unit Three

The male reproductive system

- Sperm cells are manufactured in the seminiferous tubules, which wind tightly within the testes, which descend into the scrotum. The prostate gland, seminal vesicles, and bulbourethral glands add secretions to the sperm cells to form seminal fluid. Sperm mature and are stored in the epididymis and exit through the ductus deferens. The paired ductus deferentia join in the urethra, which transports seminal fluid from the body.

-In a much broader sense, meiosis, as the mechanism of sexual reproduction, provides genetic diversity. - A population of sexually reproducing organisms is made up of individuals with different genotypes and phenotypes. - Genetic diversity may enable a population to survive an environmental challenge. - A population of asexually reproducing organisms, such as bacteria or genetically identical crops, consists of individuals with the same genome sequence. Should a new threat arise, such as an infectious disease that kills only organisms with a certain genotype, then the entire asexual population could be wiped out.

-However, in a sexually reproducing population, individuals that inherited a certain combination of genes might survive. This differential survival of certain genotypes is the basis of evolution by natural selection.

blastomere, morula

1) A day after fertilization, the zygote divides by mitosis, beginning a period of frequent cell division called cleavage. 2) resulting early cells are called blastomeres. 3) When blastomeres form a solid ball of sixteen or more cells, the embryo is called a morula.

Embryo vs fetus

1) A prenatal human is considered an embryo for the first 8 weeks. During this time, rudiments of all body parts form. The embryo in the first week is in a "pre- implantation" stage because it has not yet settled into the uterine lining. 2) Prenatal development after the eighth week is the fetal period, when structures grow and specialize. From the start of the ninth week until birth, the pre- natal human organism is a fetus.

Identical, or monozygotic (MZ), twins descend from a single fertilized ovum and therefore are genetically identi- cal. They are natural clones.

1) About a third of all MZ twins have completely separate chorions and amnions 2) two-thirds share a chorion but have separate amnions. 3) Slightly fewer than 1 percent of MZ twins share both amnion and chorion.

Before birth, a female has a million or so oocytes arrested in prophase I.

1) By puberty, about 400,000 oocytes remain. 2)After puberty, meiosis I continues in one or several oocytes each month, but halts again at metaphase II. 3) In response to specific hormonal cues each month, one ovary releases a secondary oocyte; this event is ovulation. 4) The oocyte drops into a uterine tube, where waving cilia move it toward the uterus. Along the way, if a sperm penetrates the oocyte membrane, then female meiosis completes, and a fertilized ovum forms. 5) If the secondary oocyte is not fertilized, it degenerates and leaves the body in the menstrual flow, meiosis never completed.

Metaphase I

1) Meiosis continues in metaphase I, when the homologs align down the center of the cell. Each member of a homologous pair attaches to a spindle fiber at opposite poles. 2) The pattern in which the chromosomes align during metaphase I generates genetic diversity. For each homologous pair, the pole the maternally or paternally derived member goes to is random. It is a little like the number of different ways that 23 boys and 23 girls can line up in boy-girl pairs. 3)The greater the number of chromosomes, the greater the genetic diversity generated in metaphase I. For two pairs of homologs, four (22) different metaphase alignments are possi- ble. For three pairs of homologs, eight (23) different alignments can occur. Our 23 chromosome pairs can line up in 8,388,608 (223) different ways. This random alignment of chromosomes causes independent assortment of the genes that they carry. Independent assortment means that the fate of a gene on one chromosome is not influenced by a gene on a different chromo- some

occupational hazards

1) Teratogens are present in some workplaces. 2) Researchers note increased rates of spontaneous abortion and children born with birth defects among women who work with textile dyes, lead, certain photographic chemicals, semiconductor materials, mer- cury, and cadmium. 3) Men whose jobs expose them to sustained heat, such as smelter workers, glass manufacturers, and bakers, may produce sperm that can fertilize an oocyte and then cause spontaneous abortion or a birth defect 4) A virus or a toxic chemical carried in semen may also cause a birth defect

Methyl groups

1) The cells that make up the layers of the primordial embryo, called primary germ layers, become "determined," or fated, to develop as specific cell types. 2) The cells specialize by the progressive switching off of the expression of genes important and active in the early embryo as other genes begin to be expressed. 3) Gene expression shuts off when a methyl binds certain genes and proteins. In the early embryo, methyl groups bind the proteins (called histones) that hold the long DNA molecules in a coil. 4) In the later embryo, methyl groups bind to and silence specific sets of genes.

Pronuclei formation

1) Usually only the sperm's head enters the oocyte. 2) Within 12 hours of the sperm's penetration, the ovum's nuclear membrane disappears, and the two sets of chromosomes, called pronuclei, approach one another. Within each pronucleus, DNA replicates. 3) Fertilization completes when the two genetic packages meet and merge, forming the genetic instructions for a new individual. The fertilized ovum is called a zygote.

third week

1) during the 3rd week, a band called the primtivie streak begins to develop on the back side of the embryo 2) some nations desingate day 14 of prenatal development and primitive streak formation as the point where embryo research is banned because the primitive streak is the first sign of a nervous system and at day 14 implantation completes

The reproductive organs are organized similarly in the male and female. Each system has:

1) paired structures, called gonads, where the sperm and oocytes are manufactured; 2)tubular structures that transport these cells; 3) hormones and secretions that control reproduction.

Mitosis vs Meiosis

1)One division VS Two divisions 2) Two daughter cells per cycle VS Four daughter cells per cycle 3) Daughter cells genetically identical VS Daughter cells genetically different 4) Chromosome number of daughter cells same as that of parent cell (2n) VS Chromosome number of daughter cells half that of parent cell (1n) 5) Occurs in somatic cells VS Occurs in germline cells 6) Occurs throughout life cycle VS In humans, completes after sexual maturity 7) Used for growth, repair, and asexual reproduction VS Used for sexual reproduction, producing new gene combinations

About two-thirds of all birth defects arise from a disruption during the embryonic period. More subtle defects, such as learning disabilities, that become noticeable only after infancy, are often caused by interventions during the fetal period.

A disruption in the first trimester might cause severe intellectual disability; in the seventh month of pregnancy, it might cause difficulty in learning to read.

Each sperm cell consists of a tail, body or midpiece, and a head region.

A membrane-covered area on the front end, the acrosome, contains enzymes that help the energy that will propel the sperm cell cell penetrate the protective layers around the oocyte.

corona radiata vs zona pellucida

A sperm first contacts a covering of follicle cells, called the corona radiata, that guards a secondary oocyte. The sperm's acrosome then bursts, releasing enzymes that bore through a protective layer of glycoprotein (the zona pellucida) beneath the corona radiata. Fertilization, or conception, begins when the outer membranes of the sperm and secondary oocyte meet and a pro-ein on the sperm head contacts a different protein on the oocyte.

Implantation

A week after conception, the blastocyst nestles into the uterine lining. This event, called implantation, takes about a week.

Prophase I

After interphase, prophase I (so called because it is the prophase of meiosis I) begins as the replicated chromosomes condense and become visible when stained. A spindle forms. Toward the middle of prophase I, the homologs line up next to one another, gene by gene, in an event called synapsis. A mixture of RNA and protein holds the chromosome pairs together. At this time, the homologs exchange parts, or cross over All four chromatids that comprise each homolo- gous chromosome pair are pressed together as exchanges occur. After crossing over, each homolog bears some genes from each parent. Prior to this, all of the genes on a homolog were derived from one parent. New gene combinations arise from crossing over when the parents carry different alleles. Toward the end of prophase I, the synapsed chromosomes separate but remain attached at a few points along their lengths.

Trophoblast

As implantation begins, the outermost cells of the blastocyst, called the trophoblast, secrete human chorionic gonadotropin (hCG), a hormone that prevents menstruation. This hormone detected in a woman's urine or blood is one sign of pregnancy.

Many birth defects are caused by toxic substances the pregnant woman encounters. These environmentally caused problems will not affect other family members unless they, too, are exposed to the environmental trigger.

Chemicals or other agents that cause birth defects are called teratogens. While it is best to avoid teratogens while pregnant, some women may need to continue to take potentially teratogenic drugs to maintain their own health.

Blastocyst

During cleavage, organelles and molecules from the secondary oocyte's cytoplasm still control cellular activities, but some of the embryo's genes begin to function. The ball of cells hollows out, and its center fills with fluid, creating a blastocyst. Some of the cells form a clump on the inside lining called the inner cell mass. The inner cell mass continues developing, forming the embryo.

Primary spermatocyte

During reduction division (meiosis I), each primary spermatocyte divides, forming two equal-sized haploid cells called secondary spermatocytes.

spermatid

Each spermatid then develops the characteristic sperm tail, or flagellum. The base of the tail has many mitochondria, which will split ATP molecules to release the sperm inside the female reproductive tract. After spermatid differentiation, some of the cytoplasm connecting the cells falls away, leaving mature, tadpole-shaped spermatozoa (singular, spermatozoon), or sperm.

Environmental factors

Environmental factors are also important in longevity. The very-long-lived people of Calabria, an area of southern Italy, for example, eat mostly fruits and vegetables, which may contribute to their excellent health.

adult-onset inherited disorders

Environmental factors can affect how certain genes are expressed before birth in ways that create risks that appear much later. For example, adaptations that enable a fetus to grow despite near-starvation become risk factors for certain common conditions of adulthood, when conserving calories is no longer needed. Such disorders include coronary artery dis- ease, obesity, stroke, hypertension, schizophrenia, and type 2 diabetes mellitus

Most drugs are not teratogens. Whether or not exposure to a particular drug causes birth defects may depend upon a woman's genes.

For example, certain variants of a gene that control the body's use of an amino acid called homocysteine affect whether or not the medication valproic acid causes birth defects. Valproic acid is used to prevent sei- zures and symptoms of bipolar disor- der.

dizygotic twins

Fraternal, or dizygotic (DZ), twins result when two sperm fertilize two oocytes. This can happen if ovulation occurs in two ovaries in the same month, or if two oocytes leave the same ovary and are both fertilized. DZ twins are no more alike than any two siblings, although they share a very early environment in the uterus.

Meiosis

Gametes form from special cells, called germline cells, in a type of cell division called meiosis that halves the chromosome number.

Homeotics

Genes called homeotics control how the embryo develops parts in the right places

homeobox

Homeotic genes include a 180-base-long DNA sequence, called the homeobox, which enables the encoded protein to bind other proteins that turn on sets of other genes, crafting an embryo, section by section. Homeotic genes line up on their chromosomes in the precise order in which they're deployed in development.

Anaphase I and Telophase I

Homologs separate in anaphase I and move to opposite poles by telophase I. These movements establish a haploid set of still-replicated chromosomes at each end of the stretched- out cell. Unlike in mitosis, the centromeres of each homolog in meiosis I remain together. During a second interphase, chromosomes unfold into very thin threads. Proteins are manufactured, but DNA is not replicated a second time. The single DNA replication, followed by the double division of meiosis, halves the chromosome number.

embryonic induction

In a process called embryonic induction, the specialization of one group of cells causes adjacent groups of cells to specialize. gradually, these changes mold the 3 primary germ layers into organs and organ systems

amniocentesis

In amniocentesis, a sample of amniotic fluid is taken and fetal cells in it are examined for biochemical, genetics and chromosomal anomalies

Men introduce single-gene conditions, whereas women originate chromosome imbalances because of different timetables of meiosis.

In females, oocytes exist on the brink of meiosis I for years, and the cells complete meiosis II only if they are fertilized. Mistakes occur when gametes are active, and that is when they are distributing their chromosomes. If a homologous pair doesn't separate, the result could be an oocyte with an extra or missing chromosome.

Meiosis I

In meiosis I, the primary oocyte divides into two cells: a small cell with very little cytoplasm, called a first polar body, and a much larger cell called a secondary oocyte. Each cell is haploid, with the chromosomes in replicated form.

Secondary Spermatocyte

In meiosis II, each secondary spermatocyte divides to yield two equal-sized spermatids.

Meiosis II

In meiosis II, the tiny first polar body may divide to yield two polar bodies of equal size, with unreplicated chromosomes; or the first polar body may decompose. The secondary oocyte, how- ever, divides unequally in meiosis II to produce another small polar body, with unreplicated chromosomes, and the mature egg cell, or ovum, which contains a large volume of cytoplasm.

Anatomical differences between the sexes appear at week 6, after the SRY gene is expressed in males

Male hormones stimulate male reproductive organs and glands to differentiate from existing, indifferent structures. In a female, the indifferent structures of the early embryo develop as female organs and glands, under the control of other genes. - Differences may be noticeable on ultrasound scans by 12 to 15 weeks.

two stages of meiosis

Meiosis is actually two divisions of the genetic material. The first division is called a reduction division (or meiosis I) because it reduces the number of replicated chromosomes from 46 to 23. The second division, called an equational division (or meiosis II), produces four cells from the two cells formed in the first division by splitting the replicated chromosomes.

Blighted Ovum

Most of the cytoplasm among the four meiotic products in the female ends up in only one cell, the ovum. The woman's body absorbs the polar bodies, which normally play no further role in development. Rarely, a sperm fertilizes a polar body. When this happens, the woman's hormones respond as if she is pregnant, but a disorganized clump of cells that is not an embryo grows for a few weeks, and then leaves the woman's body. This event is a type of miscarriage called a "blighted ovum."

older men

Older men are also more likely to produce gametes that have genetic errors, but the "paternal age effect" usually causes dominant single-gene diseases. That is, only one copy of the mutant gene causes the condition.

Older women

Older women are at higher risk of producing oocytes that have an extra or missing chromosome. If fertilized, such oocytes lead to offspring with the chromosomal conditions

The female reproductive system

Oocytes mature in the paired ovaries. Once a month after puberty, an ovary releases one oocyte, which is drawn into a nearby uterine tube. If a sperm fertilizes the oocyte in the uterine tube, the fertilized ovum continues into the uterus, where for 9 months it divides and develops. If the oocyte is not fertilized, the body expels it, along with the built-up uterine lining, as the menstrual flow.

Prophase Two

Prophase II marks the start of the second meiotic division. The chromosomes are again condensed and visible. In metaphase II, the replicated chromosomes align down the center of the cell. In anaphase II, the centromeres part, and the newly formed chromosomes, each now in the unreplicated form, move to opposite poles. In telophase II, nuclear envelopes form around the four nuclei, which then separate into individual cells.

SRY gene

Sex is determined at conception, when a sperm bearing an X or Y chromosome meets an oocyte, which always carries an X chromosome. An individual with two X chromosomes is female, and one with X and a Y is a male. A gene on the Y chromosome, called SRY (for "sex-determining region of the Y"), determines maleness.

Meiosis in the male has built-in protections that help prevent sperm from causing some birth defects.

Spermatogonia that are exposed to toxins tend to be so damaged that they never mature into sperm. More mature sperm cells exposed to toxins are often so damaged that they cannot swim

In contrast to oocytes, sperm develop in only 74 days. "Paternal age effect" conditions arise from stem cells in the testis that divide every 16 days, from puberty on, offering many opportunities for DNA replication to make a mistake, generating a dominant mutation.

Such paternal age effect conditions include a form of dwarfism; premature fusion of an infant's skull bones; several birth defect syndromes..etc

Sperm and egg cell

The first cell that leads to development of a new individual forms when a sperm from a male and an oocyte (also called an egg) from a female join. Sperm and oocytes are gametes, or sex cells. They mix genetic material from past generations.

epigenetics

The pattern of methyl group binding that guides the specialization of the embryo establishes the epigenome. "Epigenetics" refers to changes to DNA other than changes in DNA nucleotide base sequence.

capacitation

The woman's body helps sperm reach an oocyte. A process in the female called capacitation chemically activates sperm, and the oocyte secretes a chemical that attracts sperm. Contractions of the female's muscles and moving of sperm tails propel the sperm.

yolk sac

The yolk sac manufactures blood cells, as does the allantois, a membrane surrounding the embryo that gives rise to the umbilical blood vessles. The umbilical cord forms around these vessels and attaches to the center of the placenta

During the second week of prenatal development, a space called the amniotic cavity forms between the inner cell mass and the outer cells anchored to the uterine lining

Then the inner cell mass flattens into a two-layered embryonic disc. The layer nearest the amniotic cavity is the ectoderm layer, closer to the blastocyst cavity, is the endoderm. Shortly after, a third layer, the mesoderm, forms in the middle. This three-layered structure is called the primordial embryo, or the gastrula

age 60-85

While the environment seems to play an important role in the deaths of people ages 60 to 85, past that age, genetic effects predominate. That is, someone who dies at age 68 of lung cancer can probably blame a lifetime of cigarette smoking. But a smoker who dies at age 101 of the same disease probably had gene variants that protected against lung cancer.

Without Meiosis what would happen...

Without meiosis, the sperm and oocyte would each contain 46 chromosomes, and the fertilized ovum would have twice the normal number of chromosomes, or 92. Such a genetically overloaded cell, called a polyploid, usually does not develop far enough to be born.

sirtuins

a compo-nent of red wine called resveratrol is a member of a class of enzymes called sirtuins that regulate energy use in cells by altering the expression of certain sets of genes. Through their effect on energy metabolism, the sirtuins seem to prevent or delay several diseases that are more common in the aged, such as heart disease and neurodegenerative disorders.

Week 3

by the third week of conception, fingerlike projections called chorionic villi extend from the area of the embyonic disk close to the uterine wall. The villi project into pools of the womans blood. Her blood and the embryo's blood are seperate but nutrients and oxygen diffuse across the chorionic villi from her circulation to the embryo.

Certain nutrients ingested in large amounts, particularly vita- mins, act as drugs. The acne medicine isotretinoin (Accutane) is a vitamin A derivative that causes spontaneous abortion and defects of the heart, nervous system, and face in exposed embryos.

excess vitamin C can harm a fetus that becomes accus- tomed to the large amounts the woman takes. After birth, when the vitamin supply suddenly plummets, the baby may develop symptoms of vitamin C deficiency (scurvy), such as bruising and becoming infected easily.

chemical in cigarrete smoke stress a fetus. Carbon monoxide crosses the placenta and prevents the fetus's hemoglobin molecules from binding oxyegn.

smoking during pregenancy increases the risk of abortion and still birth

Spermatogenesis

the formation of sperm cells, begins in a diploid stem cell called a spermatogonium. This cell divides mitotically, yielding two daughter cells. One continues to specialize into a mature sperm. The other daughter cell remains a stem cell, able to self-renew and continually produce more sperm.

notorhocord and neural tube

the notochord is a structre that forms the basic framework of the skeleton. The notochord causes the overlying ecotderm to fold to form the hollow neural tube which develops the brain and spinal cord

critical period

the time when genetic abnormalities, toxic substances, or viruses can alter a specific structure is its critical period

organogenesis

transformation of 3 primary germ layers into distinct organs

An isolated population

where people have children with relatives creates a situation in which mutations are overrepresented, but the genomes are relatively uniform, making gene variants easier to identify than in more outbred populations.