Cell Division

Why do cells divide?

Growth/development, tissue repair, and reproduction.

What can happen if cell division is not properly regulated?

Conditions such as cancer or psoriasis.

Put the cell cycle stages in order.

G1 → S → G2 → M.

What happens during G1?

Cell growth plus accumulation of proteins, building blocks, and energy reserves.

What happens during S phase?

DNA is replicated, producing duplicated chromosomes.

What happens during G2?

The cell continues growing and prepares proteins/structures needed for division.

What happens during M phase?

Mitosis divides the nucleus, followed by cytokinesis.

What is interphase?

G1 + S + G2; it makes up most of the cell cycle.

What is G0?

A resting/nondividing state.

What is a genome?

The complete set of DNA in a cell or organism.

Where is most eukaryotic DNA located?

The nucleus.

What organelles also contain their own DNA?

Mitochondria and chloroplasts.

What is chromatin?

DNA associated with histone proteins.

What is a histone?

A protein around which DNA wraps.

What is a nucleosome?

A beadlike DNA-histone complex.

How is DNA packaged?

DNA → nucleosomes → chromatin fibers → condensed chromosome.

What does diploid mean?

Two sets of chromosomes, one set from each parent.

What does haploid mean?

One set of chromosomes.

What is the chromosome number of a human somatic cell?

46; 2n = 46.

What is the chromosome number of a human gamete?

23; n = 23.

What cells in humans are haploid?

Sperm and egg cells.

What are homologous chromosomes?

Matching chromosomes with the same genes in the same locations, one from each parent.

Can homologous chromosomes carry different alleles?

Yes.

How many homologous chromosome pairs do humans have?

23 pairs.

How many pairs of autosomes do humans have?

22 pairs.

What are sex chromosomes?

The remaining pair: typically XX or XY.

Why are X and Y not true homologous chromosomes?

They differ in size and carry mostly different genes.

What is a karyotype?

A display of chromosome number and appearance, usually photographed at metaphase.

What are sister chromatids?

Identical replicated copies of a chromosome made during S phase.

What joins sister chromatids together?

The centromeric region, with cohesin holding them together.

What is a centromere?

The chromosome region where sister chromatids are joined and kinetochores form.

What is a kinetochore?

A protein complex where spindle microtubules attach.

What are non-sister chromatids?

Chromatids belonging to different homologous chromosomes.

How do you count chromosomes in a drawing?

Count centromeres.

Is a duplicated chromosome one or two chromosomes?

One chromosome with two sister chromatids.

How many DNA molecules are in one duplicated chromosome?

Two.

If 2n = 20, how many homologous pairs are there?

10.

If 2n = 20, what is n?

10.

If gametes contain n = 10, what is 2n?

20.

What does homozygous mean?

The two homologs carry the same allele for a gene.

What does heterozygous mean?

The two homologs carry different alleles for a gene.

What is mitosis?

Division of the nucleus, also called karyokinesis.

What is cytokinesis?

Division of the cytoplasm.

Which comes first, mitosis or cytokinesis?

Mitosis begins first; cytokinesis completes cell division.

What are the mitosis stages in order?

Prophase → prometaphase → metaphase → anaphase → telophase.

What does PPMAT stand for?

Prophase, Prometaphase, Metaphase, Anaphase, Telophase.

What happens during prophase?

Chromosomes condense, spindle begins forming, nucleolus disappears, nuclear envelope begins breaking down.

What happens during prometaphase?

Nuclear envelope is gone; kinetochores form and spindle microtubules attach.

What happens during metaphase?

Chromosomes line up at the metaphase plate.

What happens during anaphase of mitosis?

Cohesin breaks and sister chromatids separate to opposite poles.

What happens during telophase?

Nuclear envelopes reform and chromosomes decondense.

What is the metaphase plate?

The imaginary plane at the center of the cell where chromosomes align.

What is a centrosome?

A microtubule-organizing center that helps form the spindle.

What are centrioles?

A pair of structures found inside animal-cell centrosomes.

Centromere vs centrosome?

Centromere is on a chromosome; centrosome organizes spindle microtubules.

What do kinetochore microtubules do?

Attach to kinetochores and move chromosomes.

What do polar/nonkinetochore microtubules do?

Overlap and help push the spindle poles apart.

How does a chromosome change through the cell cycle?

1 chromatid in G1 → 2 after S → condensed → sisters separate → decondensed.

When are chromosomes most condensed?

Metaphase.

How does animal-cell cytokinesis occur?

An actin contractile ring creates a cleavage furrow.

What is a cleavage furrow?

The indentation that pinches an animal cell into two daughter cells.

How does plant-cell cytokinesis occur?

Golgi vesicles form a phragmoplast and fuse into a cell plate.

What is a phragmoplast?

The vesicle/microtubule structure that helps form the plant cell plate.

What does the cell plate eventually become?

The new dividing cell wall and plasma membranes.

What is the main result of mitosis?

Two genetically identical daughter nuclei with the same chromosome number as the parent.

Why is meiosis called reduction division?

It reduces a diploid chromosome set to haploid.

How many times is DNA replicated before meiosis?

Once.

How many divisions occur in meiosis?

Two.

What are the final products of meiosis?

Four genetically different haploid cells.

What happens during meiosis I?

Homologous chromosomes pair, cross over, and then separate.

What happens during meiosis II?

Sister chromatids separate.

What are the end products of meiosis I?

Two haploid cells whose chromosomes are still duplicated.

What are the end products of meiosis II?

Four haploid cells.

What is synapsis?

Pairing of homologous chromosomes during prophase I.

What structure holds homologs together during synapsis?

The synaptonemal complex.

What is a tetrad?

A paired set of homologous chromosomes containing four chromatids.

When does crossing over occur?

Prophase I.

Between which chromatids does crossing over occur?

Non-sister chromatids of homologous chromosomes.

What is a chiasma?

A visible point where crossing over occurred.

How does crossing over increase genetic variation?

It creates recombinant chromatids with new combinations of maternal and paternal alleles.

What happens during metaphase I?

Homologous chromosome pairs line up side by side at the metaphase plate.

What happens during anaphase I?

Homologous chromosomes separate while sister chromatids stay together.

Why are cells haploid after meiosis I?

Each homologous pair has been separated, so each cell has one chromosome from each pair.

Is DNA replicated between meiosis I and meiosis II?

No.

What happens during metaphase II?

Individual duplicated chromosomes line up at the metaphase plate.

What happens during anaphase II?

Sister chromatids separate.

What separates during mitotic anaphase?

Sister chromatids.

What separates during anaphase I?

Homologous chromosomes.

What separates during anaphase II?

Sister chromatids.

Why does mitosis resemble meiosis II more than meiosis I?

Both line up individual chromosomes and separate sister chromatids.

Mitosis: how many divisions after one DNA replication?

One.

Meiosis: how many divisions after one DNA replication?

Two.

Does synapsis occur during mitosis?

No.

Does crossing over normally occur during mitosis?

No.

What lines up in mitotic metaphase?

Individual duplicated chromosomes.

What lines up in metaphase I?

Homologous pairs.

What lines up in metaphase II?

Individual duplicated chromosomes.

How many daughter cells are made by mitosis?

Two.

How many daughter cells are made by meiosis?

Four.

Are mitotic daughter cells usually genetically identical?

Yes.