Genetics Chapter 3

Zygote

First cell that leads to development of a new individual is a fertilized ovum

• Forms when sperm cells from maleand an oocyte from a female joins

Gametes

Sperm and oocytes

• Sex cells

Each reproductive system has:

1. Gonads: paired structured (where sperm and oocytes are made)

2. Tubes that transport these cells

3. Hormones and secretions that control reproduction

Testes (male gonad)

Packed with sperm-producing seminiferous tubules and cells that secrete testosterone

• Formed in abdominal cavity during embryonic development and then descends into scrotum before birth

Accessory Glands

Prostate gland, seminal vesicles, and bulbourethral gland have secretions that all combine into seminal fluid

• Fluid is alkaline and carries sperm

• Maintains sperm viability and motility

• Semen = Sperm + Seminal fluid

Where doessperm mature?

Epididymis

Ovaries (female gonad)

• Fetal development: Oocyte development begins before birth, but pauses during prophase of meiosis 1 at birth

• Puberty: Meiosis resumes once puberty begins

• Ovulation: One secondary oocyte is released from an ovary during each menstrual cycle

Oviduct

Site of fertilization

Uterus

Growth of fetus

Meiosis

• Meiosis 1 (reduction division): reduces the number of replicated chromosomes from 46 to 23

• Meiosis 2 (equational division): produces 4 haploid from the two cells from the first division by splitting the replicated chromosome

Homologous Pairs

Same genes in the same order, BUT different alleles, or variants, of the same gene

Gametes

Haploid (1n)

Somatic cell

Diploid (2n)

Meiosis only occurs in ____ cells.

Gametes

• Creates 4 genetically different cells

• Two divisions

Two major sources of genetic variation in meiosis:

1. Crossing Over

2. Independent assortment of chromosomes

When does crossing over occur?

Prophase I of meiosis.

What happens during Meiosis I?

Homologous chromosomes separate, reducing the chromosome number from diploid to haploid.

What happens during Meiosis II?

Sister chromatids separate, producing four haploid cells.

What happens during prophase?

• Chromosome condenses

• Nuclear envelope breaks down

• Spindle forms

What happens during metaphase?

Chromosomes line up at the middle of the cell.

What happens during anaphase?

Chromosomes/chromatids separate and move toward opposite poles.

What happens during Telophase?

Nuclear envelopes assemble, spindles disappears.

Cytokinesis

Divides cells.

What does Meiosis II lack?

DNA replication.

How many mature sperm come from one primary spermatocyte?

4 mature sperm

Sperm midpiece

Has many mitochondria

• Splits ATP molecules, which releases energy that powers sperm cell to swim inside female reproductive tract

Oogenesis

1. Begins with a diploid oogonium

2. Mitosis: Primary Oocyte (diploid)

3. Meiosis 1: primary oocyte divides unequally forming a small polar body (dies) and a large secondary oocyte

4. Meiosis 2 (if fertilization occurs): secondary oocyte divides to form a second polar body and a mature ovum (egg)

When is the secondary oocyte completing meiosis II?

Fertilization triggers completion of meiosis II.

How many mature ova result from one primary oocyte?

One functional ovum (egg).

When are primary oocytes arrested?

In prophase I of meiosis.

• Oocytes start meiosis 1 (replicate their DNA, pair up their homologous chromosomes, and do crossing over (recombination)) before a female is even born, but they pause partway through and stay paused for years until PUBERTY.

• AFTER Puberty: Meiosis 1 continues in one or several secondary oocytes each month, but stops again at Metaphase 2 (ovulation = when oocyte is released and unfertilized)

• Meiosis is only completed if the ovum is fertilized

Where does fertilization happen?

Uterine (fallopian) tube

• After fertilization, meiosis 2 continues and a zygote with 46 chromosomes is formed

What is the largest Human cell?

Oocyte

• can be seen with naked eye

Fertilization Steps

1. Fusion of Oocyte and sperm

2. Formation of zygote

3. Formation of the blastocyst (fluid filled ball of cells) by cell division (mitosis: blastomere → morula (ball of cells)

4. Inner cell mass will form the cells of the fetus

5. As implantation begins: Trophoblast will form the structures that support the fetus

   during pregnancy

Implantation

Blastocyst embeds into the endometrial lining of the uterus

• Trophoblast: secrete human chorionic gonadotropin (hCG), a hormone that prevents menstruation.

  • Hormone detected in woman’s urine/blood = pregnant

First Trimester

Organ Formation

Second Trimester

Organ Maturation

Third Trimester

Rapid Growth

How long does ovulation last?

12-48 hours

Acrosomal Enzymes

Aid sperm penetrate the protective layers around the oocyte

• Bore through protective layer (zona pellucida)

Capacitation

Capacitation: activates sperm and the oocyte secretes a chemical that attracts sperm

• Chemical and electrical changes in the oocyte surface

  block entry of more sperm

Gastrula

• Three-layered embryo primary germ layer form)

• End of second week

Dizygotic Twins (DZ)

Fraternal twins

• 2 Sperm fertilize 2 oocytes

• Same genetic relationship as any two siblings

Monozygotic Twins (MZ)

Identical twins

• Single fertilized ovum

Types of Identical Twins

1. Identical twins with separate amnions AND and chorions (1/3)

2. Identical twins that share an amnion and chorion (1 percent)

3. Identical twins that share a chorion BUT have separate amnion (2/3)

Conjoined Twins

RARE

• Embryo divides into twins after the point at which the two groups of cells can develop as two individuals

• Between at 13-15

Organogenesis

Transformation of simple three germ layers into distinct organs

Primitive Streak

Third week of prenatal development, a band appears along back of embryo

• First sign of nervous system

• Day 14: implantation completes

• Rise to connective tissue progenitor cells and the notochord

Notochord

Induces sheet of overlying ectoderm to fold into the hollow neural tube

• Neural tube = brain and spinal cord (Central Nervous system)

Week 8 of Gestation

All organs will be present in the newborn have begun to develop

• It is now a fetus

Fetal Period

• Body proportions approach those of a newborn

• Bone replaces softer cartilage

• Nerve and muscle functions become coordinated, fetus moves

SRY Gene

Sex-determining region of the Y

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

Week 12 (3 months)

Sucks thumb, kicks, makes fists and faces, and has the beginnings of teeth

Week 16 (4 months)

Has hair, eyebrows, lashes, nipples, and nails

Week 18

Vocal cords formed

• Fetus makes no sound because it doesn’t breath air

End of Second Trimester (27 weeks)

Woman feels kicks and jabs

• May detect fetal hiccup

Final Trimester

Fetal brain cells link into networks as organs elaborate and grow

• Fat fills out the skin

• Digestive and respiratory system mature last

Birth

Hormonally induced

• Oxytocin: uterine contraction

• Cervical dilation

• Uterine contractions: expel the fetus and the placenta

Critical Period

Time during prenatal development when genetic variants, toxic substances, or viruses can alter a specific structure

• 2/3 of birth defects develop during the embryonic period (3-8 weeks)

  • MORE SEVERE than those that arise during the fetal period

• 
  

Phocomelia

Congenital disorder caused by a mutation that acts at a specific point in prenatal development

• Mutation halts limb development from 3rd to 5th week of the embryonic period (no arms & legs)

Teratogens

Chemicals or other agents that cause birth defects (congenital disorders)

EX:

1. Thalidomide (no limbs)

2. opioids

3. alcohol: Fetal alcohol syndrome (FAS) → small head & flat faces (behavior problems)

4. Nutrient (vitamins): Accutane (vit A) causes spontaneous abortion and hear, nervous and face problems

5. Viral Infections: zika virus (microcephaly: small head), HIV (prematurity, low weight, stillbirth), rubella (congenital disorder & stillbirths)

Aging

Age 30 = turning point for decline

• Human body becomes less efficient by 0.8 percent each year

• Diseases that begin in adulthood have genetic components (COMPLEX)

  • Many years for environmental exposures to alter gene expression

Malnutrition

Powerful environmental influence: create risks that appear later

• Increase of cardiovascular and diabetes later in life

Single-Gene disease

Strike in childhood, tend to be recessive

• Delayed effects of fetal malnutrition

Dominantly Inherited Conditions

Affects health in early to middle adulthood

Progeroid syndromes

single-gene disorders that speed aging-associated changes

• Progerias: shorten life expectancy (SEVERE)

• Inability to adequately repair DNA

Hutchinson-Gilford Progeria Syndrome

Most SEVERE rapid aging condition

• Normal cells divide 50 times before dying

• BUT cells from syndrome die after 10-30 divisions

• Caused by mutation in LMNA (lamin) gene

• Lamin A: called progerin triggers rapid programmed cell death (apoptosis)

  • Telomeres shrink → cell ages quick → nuclear membrane collapse