(27.1) Common Features of the Female & Male Reproductive System

T/F: Reproductive system functions continuously

→ FALSE

• Reproductive system DOES NOT function continuously like organ system

• Does nor become active until puberty

T/F: Many male and female structures are homologous structures, and share common origin

→ TRUE

Name the Primary Sex Organs in Males and Females

• Primary sex organs are the gonads

  • Male → Testes

  • Female → Ovaries

Role of Gonads

Produce two products: gametes (sperm and eggs) and sex hormones

List Accessory Reproductive organs

1. Ducts

2. Glands

3. External genitalia

List Four Tasks shared by the Female and Male Reproductive Organs

1. Form specialized cells for sexual reproduction call gametes

2. Bring gametes from male and female together through sexual intercourse (copulation)

3. Combine genetic information contained within gametes through fertilization

4. Support development of fetus (gestation) and birth of baby (parturition)

Name Gametes in Females and Males

• Ova (eggs)

  • Gametes in females

• Sperm

  • Gametes in males

Define Zygote

Is formed when sperm and egg fuse

T/F: Zygote is first cell of new individual and all body cells arise from it

→ TRUE

Describe the HPG axis

• Production of gametes and sex hormones is regulated by sequence of hormonal events involving hypothalamus, anterior pituitary gland, and testes or ovaries

• Involves interacting hormones:

  1. GnRH

  2. FSH

  3. LH

  4. Testosterone

  5. Inhibin

Describe the Secretion of GnRH, FSH, LH, Testosterone, and Inhibin

1. Gonadotropin-Releasing Hormone

   • Released from hypothalamus reaches anterior pituitary cells via hypophyseal portal system

   • Stimulates testes and ovaries through affecting the release of FSH and LH

2. Follicle-Stimulating Hormone (FSH)

   • Gonadotropins released from anterior pituitary

3. Luteinizing Hormone (LH)

   • Gonadotropins released from anterior pituitary

4. Sex Hormones

   • Testosterone (males) & Estrogen and Progesterone (female) → act at target tissues in body

   • Exert negative feedback on hypothalamus and anterior pituitary

5. Inhibin

   • Released from the gonads of both male and female

   • Exert negative feedback on FSH release from anterior pituitary

Define Puberty

Period of life when reproductive organs grow to adult size and become functional

• Earliest time that reproduction is possible

Describe the Activation of the HPG Axis at puberty

• Before puberty → low levels of circulating levels of steroidal sex hormones are enough to suppress secretion of GnRH by hypothalamus

• As puberty approaches → hypothalamus becomes less sensitive to inhibition by sex hormones

  1. Hypothalamus releases GnRH in pulse like manner

  2. Anterior pituitary releases FSH and LH in response to increased GnRH levels

  3. FSH and LH stimulate gonads to release more sex hormones from gonads

  4. Sex hormones are released until adult pattern of hormone interaction is achieved

Define Meiosis

Unique kind of nuclear division that occurs ONLY in gonads

T/F: Meiosis is same process for males and females

→ TRUE

Role of Meiosis in Reproduction

Sexual reproduction CANNOT be take place without meiosis → Reduces number of chromosomes in gametes by half so zygote does nor end up with 2x as many chromosomes

Explain Sequence of Events in Meiosis

• Meiosis consists of 2 consecutive nuclear divisions referred to as meiosis I and meiosis II

  • DNA replication only occurs once

Name the Four Steps of Meiosis

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

SUMMARY of Sequence of Events in Meiosis

1. Interphase

   • Preceding meiosis, all chromosomes are replicated

2. Meiosis I

   • Homologous pairs separate from each other, reducing number of chromosomes from 2n to n

   • Reduction division of meiosis

3. Meiosis II

   • Sister chromatids separate and each daughter cell gets equal share of chromosomes

   • Equational division of meiosis

Sister Chromatids vs Homologous Pairs of Chromosomes

Sister Chromatids

• Cell’s DNA is replicated during interphase

  • Each chromosome has two sister chromatids that are identical and jointed together by centromere

Homologous Pairs of Chromosomes

• One member of each pair is from father (paternal chromosome) and other is from mother (maternal chromosome)

  • Look alike and carry same genes that code for same traits

Define Nonsister Chromatids

• Within homologous pairs, chromatids of one chromosome are non-sister chromatids to other chromosome in pair

  • Non-sister chromatids exchange DNA with each other in process called crossing over

  • Process INCREASES genetic variability

Define Diploid vs Haploid

Diploid

• Diploid chromosomal number is normal number of chromosomes in most body cells

  • Consists of two sets of chromosomes (one from each parent) and is symbolized as 2n

Haploid

• Haploid chromosomal number, n, is the number of chromosomes in a gamete

  • Gametes contain only one member of each homologous pair, either maternal and paternal

What is human diploid number?

• Humans diploid # is 46

• Diploid cells contain 23 pairs of homologous chromosomes

What is human haploid number?

• Humans haploid chromosomal # is 23

• When sperm and egg fuse, they form a diploid, 2n, zygote

Compare and contrast Meiosis to Mitosis

Meiosis

• DNA replication

  • Occurs during interphase, before meiosis I but nor meiosis II

• # of divisions

  • Two, each consisting of prophase, metaphase, anaphase, and telophase

• Synapsis of homologous chromosomes

  • Occurs during prophase I

• # of daughter cells

  • Four

  • Each haploid (n) cell contains half as many chromosomes as the parent cell and is genetically different from the parent cell

• Roles in the body

  • Produces cells for reproduction (gametes)

  • Introduces genetic variability in gametes

  • Reduces chromosomal number by half so that fertilization can restore the normal diploid chromosome number (2n = 46)

Mitosis

• DNA replication

  • Occurs during interphase, before mitosis begins

• # of divisions

  • One, consisting of prophase, metaphase, anaphase, and telophase

• Synapsis of homologous chromosomes

  • Does not occur

• # of daughter cells

  • Two

  • Each diploid (2n) cell is identical to the parent cell

• Roles in the body

  • Development of multicellular adult from zygote

  • Tissue repair and renewal

  • Ensures constant genetic makeup of all body cells

During cellular division, the replication of DNA occurs __________.

→ before mitosis and meiosis I

T/F: After DNA replication, each individual chromosome becomes a homologous pair.

→ FALSE

• After DNA replication, each chromosome is composed of two sister chromatids.

During prophase I of meiosis,

→ Homologous chromosomes stick together in pairs

During meiosis, segments of nonsister chromatids can trade places. This recombination of maternal and paternal genetic material is a key feature of meiosis. During what phase of meiosis does recombination occur?

→ Prophase I

• Segments of nonsister chromatids trade places during prophase I, resulting in recombination.

During what stage of meiosis do chromosomes align on the spindle equator in homologous pairs?

→ Metaphase I

Why do chiasmata form during meiosis?

→ To form gametes that are genetically unique

• Unlike mitosis, meiosis has the purpose of producing haploid cells that are all genetically unique. Chiasmata form and genetic material is exchanged between chromatids of homologous chromosomes to provide genetic variation in each daughter cell.

Chromosomes can exchange genetic information during a process called "crossing over."  This occurs when homologous chromosomes are lined up in pairs.  When does this happen?

→ Prophase I

• During prophase I, homologous chromosomes align with each other and undergo crossing over

When does the total number of chromosomes get reduced from 46 pairs to 23 individual chromosomes?

→ during meiosis I

Which of the following is a likely result of meiosis?

→ 1 eggs, each with 23 chromosomes

→ 4 sperm, each with 23 chromosomes

What is the function of meiosis?

→ to produce cells with half the chromosome number of the original spermatogonium or oogonium

At what point during meiosis do homologous chromosomes pair up?

→ Prophase I

Which of these meiosis stages is functionally the most similar to mitosis?

→ Meiosis II

• Meiosis II is fundamentally identical to mitosis. Prophase II, anaphase II, metaphase II, and telophase II have the same events as their corresponding stages in mitosis.

T/F: Crossing over is a source of genetic variability seen in meiosis while independent assortment is a source of genetic variability seen in mitosis.

→ FALSE

T/F: A human egg or sperm contains 23 pairs of chromosomes.

→ FALSE