bio lect 10/11

Cell theory

All cellular life consists of one or more cells, cells are the basic organizational and functional units of life, and all cells come from preexisting cells.

Why is cell division important?

It distributes genetic material into daughter cells and reproduces cells; it enables reproduction in unicellular organisms and growth, development, and repair in multicellular organisms.

Genome

The complete set of genetic material in a cell.

Chromosome

A molecule of DNA that carries genetic information.

Chromatin

DNA associated with proteins in a dispersed, less-condensed state, typical when a cell is not actively dividing.

Condensed chromosome

The highly packaged form of DNA and proteins visible during cell division.

Prokaryotic genome organization

Prokaryotes usually have one circular DNA molecule.

Eukaryotic genome organization

Eukaryotes usually have more than one linear DNA molecule.

Haploid (n)

A cell containing one complete set of chromosomes, with one of each chromosome type.

Diploid (2n)

A cell containing two complete chromosome sets, with two homologous copies of each chromosome type.

Homologous chromosomes

A chromosome pair with the same length, structural features, and genes; the two chromosomes may carry different versions of those genes.

Gamete

A reproductive cell that transmits genes to the next generation; gametes are haploid.

Somatic cell

A non-reproductive body cell; human somatic cells are diploid.

Chromosome vs. chromatid

A chromosome is a DNA molecule; after DNA replication, one duplicated chromosome consists of two sister chromatids.

Sister chromatids

Identical copies produced when a chromosome replicates; they remain joined at the centromere until they separate.

Centromere

The region joining sister chromatids in a duplicated chromosome.

Ploidy

How many complete sets of chromosomes a cell contains.

Does DNA replication change ploidy?

No. DNA replication doubles the DNA content and produces sister chromatids but does not change the number of chromosome sets.

Cell cycle

The sequence of growth, DNA replication, preparation, and division that a eukaryotic cell undergoes.

Binary fission

The simpler cell-division process used by prokaryotes.

Interphase

The period between cell divisions when a cell grows, performs normal functions, carries out metabolism, and replicates DNA.

How much of the cell cycle is typically spent in interphase?

At least 90%.

G1 phase

Interphase stage for cell growth, development, and normal cellular functions; many cells spend most of their lives here.

S phase

Interphase stage when DNA is replicated, producing duplicated chromosomes with two sister chromatids each.

G2 phase

Interphase stage of high metabolic activity and preparation for mitosis; DNA remains in chromatin form.

M phase

The shorter cell-cycle phase that includes mitosis and cytokinesis.

Mitosis

Nuclear division that separates duplicated chromosomes into two genetically identical nuclei.

Prophase of mitosis

Chromatin condenses into chromosomes, the nucleus breaks down, and the mitotic spindle forms.

Mitotic spindle

Fibers extending between cell poles that guide chromosome movement during mitosis.

Metaphase of mitosis

Duplicated chromosomes align at the metaphase plate.

Metaphase plate

The location where chromosomes line up during metaphase; it is not a physical structure.

Anaphase of mitosis

Sister chromatids separate and move toward opposite cell poles.

Kinetochore

Protein structure attached to the centromere that helps pull chromosomes during separation.

Telophase of mitosis

Chromosomes decondense and nuclear envelopes reform, producing two nuclei with genetic information matching the parent nucleus.

Cytokinesis

Division of the cytoplasm that separates one parent cell into two daughter cells; it is distinct from mitosis and often overlaps with telophase.

Animal-cell cytokinesis

The cell membrane forms a cleavage furrow that pinches inward until the cell separates into two daughter cells.

Plant-cell cytokinesis

A cell plate forms between the daughter cells and fuses with the existing plasma membrane to separate them.

Order of mitosis

Prophase → metaphase → anaphase → telophase.

Purpose of mitosis

To distribute duplicated chromosomes into two genetically identical nuclei while preserving chromosome-set number.

Heredity

The transmission of traits from one generation to the next.

Variation

Differences among individuals.

Genetics

The study of heredity and hereditary variation.

Asexual reproduction

Reproduction involving one parent, producing offspring that are genetically identical to the parent and one another under the outline's description.

Asexual reproduction in eukaryotes

A diploid parent can produce two diploid offspring, or a haploid parent can produce two haploid offspring.

Advantages of asexual reproduction

It is fast, requires less energy, can produce many offspring, avoids some risks of finding a mate, and preserves existing adaptations without genome dilution.

Sexual reproduction

Reproduction involving the fusion of two gametes to form a zygote.

Fertilization

The fusion of two gametes.

Zygote

The diploid cell formed when two gametes fuse.

Why does sexual reproduction increase genetic variation?

Offspring inherit novel combinations of genes from the gametes, so they are not genetically identical to their parents or one another.

Advantage of genetic variation

A population with greater variation is more likely to contain individuals able to survive environmental change or stress.

Costs of sexual reproduction

It is slower, requires more energy, can be riskier, usually produces fewer offspring, and involves genome dilution.

Why is meiosis necessary for sexual reproduction?

It halves the chromosome-set number in gametes so fertilization does not double chromosome number in every generation.

Meiosis

Reduction division involving two rounds of cell division that turns one diploid cell into four haploid cells.

Reduction division

The reduction of chromosome-set number from diploid to haploid during meiosis I.

What happens before meiosis begins?

During interphase, DNA replicates so each chromosome consists of two sister chromatids.

Human chromosome count entering meiosis

One diploid human cell enters meiosis with 46 duplicated chromosomes and 92 chromatids.

Meiosis I

The first meiotic division, in which homologous chromosomes separate and the chromosome-set number is reduced.

Prophase I

Homologous chromosomes pair by synapsis, forming tetrads; crossing over can exchange DNA between non-sister chromatids.

Synapsis

The pairing of homologous chromosomes during prophase I.

Tetrad

A paired set of homologous chromosomes containing four chromatids in total.

Crossing over

The exchange of DNA between non-sister chromatids of homologous chromosomes during prophase I.

Why does crossing over increase genetic variation?

It creates new combinations of genes on chromosomes.

Metaphase I

Homologous chromosome pairs align as tetrads at the metaphase plate, with the homologs oriented toward opposite poles.

Anaphase I

Homologous chromosomes separate toward opposite poles; sister chromatids remain joined.

Telophase I and cytokinesis

The cell divides into two haploid cells, each containing duplicated chromosomes.

What is the key outcome of meiosis I?

One diploid cell produces two haploid cells; homologous chromosomes separate, reducing ploidy while sister chromatids remain together.

Interkinesis

The usually short interval between meiosis I and meiosis II.

Does DNA replicate during interkinesis?

No. There is no S phase or DNA replication between the two meiotic divisions.

Meiosis II

The second meiotic division, in which sister chromatids separate; its stages resemble mitosis.

Anaphase II

Sister chromatids separate and move to opposite poles.

Final outcome of meiosis

One diploid starting cell produces four haploid cells after meiosis I and meiosis II.

Human chromosome count after meiosis I

Each of the two cells has 23 duplicated chromosomes and 46 chromatids.

Human chromosome count after meiosis II

Each of the four final cells has 23 chromosomes and 23 chromatids.

Chromosome count in a human diploid somatic cell

46 chromosomes arranged as 23 homologous pairs.

Chromosome count in a human haploid cell

23 chromosomes, one from each homologous pair.

Chromosome count during human meiosis

Before replication: 46 chromosomes and 46 chromatids; after replication: 46 chromosomes and 92 chromatids; after meiosis I: 23 chromosomes and 46 chromatids per cell; after meiosis II: 23 chromosomes and 23 chromatids per cell.

What separates during mitotic anaphase?

Sister chromatids separate.

What separates during meiotic anaphase I?

Homologous chromosomes separate; sister chromatids remain joined.

What separates during meiotic anaphase II?

Sister chromatids separate.

Mitosis vs. meiosis: number of divisions

Mitosis has one nuclear division; meiosis has two divisions.

Mitosis vs. meiosis: final products

Mitosis produces two daughter cells; meiosis produces four haploid cells from one diploid starting cell.

Mitosis vs. meiosis: chromosome number

Mitosis preserves the parent's ploidy; meiosis reduces ploidy during meiosis I.

Mitosis vs. meiosis: genetic outcome

Mitosis produces genetically identical nuclei; meiosis produces haploid cells with variation, including variation from crossing over.

Mitosis vs. meiosis: chromosome separation

Mitosis separates sister chromatids; meiosis I separates homologous chromosomes, and meiosis II separates sister chromatids.

Mitosis vs. meiosis: main role

Mitosis supports growth, development, repair, and asexual cell reproduction; meiosis produces haploid cells for sexual reproduction.

Asexual vs. sexual reproduction

Asexual reproduction uses one parent and typically produces genetically identical offspring; sexual reproduction uses gamete fusion and creates genetically varied offspring.

Why does meiosis prevent chromosome number from doubling each generation?

It produces haploid gametes, so fertilization restores the diploid number rather than doubling it.

How does fertilization restore diploidy?

Two haploid gametes fuse, combining their chromosome sets to form a diploid zygote.

What is the difference between synapsis and crossing over?

Synapsis is the pairing of homologous chromosomes; crossing over is the exchange of DNA between non-sister chromatids in the paired chromosomes.

What is the difference between homologous chromosomes and sister chromatids?

Homologous chromosomes carry the same genes and come as a pair; sister chromatids are replicated copies of a single chromosome.

Why does DNA replication not double chromosome number?

The two sister chromatids remain joined and are counted as one duplicated chromosome until they separate.

Why are the products of meiosis haploid?

Homologous chromosomes separate during meiosis I, leaving each resulting cell with one chromosome from each homologous pair.