Chromosomes and Cellular Reproduction Overview

Prokaryotic cell

No nucleus, no paired chromosomes (haploid), typically single circular chromosome.

Eukaryotic cell

Nucleus present, paired chromosomes common (diploid), typically multiple linear chromosomes.

Cell reproduction fundamental events

A cell's genetic information must be copied, the copies of the genetic information must be separated from one another, and the cell must divide into two cells.

Centromere

Serves as the point of attachment for the kinetochore to which spindle fibers (microtubules) attach.

Telomeres

Serve to stabilize the ends of the chromosome and limit cell division.

Origins of replication

Serve as the starting place for DNA synthesis.

G1 (Gap 1)

In this phase, the cell grows and synthesizes proteins necessary for cell division. During G1, the G1/S checkpoint takes place.

S phase

During S phase, DNA replication takes place.

G2 (Gap 2)

In G2, additional biochemical reactions take place that prepare the cell for mitosis. Near the end of G2 is the G2/M checkpoint.

G0 phase

A nondividing stage some cells may exit the active cell cycle.

Checkpoints

Function to ensure that all the cellular components are present and functioning before the cell moves to the next stage of the cell cycle.

G1/S checkpoint

Occurs during G1 prior to the S phase.

G2/M checkpoint

Occurs in G2 prior to mitosis.

Spindle-assembly checkpoint

Occurs during mitosis.

Prophase

The chromosomes condense and become visible. The mitotic spindle forms. The centrosomes move apart and microtubules form from the centrosomes.

Prometaphase

The nuclear membrane disintegrates. Spindle microtubules enter the nuclear region and attach to the chromosomes.

Metaphase

The chromosomes become arranged on the metaphase plate of the cell.

Anaphase

The sister chromatids separate, and the resulting chromosomes move to the opposite poles of the cell.

Telophase

The chromosomes arrive at the spindle poles. The nuclear membrane reforms around each set of chromosomes.

Genetically important results of the cell cycle and mitosis

The outcomes of these processes contribute to genetic diversity and cell function.

Cell Cycle

The cell cycle produces two cells that are genetically identical and that contain a full complement of chromosomes; there is no net reduction or increase in chromosome number.

Genetic Identity of Cells

The two cells are genetically identical because an exact copy of each DNA molecule was created during S phase.

Sister Chromatids

These exact copies give rise to the two identical sister chromatids.

Mitosis

Mitosis ensures that each new cell receives one of the identical sister chromatids.

Meiosis I

Separation of homologous chromosomes.

Prophase I

The chromosomes condense and homologous pairs of chromosomes undergo synapsis. While the chromosomes are synapsed, crossing over occurs.

Metaphase I

The homologous pairs of chromosomes line up on the metaphase plate.

Anaphase I

Homologous chromosomes separate and move to opposite poles of the cell.

Telophase I

The separated homologous chromosomes reach the spindle poles and are at opposite ends of the cell.

Cytokinesis after Meiosis I

Results in the division of the cytoplasm and the production of two haploid cells.

Meiosis II

Separation of sister chromatids.

Prophase II

Chromosomes condense, the nuclear envelope breaks down, and the spindle fibers form.

Metaphase II

Chromosomes line up at the metaphase plate.

Anaphase II

The sister chromatids separate and are pulled to opposite poles.

Telophase II

The chromosomes arrive at the spindle poles. The nuclear membrane reforms, and the spindle fibers break down.

Cytokinesis after Meiosis II

Takes place following meiosis II.

Results of Meiosis

Meiosis involves two cell divisions, thus producing four new cells. The chromosome number is reduced by half.

Genetic Variation in Meiosis

Crossing over occurs in prophase I and the random separation of homologous chromosomes takes place in anaphase I.

Anaphase I vs. Anaphase of Mitosis

In anaphase I of meiosis, homologous chromosomes separate, whereas in anaphase of mitosis, sister chromatids separate.

Spermatogenesis

Occurs in the testes; primordial diploid germ cells divide mitotically to produce diploid spermatogonia.

Primary Spermatocytes

Diploid cells that enter prophase I and complete meiosis I to become two secondary spermatocytes.

Secondary Spermatocytes

Haploid cells that complete meiosis II, producing a total of four haploid spermatids.

Oogenesis

The process by which female animals produce eggs.

Primary Oocyte

A diploid cell that has entered prophase I during oogenesis.

Secondary Oocyte

The haploid cell that receives most of the cytoplasm after meiosis I.

First Polar Body

The smaller haploid cell that receives only a small portion of the cytoplasm after meiosis I.

Ovum

The cell that receives most of the cytoplasm from the secondary oocyte after meiosis II.

Second Polar Body

The smaller haploid cell produced during oogenesis that does not develop into an ovum.

Polar Bodies

Typically, the polar bodies disintegrate, and only the ovum is capable of being fertilized.

Meiosis II in Humans

In humans and many other mammals, meiosis II is not completed until the sperm penetrates the secondary oocyte.

Gametophyte

A multicellular haploid stage in plants.

Sporophyte

A multicellular diploid stage in plants.

Meiosis in Plants

Meiosis in the diploid sporophyte stage of plants produces haploid spores that develop into the gametophyte.

Microsporocytes

Found in the stamen of the flower, they undergo meiosis to produce four haploid microspores.

Pollen Grain

Each microspore divides by mitosis to produce the pollen grain, which contains two haploid nuclei.

Sperm Cells

One of the haploid nuclei in the pollen grain divides by mitosis to produce two sperm cells.

Pollen Tube

The other haploid nucleus in the pollen grain directs the formation of the pollen tube.

Megasporocytes

Diploid megasporocytes found within the ovary divide by meiosis to produce four megaspores.

Embryo Sac

The remaining megaspore divides mitotically three times to produce eight haploid nuclei that form the embryo sac (or female gametophyte).

Egg Cell

Of the eight nuclei formed in the embryo sac, one will become the egg.

Homologous Pair

The two pairs (four socks in all) of each color represent the two chromosomes of a homologous pair, each with two sister chromatids.

Cohesin

The thread that connects the two socks of a pair represents cohesin.

Archaea

This cell is most likely an archaea because it lacks a nuclear membrane and has a single circular chromosome.

Viruses

Viruses are neither prokaryotes nor eukaryotes, because they do not possess a cellular structure.

Chromosome 1

The centromere in chromosome 1 is centrally located, so it is metacentric.

Chromosome 4

The centromere of chromosome 4 is located between the center and the end of the chromosome, so it is submetacentric.

Cell Cycle Duration

If the complete cell cycle requires 24 hours, the average duration of the M phase and metaphase can be calculated based on the proportion of cells in each stage.

Average Duration Calculation

To calculate the time required for a given stage, multiply 24 hours by the proportion of cells at that stage.

Interphase

Stage of the cell cycle where the cell prepares for division, with 160 cells counted, 0.80 proportion of cells, and an average duration of 19.2 hours.

M phase

The phase of the cell cycle that lasts 4.8 hours, determined by adding up the hours spent in each stage of mitosis.

Metaphase duration

Metaphase requires 0.24 hours, or 14.4 minutes.

Preformationism

Theory proposing that a tiny, fully formed adult (the homunculus) exists in the egg or sperm, predicting all traits are inherited from one parent.

Homunculus

The tiny, fully formed adult proposed by preformationism that enlarges during development.

Meiosis

A type of cell division that reduces the chromosome number by half, producing haploid cells.

G1 phase

The first gap phase of the cell cycle where the cell grows and prepares for DNA replication, with 4 chromosomes.

G2 phase

The second gap phase of the cell cycle where the cell prepares for mitosis, containing 12 chromosomes and 24 DNA molecules.

Prophase of mitosis

Stage of mitosis where the cell contains 12 chromosomes and 24 DNA molecules.

Anaphase I of meiosis

Stage of meiosis where homologous chromosomes separate, with 12 chromosomes and 24 DNA molecules.

Anaphase II of meiosis

Stage of meiosis where sister chromatids separate, resulting in 12 chromosomes and 12 DNA molecules.

Prophase II of meiosis

Stage of meiosis where haploid cells contain 6 chromosomes and 12 DNA molecules.

Cytokinesis after mitosis

Each cell will contain 12 chromosomes and 12 DNA molecules.

Diploid number of chromosomes in the plant

There are six chromosomes in this species.

Cell 1 during meiosis

Undergoing anaphase of meiosis I, with six chromosomes, each with two chromatids.

Cell 2 during mitosis

In anaphase of mitosis, where six chromosomes have separated into 12 chromosomes, each with a single chromatid.

Cell 3 during meiosis

In anaphase II of meiosis, with six chromosomes present.

DNA molecules in Cell 1

There are 12 DNA molecules, as there are six chromosomes each with two chromatids.

DNA molecules in Cell 2

There are 12 DNA molecules, as there are 12 chromosomes and sister chromatids are not present.

DNA molecules in Cell 3

There are six DNA molecules, as there are six chromosomes and sister chromatids are not present.

Amount of DNA in G1

7.3 pg, occurring prior to S phase and the doubling of DNA.

Amount of DNA in Prophase I

14.6 pg, as the amount of DNA is doubled during S phase.

Amount of DNA in G2

14.6 pg, as it takes place directly after S phase where the amount of DNA is doubled.

Amount of DNA following telophase II and cytokinesis

3.7 pg, as the chromosome number is reduced by half in meiosis I and then again in telophase II.

Amount of DNA in Anaphase I

14.6 pg, as the amount of DNA does not change from G2 to anaphase I.

Cytokinesis in spermatogenesis

Cytokinesis is equal, resulting in haploid cells of similar sizes.

Cytokinesis in oogenesis

Cytokinesis is unequal, resulting in a larger secondary oocyte and smaller polar bodies.

Homologous pairs of chromosomes

Pairs of chromosomes that are similar in shape, size, and genetic content.

7.3 pg

The amount of DNA present in metaphase II, which is half that in G2.

Oculocutaneous type 2 albinism

A recessive condition where a person has a copy of the DNA sequence that causes albinism on each of two homologous chromosomes.

Anaphase of mitosis

During this stage, there will be four copies of the DNA sequence causing albinism present in the cell.

G1 of interphase

In this stage, there will be two copies of the DNA sequence causing albinism.