10 Cell Cycle

Cell Cycle

ordered series of events involving cell growth and cell division that produces two new daughter cells

Mitotic Phase

period of the cell cycle during which duplicated chromosomes are distributed into two nuclei and cytoplasmic contents are divided; includes karyokinesis (mitosis) and cytokinesis

Interphase

period of the cell cycle leading up to mitosis; includes G1, S, and G2 phases (the interim period between two consecutive cell divisions

Gene

physical and functional unit of heredity, a sequence of DNA that codes for a protein.

Locus

position of a gene on a chromosome

Attachment of the mitotic spindle fibers to the kinetochores is a characteristic of which stage of mitosis?

prometaphase

The chromosomes become visible under a light microscope during which stage of mitosis?

prophase

Kinetochore

protein structure associated with the centromere of each sister chromatid that attracts and binds spindle microtubules during prometaphase

Condensin

proteins that help sister chromatids coil during prophase

Quiescent

refers to a cell that is performing normal cell functions and has not initiated preparations for cell division

Centromere

region at which sister chromatids are bound together; a constricted area in condensed chromosomes

Centriole

rod-like structure constructed of microtubules at the center of each animal cell centrosome

S Phase

second, or synthesis, stage of interphase during which DNA replication occurs

Which of the following events does not occur during some stages of interphase?

separation of sister chromatids

Chromatid

single DNA molecule of two strands of duplicated DNA and associated proteins held together at the centromere

Identical copies of chromatin held together by cohesin at the centromere are called _____.

sister chromatids

Metaphase

stage of mitosis during which chromosomes are aligned at the metaphase plate

Telophase

stage of mitosis during which chromosomes arrive at opposite poles, decondense, and are surrounded by a new nuclear envelope

Prophase

stage of mitosis during which chromosomes condense and the mitotic spindle begins to form

Anaphase

stage of mitosis during which sister chromatids are separated from each other

Prometaphase

stage of mitosis during which the nuclear membrane breaks down and mitotic spindle fibers attach to kinetochores

Cell Plate

structure formed during plant cell cytokinesis by Golgi vesicles, forming a temporary structure (phragmoplast) and fusing at the metaphase plate; ultimately leads to the formation of cell walls that separate the two daughter cells

Nucleosome

subunit of chromatin composed of a short length of DNA wrapped around a core of histone proteins

Unpacking of chromosomes and the formation of a new nuclear envelope is a characteristic of which stage of mitosis?

telophase

Cell Cycle

ordered sequence of events that a cell passes through between one cell division and the next

G1 Phase

(also, first gap) first phase of interphase centered on cell growth during mitosis

Mitosis

(also, karyokinesis) period of the cell cycle during which the duplicated chromosomes are separated into identical nuclei; includes prophase, prometaphase, metaphase, anaphase, and telophase

G2 Phase

(also, second gap) third phase of interphase during which the cell undergoes final preparations for mitosis

Compare and contrast a human somatic cell to a human gamete.

Human somatic cells have 46 chromosomes: 22 pairs and 2 sex chromosomes that may or may not form a pair. This is the 2n or diploid condition. Human gametes have 23 chromosomes, one each of 23 unique chromosomes, one of which is a sex chromosome. This is the n or haploid condition.

Briefly describe the events that occur in each phase of interphase.

During G1, the cell increases in size, the genomic DNA is assessed for damage, and the cell stockpiles energy reserves and the components to synthesize DNA. During the S phase, the chromosomes, the centrosomes, and the centrioles (animal cells) duplicate. During the G2 phase, the cell recovers from the S phase, continues to grow, duplicates some organelles, and dismantles other organelles.

What cell cycle events will be affected in a cell that produces mutated (non-functional) cohesin protein?

If cohesin is not functional, chromosomes are not packaged after DNA replication in the S phase of interphase. It is likely that the proteins of the centromeric region, such as the kinetochore, would not form. Even if the mitotic spindle fibers could attach to the chromatids without packing, the chromosomes would not be sorted or separated during mitosis.

List some reasons why a cell that has just completed cytokinesis might enter the G0 phase instead of the G1 phase.

Many cells temporarily enter G0 until they reach maturity. Some cells are only triggered to enter G1 when the organism needs to increase that particular cell type. Some cells only reproduce following an injury to the tissue. Some cells never divide once they reach maturity.

Chapter Summary

Prokaryotes have a single circular chromosome composed of double-stranded DNA, whereas eukaryotes have multiple, linear chromosomes composed of chromatin surrounded by a nuclear membrane. The 46 chromosomes of human somatic cells are composed of 22 pairs of autosomes (matched pairs) and a pair of sex chromosomes, which may or may not be matched. This is the 2n or diploid state. Human gametes have 23 chromosomes or one complete set of chromosomes; a set of chromosomes is complete with either one of the sex chromosomes. This is the n or haploid state. Genes are segments of DNA that code for a specific protein. An organism's traits are determined by the genes inherited from each parent. Duplicated chromosomes are composed of two sister chromatids. Chromosomes are compacted using a variety of mechanisms during certain stages of the cell cycle. Several classes of protein are involved in the organization and packing of the chromosomal DNA into a highly condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure is necessary for chromosomal segregation during mitosis.

Chromosomes are duplicated during what stage of the cell cycle?

S phase

Eukaryotic chromosomes are thousands of times longer than a typical cell. Explain how chromosomes can fit inside a eukaryotic nucleus.

The DNA double helix is wrapped around histone proteins to form structures called nucleosomes. Nucleosomes and the linker DNA in between them are coiled into a 30-nm fiber. During cell division, chromatin is further condensed by packing proteins.

What is the relationship between a genome, chromosomes, and genes?

The genome consists of the sum total of an organism's chromosomes. Each chromosome contains hundreds and sometimes thousands of genes, segments of DNA that code for a polypeptide or RNA, and a large amount of DNA with no known function.

Chemotherapy drugs such as vincristine and colchicine disrupt mitosis by binding to tubulin (the subunit of microtubules) and interfering with microtubule assembly and disassembly. Exactly what mitotic structure is targeted by these drugs and what effect would that have on cell division?

The mitotic spindle is formed of microtubules. Microtubules are polymers of the protein tubulin; therefore, it is the mitotic spindle that is disrupted by these drugs. Without a functional mitotic spindle, the chromosomes will not be sorted or separated during mitosis. The cell will arrest in mitosis and die.

Describe the similarities and differences between the cytokinesis mechanisms found in animal cells versus those in plant cells.

There are very few similarities between animal cell and plant cell cytokinesis. In animal cells, a ring of actin fibers is formed around the periphery of the cell at the former metaphase plate (cleavage furrow). The actin ring contracts inward, pulling the plasma membrane toward the center of the cell until the cell is pinched in two. In plant cells, a new cell wall must be formed between the daughter cells. Due to the rigid cell walls of the parent cell, contraction of the middle of the cell is not possible. Instead, a phragmoplast first forms. Subsequently, a cell plate is formed in the center of the cell at the former metaphase plate. The cell plate is formed from Golgi vesicles that contain enzymes, proteins, and glucose. The vesicles fuse and the enzymes build a new cell wall from the proteins and glucose. The cell plate grows toward and eventually fuses with the cell wall of the parent cell.

Separation of the sister chromatids is a characteristic of which stage of mitosis?

anaphase

Mitotic Spindle

apparatus composed of microtubules that orchestrates the movement of chromosomes during mitosis

The fusing of Golgi vesicles at the metaphase plate of dividing plant cells forms what structure?

cell plate

Haploid

cell, nucleus, or organism containing one set of chromosomes (n)

Diploid

cell, nucleus, or organism containing two sets of chromosomes (2n)

The mitotic spindles arise from which cell structure?

centrosome

Homologous Chromosomes

chromosomes of the same morphology with genes in the same location; diploid organisms have pairs of homologous chromosomes (homologs), with each homolog derived from a different parent

Cleavage

constriction formed by an actin ring during cytokinesis in animal cells that leads to cytoplasmic division

G0 Phase

distinct from the G1 phase of interphase; a cell in G0 is not preparing to divide

Cytokinesis

division of the cytoplasm following mitosis that forms two daughter cells.

Metaphase Plate

equatorial plane midway between the two poles of a cell where the chromosomes align during metaphase

An organism's traits are determined by the specific combination of inherited _____.

genes

Gamete

haploid reproductive cell or sex cell (sperm, pollen grain, or egg)

The first level of DNA organization in a eukaryotic cell is maintained by which molecule?

histone

Karyokinesis

mitotic nuclear division

Histone

one of several similar, highly conserved, low molecular weight, basic proteins found in the chromatin of all eukaryotic cells; associates with DNA to form nucleosomes

Genome

total genetic information of a cell or organism

A diploid cell has_______ the number of chromosomes as a haploid cell.

twice