oocytes before menopause.

Stages of Early Prenatal Development

• Fertilization: Sperm and oocyte unite.
• Cleavage: Rapid cell divisions.
• Morula: Solid ball of cells.
• Blastocyst: Hollow ball of cells with inner cell mass and outer trophoblast.
• Gastrula: Three primary germ layers form.

Embryo Formation

• Primary germ layers (ectoderm, mesoderm, endoderm) form the developing embryo.
• Gastrulation is the process where these layers form in the second week following fertilization.
• Cells in each layer begin forming organs.
• This three-layered structure is the gastrula.
• Epigenetic effects regulate differentiation.

Prenatal Development

• An embryo is the developing human from conception up to eight weeks. Rudiments of all major body parts are present.
• The preimplantation stage occurs during the first week after conception.
• The fetal period begins at week 9 and continues until birth.
• Fetal structures grow and specialize during this period.

Supportive Structures

• Structures to aid and protect the embryo include: chorionic villi, yolk sac, allantois, umbilical cord, and amniotic sac.
• Placenta, formed by chorionic villi, secretes pregnancy-sustaining hormones and nutrient delivery to the fetus.

Multiple Births

• Dizygotic twins arise from two separate fertilized ova.
• Monozygotic twins arise from one fertilized ovum that splits early in development.
• Various types of monozygotic twinning occur based on timing of the division and whether embryonic tissues and membranes (chorion and amnion) share common origins.

Birth Defects

• Critical periods during embryonic development are times when genetic abnormalities, toxic substances, or viruses can alter specific structures.
• Most birth defects arise during the embryonic period.
• Teratogens are harmful agents that cause birth defects.

Fetal Alcohol Syndrome and Other Syndromes

• Fetal Alcohol Syndrome results from maternal alcohol consumption during pregnancy.
• Progeroid syndromes are single-gene disorders that accelerate aging.
• These syndromes can significantly shorten lifespan.

Studying Human Embryos and Fetuses

• Difficulties in studying prenatal human development lead to the use of animal models and donated/created tissues, cells, and embryos.

The Making of Oocytes, Summary

• Oocytes maturation and ovulation are cyclical processes regulated by hormones, controlled by the cycles of the uterus and the overall female reproductive system.

Maturation and Aging, Summary

• Aging entails the biological changes occurring through life.
• Age 30 is a key transition.
• Many diseases related to adulthood or aging have genetic components, while influences from the environment will shape gene expression.

Adult-Onset Inherited Disorders

• Genes can influence health across a lifetime.
• Environmental factors, such as malnutrition, can impact gene expression and later-life health risks.
• Some adult-onset disorders follow recessive patterns from single genes from the prior generations, while some follow a dominant pattern.

Syndromes Resembling Accelerated Aging, Summary

• Genes play dual roles in influencing aging, passively via structural breakdown and actively via stimulating new processes.
• Progeroid syndromes—specifically progerias—are single-gene disorders that accelerate aging, thus shortening life expectancy.
• Causes of premature aging include problems in cellular mechanisms to repair DNA damages.

Longevity Genes, Summary

• Specific genes are associated with enhanced longevity, acting as protectors against diseases like hypertension and cardiovascular issues.
• Genetic variations and environmental factors play combined roles influencing overall aging processes and longevity.
• Longevity genes are studied among people who live to be centennials.