Chapter 11: Sexual Reproduction and Meiosis

Exam #3

What is sexual reproduction?

Involves the exchange of genetic material (DNA) between gametes or germ cells of two different types, such as egg and sperm.

What is sexual reproduction and how does it differ from somatic cell production?

• Sexual reproduction: exchange of genetic material (DNA) between gametes (egg and sperm) from 2 different parents.

• Gametes are produced by meiosis, not mitosis.

• Somatic cells are produced by mitosis.

• Meiosis differs from mitosis (reduces chromosome number and creates genetic diversity).

What happens during mitosis?

During mitosis, a parent cell divides only once, going through one round of cell division to produce two daughter cells. These daughter cells are genetically identical to each other and to the parent cell. The cells in our body that divide by mitosis are called somatic cells (these daughter cells are somatic cells). Each daughter somatic cell at the end of mitosis has the same number of chromosomes as the parent cell. In mitosis, the number of chromosomes does not change.

What are the key events and outcomes of mitosis?

• One cell division: Parent cell divides once.

• Two daughter cells: Mitosis produces 2 cells.

• Genetically identical: Daughter cells are identical to each other and to the parent cell.

• Somatic cells: Cells that divide by mitosis are called somatic cells.

• Chromosome number stays the same: Each daughter cell has the same number of chromosomes as the parent.

What happens during meiosis?

• A parent cell goes through two rounds of cell division → produces 4 daughter cells (not 2 like in mitosis)

• The 4 daughter cells are genetically different from each other and from the parent cell

• Daughter cells are called gametes (germ cells) → can be egg or sperm, depending on the organism

• Each gamete has half the number of chromosomes as the parent cell

• Chromosome number is reduced by half in meiosis, unlike mitosis where it stays the same

How do mitosis and meiosis differ?

• Number of cell divisions: Mitosis = 1, Meiosis = 2

• Number of daughter cells: Mitosis = 2, Meiosis = 4

• Genetic similarity: Mitosis = identical to parent and each other, Meiosis = genetically different

• Type of cells produced: Mitosis = somatic cells, Meiosis = gametes (egg or sperm)

• Chromosome number: Mitosis = same as parent, Meiosis = half of parent

How are body cells (somatic cells) produced and what is their chromosome number?

• Body cells (somatic cells) are produced via mitosis

• Somatic cells are diploid (2n), meaning they have two sets of chromosomes

• In humans, 2n = 46 chromosomes → each somatic cell has two sets of chromosomes

Body cells (somatic cells) are produced via mitosis and are ______ (2n).

Diploid

What does diploid mean in relation to somatic cells and mitosis?

• Diploid (2n) means a cell has two sets of chromosomes

• Somatic cells are diploid

• During mitosis, somatic cells divide to produce two diploid daughter cells with the same number of chromosomes as the parent

Why does each somatic cell have two sets of chromosomes?

• Each somatic cell is diploid (2n) → has two sets of chromosomes

• One set comes from the mother (maternal) and one set comes from the father (paternal)

• In humans, this means each somatic cell has 46 chromosomes

How are germ cells (gametes) produced and what is their chromosome number?

• Germ cells (gametes), including egg and sperm, are produced via meiosis

• Gametes are haploid (n) → have one set of chromosomes

• In humans, n = 23 chromosomes → egg has 23, sperm has 23

• Gametes are required for sexual reproduction

What are germ-line cells and how do they produce gametes?

• Females have ovaries, males have testes; both contain germ-line cells

• Germ-line cells are diploid (46 chromosomes in humans)

• Germ-line cells undergo meiosis to produce gametes (germ cells) → egg in females, sperm in males

• Eggs and sperm are called gametes

• Diploid germ-line cells (46)→ meiosis → haploid gametes (germ cells)(23).

What is the difference between germ-line cells and germ cells?

• Germ-line cells are diploid (46 chromosomes in humans)

• Germ cells (gametes) are haploid (23 chromosomes in humans)

• Germ cells are derived from germ-line cells

• Do not confuse germ-line cells with germ cells

• Diploid germ-line cells (46) → meiosis → haploid gametes (23).

What happens when egg and sperm fuse during fertilization?

• Fertilization (syngamy): Egg and sperm fuse to form a zygote

• Zygote is diploid (2n) → has two sets of chromosomes

  • One set from mom (egg)

  • One set from dad (sperm)

• Another name for a diploid zygote is a fertilized egg

• Unfertilized egg is haploid (23 chromosomes), fertilized egg is diploid (46 chromosomes in humans)

What happens to a zygote after fertilization?

• The zygote undergoes mitosis to produce an embryo

• The embryo is diploid (46 chromosomes in humans)

• In mitosis, the number of chromosomes stays the same, so if the zygote is diploid, the embryo is also diploid

How does an embryo develop into a baby and then an adult?

• Embryo undergoes mitosis → develops into a baby/fetus (diploid)

• Baby continues to undergo many rounds of mitosis → grows into an adult

• Diploid status is maintained throughout because mitosis does not change chromosome number

What can germ-line cells do in the human life cycle?

• Germ-line cells are diploid (46 chromosomes)

• They can divide by mitosis → produce more germ-line cells

• They can divide by meiosis → produce haploid gametes (egg or sperm)

What happens during interphase and when does it occur?

• Interphase occurs before mitosis and before meiosis I

• Consists of 3 phases: G1, S, G2

  • G1 phase: Cell grows

  • S phase (S = synthesis): Cell replicates DNA

  • G2 phase: Cell continues to grow

What happens between meiosis I and meiosis II, and what are the outcomes of meiosis II?

• Before meiosis II, cells go through a special interphase:

  • No DNA replication occurs

  • Cell grows and duplicates centrosomes

  • DNA replication isn’t needed because each chromosome already has two sister chromatids after meiosis I

• At the end of meiosis II, 4 daughter cells are produced

  • Genetically different from each other and from the parent cell

  • Haploid (n), just like after meiosis I

Why is meiosis called a reduction division?

• In meiosis, the number of chromosomes is reduced by half

• Diploid cells (2n = 46 in humans) produce haploid gametes (n = 23 in humans)

What is meiosis and how does it produce gametes in humans?

• Meiosis: Cell goes through 2 rounds of division → produces 4 daughter cells

• Daughter cells are gametes → can be egg or sperm

• Gametes are derived from germ-line cells in ovaries and testes

• Germ-line cells are diploid (46 chromosomes in humans)

• After meiosis I and II, producing gametes which are haploid (23 chromosomes)

Do meiosis I and meiosis II include the stages PMAT?

Yes, both meiosis I and meiosis II go through the stages:

• Prophase

• Metaphase

• Anaphase

• Telophase

How do you distinguish prophase in mitosis vs meiosis?

• Mitosis: Just prophase (no Roman numeral)

• Meiosis I: Prophase of the first meiosis (Prophase I)

• Meiosis II: Prophase of the second meiosis (Prophase II)

• Note: There is no pro-metaphase in meiosis II or mitosis I

Which phase is present in mitosis but not in meiosis?

• Pro-metaphase is present in mitosis

• Meiosis I and meiosis II do not have pro-metaphase

Explain meiosis in humans, including chromosome and DNA numbers.

Back:

1. Starting cell: Germ-line cell in ovaries or testes, diploid (2n = 46 chromosomes), 46 DNA molecules

2. Interphase before meiosis I: DNA replicates, cell has 46 chromosomes, 92 chromatids, 92 DNA molecules

3. Meiosis I:

   • Produces 2 daughter cells, genetically different from each other and parent

   • Haploid (n = 23 chromosomes), each chromosome still has 2 chromatids → 46 chromatids, 46 DNA molecules

4. Interphase before meiosis II (special):

   • Cell grows, duplicates centrosomes, DNA is NOT replicated

   • Each cell has 23 chromosomes, 46 chromatids, 46 DNA molecules, and 2 centrosomes

5. Meiosis II:

   • Produces 4 daughter cells, genetically different from each other and parent

   • Haploid (n = 23 chromosomes), each chromosome is single → 23 DNA molecules

6. End result: 4 haploid gametes (egg or sperm)

• Key point: Meiosis reduces chromosome number by half (2n → n)

What happens in S phase of interphase?

• During S phase, the cell replicates its DNA in preparation for cell division.

• You start with 2 chromosomes (one maternal, one paternal), and each chromosome has 1 chromatid and 1 DNA molecule.

• At the end of S phase, you still have 2 chromosomes, but now each chromosome has 2 sister chromatids held together by cohesion proteins.

• Each chromatid has its own DNA, so after S phase each chromosome contains 2 DNA molecules.

Key point:
Chromatids double, but the chromosome number stays the same.

What happens during Prophase I of Meiosis I?

Interphase is followed by Meiosis I, which begins with Prophase I.
In Prophase I:

• Mitotic spindle begins to form.

• Nuclear envelope breaks apart.

• Chromosomes become tightly packed and condensed due to condensin proteins.

• Each chromosome consists of two sister chromatids held together by cohesion proteins.

• Homologous chromosomes pair up — this pairing is called synapsis.

• Synapsis forms tetrads (also called bivalents).

• A tetrad = 4 chromatids = 2 homologous chromosomes.

Example: If a cell has 4 chromosomes, then after DNA replication it has 8 chromatids, forming 2 tetrads.

When does the nuclear envelope break in mitosis and meiosis?

• Mitosis: The nuclear envelope breaks in prometaphase.

• Meiosis: The nuclear envelope breaks in Prophase I and Prophase II.

What is a tetrad?

A tetrad is a structure formed during Prophase I of meiosis when two homologous chromosomes pair up, and each chromosome has two sister chromatids.
So, a tetrad consists of 4 chromatids total (2 chromosomes × 2 sister chromatids each).

Key Point: Tetrad = 4 chromatids (2 homologous chromosomes)

How many tetrads are visible in a human germ-line cell during Prophase I?

• Humans have 46 chromosomes → after DNA replication, there are 92 chromatids.

• Each tetrad = 4 chromatids (2 homologous chromosomes × 2 sister chromatids each).

• Calculation: 92 ÷ 4 = 23 tetrads

• Key point: Number of tetrads = number of homologous chromosome pairs = 23 in humans

When does synapsis and tetrad formation occur, and when does it NOT occur?

• Synapsis and tetrad formation are unique to Prophase I of Meiosis I.

• During this stage, homologous chromosomes pair up, forming tetrads.

• You will NOT see synapsis or tetrads in:

  • Prophase of mitosis

  • Meiosis II (Prophase II)

Key Point: Tetrads and synapsis = exclusive to Prophase I of Meiosis I.

What is a tetrad and how are its chromatids held together?

• A tetrad consists of 2 homologous chromosomes (one maternal, one paternal).

• Each chromosome has 2 identical sister chromatids held together by cohesion proteins.

• The maternal and paternal homologous chromosomes, which are not identical, are held together by the synaptonemal complex, a protein layer that stabilizes the homolog pairing.

Key Point: Tetrad = 4 chromatids (2 sister chromatids per homologous chromosome) with cohesion proteins holding sister chromatids and synaptonemal complex holding homologs together.

What is the synaptonemal complex?

• The synaptonemal complex is a layer of proteins that forms between homologous chromosomes during Prophase I of meiosis.

• It holds the maternal and paternal homologous chromosomes together to allow synapsis and crossing over.

Key Point: Synaptonemal complex = protein “zipper” connecting homologous chromosomes during meiosis I.

What is the difference between cohesion proteins and the synaptonemal complex in a tetrad?

• Cohesion proteins: Hold sister chromatids of a single chromosome together within a tetrad.

• Synaptonemal complex: Holds non-sister (homologous) chromatids together, connecting maternal and paternal chromosomes during Prophase I of meiosis.

• Key Point: Cohesion = sister chromatids; Synaptonemal complex = homologous chromosomes.

Why do homologous chromosomes need to be held together during Prophase I of meiosis?

Homologous chromosomes are held together by the synaptonemal complex so that crossing over (exchange of genetic material) can occur between non-sister chromatids.

What is crossing over and why is it important? (Complicated)

• Crossing over is the exchange of DNA between non-identical (non-sister) chromatids of homologous chromosomes during Prophase I of meiosis.

• Homologous chromosomes are held together by the synaptonemal complex, allowing the exchange to occur.

• During crossing over, chromatids physically break and swap DNA segments, then rejoin.

• The site of crossover is called a chiasma (plural: chiasmata).

• Important:

  • Occurs only between non-sister chromatids, not identical sister chromatids.

  • Creates genetic variation — chromatids are no longer identical, and this is why offspring do not look identical to their parents.

  • Example: One chromatid of paternal chromosome 10 may have a piece of maternal chromosome 10 attached after crossing over.

What is crossing over? (simple)

Crossing over is the exchange of DNA between non-identical (non-sister) chromatids of homologous chromosomes during Prophase I of meiosis.

Between which chromatids does crossing over occur?

Crossing over occurs between non-sister chromatids of homologous chromosomes, not between identical sister chromatids.

What protein structure holds homologous chromosomes together to allow crossing over?

The synaptonemal complex holds homologous chromosomes together during Prophase I, allowing crossing over to occur.

What is a chiasma (plural: chiasmata)?

A chiasma is the site where crossing over occurs — where chromatids break and exchange DNA segments.

How does crossing over contribute to genetic variation?

Crossing over swaps DNA between homologous non-sister chromatids, so chromatids are no longer identical. This ensures offspring are genetically different from their parents.

In a tetrad, what happens to a paternal chromatid after crossing over?

A piece of the maternal homologous chromatid may attach to the paternal chromatid, making it genetically unique.

What is the only stage crossing over occurs in meiosis?

Crossing over occurs only during Prophase I of meiosis. You will never see it in mitosis or Meiosis II.

What processes are unique to Prophase I of meiosis?

Synapsis, formation of tetrads, and crossing over are unique to Prophase I. For tetrads to form, homologous chromosomes must come together, and for crossing over to occur, tetrads must be present.

What happens during Metaphase I of meiosis, and how is it different from metaphase in mitosis?

• In Metaphase I of meiosis, tetrads (pairs of homologous chromosomes) align on the metaphase plate.

• Microtubules from one pole attach to the kinetochore of one chromosome in each tetrad, and microtubules from the other pole attach to the kinetochore of the other chromosome.

• In mitosis, individual chromosomes (not tetrads) align on the metaphase plate. (There are no tetrads involved, and microtubules attach to each sister chromatid's kinetochore independently.)

How do kinetochore attachments differ between Metaphase of mitosis and Metaphase I of meiosis I?

In Metaphase of mitosis, kinetochore microtubules are attached to the kinetochores of all chromosomes.
However, in Metaphase I of meiosis, kinetochore microtubules are attached to the kinetochores of only one half of the chromatids.

Why do daughter cells look different from the parent cell after meiosis?

Even if crossing over doesn’t occur, daughter cells are still different because of the random orientation of tetrads on the metaphase plate — this is called independent assortment of chromosomes.

• Crossing over happens in Prophase I.

• Independent assortment happens in Metaphase I.
  Together, these processes create genetic variation, which is why we don’t look identical to our parents.