LECTURE 6: The Role of Sex in Evolution

What is heredity?

The transmission of traits from one generation to the next through inheritance.

What are gametes?

Reproductive cells, such as sperm and eggs, that transmit genes from parents to offspring.

What is a homologous chromosome pair?

Two corresponding chromosomes, one inherited from each parent, that carry genes for the same characteristics.

What are sister chromatids?

Two identical copies of a chromosome produced during DNA replication and joined together.

What is asexual reproduction?

Reproduction in which one individual passes its genes to offspring without the fusion of gametes.

What is a clone?

An individual that is genetically identical to its parent or other individuals produced from the same parent.

What is sexual reproduction?

Reproduction in which two gametes combine, producing offspring with unique combinations of genes.

What is a major benefit of asexual reproduction?

It can reproduce without finding a mate and avoids some of the energetic costs associated with sexual reproduction.

What is a major cost of asexual reproduction?

Offspring have little genetic variation because they receive their genes from a single parent.

What is a major benefit of sexual reproduction?

It generates genetically diverse offspring through meiosis and fertilization.

What is a major cost of sexual reproduction?

It generally requires more energy and investment in reproduction and involves producing/finding compatible gametes or mates.

Why can genetic variation produced by sexual reproduction be beneficial?

Genetically different offspring provide variation on which natural selection can act, so some individuals may be better suited to environmental conditions than others.

What is a life cycle?

The generation-to-generation sequence of stages in an organism's reproductive history.

What does diploid (2n) mean?

A cell contains two sets of chromosomes, with one set originating from each parent.

What does haploid (n) mean?

A cell contains one set of chromosomes.

What is the human diploid chromosome number?

46 chromosomes, or 2n = 46.

What is the human haploid chromosome number?

23 chromosomes, or n = 23.

What is meiosis?

A form of cell division that reduces chromosome sets from diploid to haploid and ultimately produces four genetically different cells.

Why must meiosis reduce chromosome number?

Gametes must contain half the diploid chromosome number so fertilization can restore the diploid number without chromosome number doubling every generation.

What happens to DNA before meiosis begins?

DNA replicates once during interphase, producing chromosomes made of two sister chromatids.

How many times is DNA replicated during meiosis?

Once, before meiosis I.

How many cell divisions occur during meiosis?

Two: meiosis I and meiosis II.

What is the overall result of meiosis?

One diploid parent cell produces four genetically different haploid cells.

What is the main purpose of meiosis I?

To separate homologous chromosome pairs, reducing the chromosome sets from diploid to haploid.

What happens during prophase I?

Homologous chromosomes pair through synapsis, crossing over occurs between nonsister chromatids, and the spindle begins forming.

What is synapsis?

The pairing of homologous chromosomes during prophase I.

What is crossing over?

The exchange of corresponding DNA segments between nonsister chromatids of homologous chromosomes.

Why is crossing over important?

It creates recombinant chromosomes with new combinations of maternal and paternal DNA, increasing genetic variation.

What happens during metaphase I?

Homologous chromosome pairs line up at the metaphase plate.

Why is metaphase I important for genetic variation?

Each homologous pair can orient independently of the others, producing independent assortment of maternal and paternal chromosomes.

What happens during anaphase I?

Homologous chromosomes separate and move toward opposite poles while sister chromatids remain attached.

What happens during telophase I and cytokinesis?

The cell divides into two haploid cells whose chromosomes are still composed of two sister chromatids.

Why are cells haploid after meiosis I even though each chromosome still has two chromatids?

Each cell has only one chromosome from each homologous pair, so it has only one chromosome set.

What is the main purpose of meiosis II?

To separate sister chromatids.

Does DNA replicate again between meiosis I and meiosis II?

No. DNA is replicated only once before meiosis I.

What happens during prophase II?

A spindle forms in each haploid cell and the chromosomes prepare for another division.

What happens during metaphase II?

Chromosomes line up individually at the metaphase plate.

What happens during anaphase II?

Sister chromatids separate and move toward opposite poles.

What happens during telophase II and cytokinesis?

Nuclei form and the cells divide, producing four genetically different haploid cells.

What is the easiest way to remember the major difference between meiosis I and meiosis II?

Meiosis I separates HOMOLOGOUS chromosomes; meiosis II separates SISTER CHROMATIDS.

How does meiosis reduce a diploid cell to haploid cells?

During meiosis I, homologous chromosome pairs separate so each daughter cell receives only one chromosome from each pair. Meiosis II then separates the sister chromatids without changing the haploid chromosome set.

What is fertilization?

The union of two haploid gametes.

What is a zygote?

The diploid cell produced when two haploid gametes fuse during fertilization.

How do meiosis and fertilization alternate in a sexual life cycle?

Meiosis reduces diploid cells (2n) to haploid cells (n), while fertilization combines two haploid gametes to restore the diploid state (2n).

Why must meiosis and fertilization alternate?

Their alternation maintains the species' chromosome number across generations.

What happens to chromosome number during meiosis and fertilization?

Meiosis halves chromosome sets from 2n → n; fertilization restores them from n + n → 2n.

What are the three major sources of genetic variation in sexual reproduction?

Independent assortment, crossing over, and random fertilization.

What is independent assortment?

The random orientation and distribution of homologous chromosome pairs during meiosis I.

How does independent assortment increase genetic variation?

Each homologous pair independently determines whether the maternal or paternal chromosome enters a gamete, creating many possible chromosome combinations.

How many chromosome combinations can independent assortment produce in human gametes?

2^23, or about 8 million possible chromosome combinations.

How does crossing over increase genetic variation?

Homologous chromosomes exchange DNA between nonsister chromatids, creating chromosomes containing new combinations of maternal and paternal DNA.

What is random fertilization?

Any genetically unique sperm can potentially fertilize any genetically unique egg, producing enormous numbers of possible genetic combinations.

How many possible chromosome combinations can random fertilization produce in humans before even considering crossing over?

About 64 trillion combinations because roughly 8 million possible sperm combinations can combine with roughly 8 million possible egg combinations.

Why are the four cells produced by meiosis genetically different?

Crossing over and independent assortment create different chromosome combinations in the resulting cells.

What is the main purpose of mitosis?

To produce genetically identical cells while maintaining the same chromosome number.

What is the main purpose of meiosis?

To produce genetically diverse haploid cells for sexual reproduction.

How many daughter cells does mitosis produce?

Two.

How many daughter cells does meiosis produce?

Four.

How many divisions occur in mitosis versus meiosis?

Mitosis has one division; meiosis has two divisions.

How does chromosome number change in mitosis versus meiosis?

Mitosis maintains chromosome number, while meiosis reduces chromosome sets from diploid to haploid.

What happens during metaphase of mitosis versus metaphase I of meiosis?

In mitosis, individual chromosomes line up; in meiosis I, homologous chromosome pairs line up.

What separates during mitosis versus meiosis I?

Sister chromatids separate during mitosis, while homologous chromosomes separate during meiosis I.

What separates during meiosis II?

Sister chromatids, similar to what occurs during mitosis.

Does crossing over normally occur during mitosis?

No. Synapsis and crossing over occur during prophase I of meiosis.

What are three processes that occur in meiosis but not mitosis?

Synapsis and crossing over in prophase I, alignment of homologous pairs during metaphase I, and separation of homologous chromosomes during anaphase I.

What is the final genetic difference between mitosis and meiosis?

Mitosis produces two genetically identical cells with the same chromosome number as the parent; meiosis produces four genetically different haploid cells with half the chromosome sets.