Chapter 7 - The cell cycle and cell division

Clones

(1) Genetically identical cells or organisms produced from a common ancestor by asexual means. (2) To produce many identical copies of a DNA sequence by its introduction into, and subsequent asexual reproduction of, a cell or organism.

Homologs

(1) In cytogenetics, one of a pair (or larger set) of chromosomes having the same overall genetic composition and sequence. In diploid organisms, each chromosome inherited from one parent is matched by an identical (except for mutational differences) chromosome—its homolog—from the other parent. (2) In evolutionary biology, one of two or more features in different species that are similar by reason of descent from a common ancestor.

Mutations

A change in the genetic material that is not caused by recombination.

growth factors

A chemical signal that stimulates cells to divide.

aneuploidy

A condition in which the number of one or more chromosomes is either lacking or in excess.

Homologous pairs

A pair of matching chromosomes made up of a chromosome from each of the two sets of chromosomes in a diploid organism.

centrioles

A paired organelle that helps organize the microtubules in animal and protist cells during nuclear division.

cyclin-dependent kinases (CDKs)

A protein kinase whose target proteins are involved in transitions in the cell cycle and which is active only when complexed with additional protein subunits, called cyclins.

cyclin

A protein that activates a cyclin-dependent kinase, bringing about transitions in the cell cycle.

Kinetochores

A specialized structure on a centromere to which microtubules attach.

somatic cell

All the cells of the body other than the germ cells and gametes. See germ cells

chiasmata

An X-shaped connection between paired homologous chromosomes in prophase I of meiosis. A chiasma is the visible manifestation of crossing over between homologous chromosomes.

1. the cell is no longer functioning appropriately. In many cases, cells are replaced as they age or if they are damaged. The longer a cell has been around, the more damage it has likely sustained. Cells are able to monitor their own health and initiate apoptosis when they show symptoms of reduced functionality. 2. The cell is no longer needed by the organism. In some cases, cells are present at certain times during development and then later are no longer needed. One example occurs before birth in humans. A human fetus has weblike hands, with connective tissue between the fingers. As development proceeds, this unneeded tissue disappears as the cells between the digits undergo apoptosis in response to specific signals, producing our familiar unwebbed hand. 3. Cells have reached the limit of how many times they can divide in laboratory cell cultures, a phenomenon known as the Hayflick limit. A major reason involves chromosome shortening at the ends during DNA replication (see Key Concept 9.2). In many organisms, populations of cells called stem cells divide without limit, as do cancer cells. 4. The cell is responding to an infection. Death of the cell prevents the infection from spreading throughout the rest of the organism. You will see examples of this in plants and animals

Apoptosis (Greek, "falling apart") is a genetically programmed series of events that result in cell death. Why would a cell initiate apoptosis, which is essentially cell suicide? In multicellular organisms there are at least four possible reasons:

Spindle

Array of microtubules emanating from both poles of a dividing cell during mitosis and playing a role in the movement of chromosomes at nuclear division. Named for its shape.

Reproductive Cell division in bacteria

Binary fission is the standard model of cell division in prokaryotes like bacteria... following doubling in size & copying of the chromosome, a mother cell divides into two identical daughter cells

Growth factor→ cyclin synthesis →CDK activation →cell cycle phase-specific events

Cyclins are synthesized in response to various molecular signals, including growth factors. This starts a chain reaction:

Meiosis

Division of a diploid nucleus to produce four haploid daughter cells. The process consists of two successive nuclear divisions with only one cycle of chromosome replication. In meiosis I, homologous chromosomes separate but retain their chromatids. The second division meiosis II, is similar to mitosis, in which chromatids separate.

Independent assortment

During meiosis, the random separation of genes carried on nonhomologous chromosomes into gametes so that inheritance of these genes is random. This principle was articulated by Mendel as his second law.

Homologous chromosomes pair

During prophase I, homologous, replicated chromosomes pair with each other along their entire lengths. Pairing is facilitated by special proteins that glue the homologs together. No such pairing of homologs occurs in mitosis (although sister chromatids do remain together until anaphase). This homolog pairing remains through metaphase I.

sister chromatids

Each of a pair of newly replicated chromatids.

nondisjunction

Failure of sister chromatids to separate in meiosis II or mitosis, or failure of homologous chromosomes to separate in meiosis I. Results in aneuploidy.

Homologous chromosomes have separated and are located in the different daughter cells

Following Meiosis I: A.The chromosomes are duplicated before prophase of Meiosis II. B.Sister chromatids have separated and are located in the different daughter cells. C.Homologous chromosomes have separated and are located in the different daughter cells. D.The diploid daughter cells prepare to divide again.

Haploid

Having a chromosome complement consisting of just one copy of each chromosome; designated 1n or n

diploid

Having a chromosome complement consisting of two copies (homologs) of each chromosome.

Chromosome arm

In linear, eukaryotic chromosomes, the part between a centromere and a telomere.

chromatids

In meiosis, a chromatid after crossing over that contains part of a sister chromatid, typically having exchanged that part with the sister chromatid.

recombinant

In meiosis, a chromatid after crossing over that contains part of a sister chromatid, typically having exchanged that part with the sister chromatid.

G2 phase (Once DNA synthesis is complete, cells enter G2 phase, where they prepare for mitosis.)

In the cell cycle, the gap between S phase and the onset of mitosis.

G1 (or G0) phase (Cells that are preparing to undergo cell division are in G1 phase of the cell cycle. G1 is the phase in which most eukaryotic cells are found, and this is where they carry out their normal functions. Cells that are not yet dividing are in an arrested state termed G0. Cells remain in G0 or G1 unless they receive appropriate signals to enter the next phase of the cell cycle.) (G0 - A resting phase in which a cell is not preparing for the cell division cycle.)

In the cell cycle, the gap between the end of mitosis and the onset of S phase.

Interphase

In the cell cycle, the period between successive nuclear divisions during which the chromosomes are diffuse and the nuclear envelope is intact. During interphase the cell is most active in transcribing and translating genetic information.

S phase (Cells in S phase (synthesis phase) of the cell cycle are actively replicating their DNA.)

In the cell cycle, the stage of interphase during which DNADNA is replicated.

There is no centromere division

Instead of centromeres dividing as in mitosis, in anaphase I the homologous chromosome pairs separate. Anaphase I begins when the glue holding homologs together loosens. Homologs separate and then move to opposite poles.

Sexual reproduction

Many plants are capable of self-fertilization. That is, haploid pollen grains produce male gametes that can fertilize female gametes produced by the ovule of the same plant. This is an example of: A.Asexual reproduction. B.Sexual reproduction.

homologous chromosomes pair and there is no centromere division

Meiosis replaces mitosis in the eukaryotic cell cycle in diploid cells that are destined to produce haploid daughter cells. As noted above, meiosis consists of two nuclear divisions, meiosis I and meiosis II. Two features characterize meiosis I:

Premature cell death caused by external agents such as toxins.

Necrosis

Mitosis

Nuclear division in eukaryotes leading to the formation of two daughter nuclei, each with a chromosome complement identical to that of the original nucleus.

1. The G1, or G1 to S, checkpoint (restriction point), which is triggered by DNA damage 2. The S checkpoint, which is triggered by incomplete replication or DNA damage 3. The G2 checkpoint, which is triggered by DNA damage 4. The metaphase checkpoint, which is triggered by a chromosome that fails to attach to the spindle

Points of transition between different phases of the cell cycle, which are regulated by cyclins and cyclin-dependent kinases (CDKs).

cell cycle checkpoints

Points of transition between different phases of the cell cycle, which are regulated by cyclins and cyclin-dependent kinases (CDKs).

Apoptosis

Programmed cell death. A controlled cell death important in morphogenesis during animal development and growth (e.g., removal of tissue between digits, metamorphosis of frog tadpole tail and gut).

Haplontic

Referring to a life cycle in which the mature organism is haploid and the zygote is the only diploid stage.

diplontic

Referring to a life cycle in which the organism is diploid and the gametes are the only haploid stage

Haplo-diplontic

Referring to a life cycle in which the organism spends significant time in both the haploid and diploid stages.

Sexual reproduction

Reproduction in which the genes of two individuals are combined to produce offspring, typically involving the union of male and female gametes.

Binary fission

Reproduction of a prokaryote by division of a cell into two comparable progeny cells.

DNA replication

The creation of a new strand of DNA in which DNA polymerase catalyzes the exact reproduction of an existing (template) strand of DNA.

Cytokinesis

The division of the cytoplasm of a dividing cell.

polyspermy

The fertilization of a single female gamete by more than one male gamete.

zygote

The fertilized egg. The cell created by the union of two gametes, in which the gamete nuclei are fused. The earliest stage of the diploid generation.

Telophase

The final phase of mitosis or meiosis during which chromosomes become diffuse, nuclear envelopes re-form, and nucleoli begin to reappear in the daughter nuclei.

Prophase

The first stage of nuclear division, during which chromosomes condense from diffuse, threadlike material to discrete, compact bodies.

Asexual reproduction

The formation of new individuals without the union of genetic material from two different parents. Often takes place by budding or fragmentation. The offspring are genetically identical to their parent.

centrosomes

The major microtubule organizing center of an animal cell.

Gametes

The mature sexual reproductive cell: the egg or the sperm.

Hayflick limit

The maximum number of cell divisions for a normal (human) cell before apoptosis occurs (due to telomere shortening).

crossing over

The mechanism by which linked genes undergo recombination. In general, the term refers to the reciprocal exchange of corresponding segments between two homologous chromatids.

chromatin

The nucleic acid-protein complex that makes up eukaryotic chromosomes.

Karyotype

The number, forms, and types of chromosomes in a cell.

Cell division signals

The one or more signals required to initiate cell division. The signals may originate from either inside or outside the cell.

prometaphase

The phase of nuclear division that begins with the disintegration of the nuclear envelope.

M phase (Finally cells enter M phase, which is when DNA segregation in mitosis and cytokinesis occur. Together, G1, S, and G2 are termed interphase; the cell cycle can be partitioned into M phase and interphase.)

The portion of the cell cycle in which mitosis takes place.

polyploidy

The possession of more than two entire sets of chromosomes.

Centromere

The region where sister chromatids join.

Cell division

The reproduction of a cell to produce two new cells. In eukaryotes, involves nuclear division (mitosis) and cytoplasmic division (cytokinesis).

DNA segregation

The separation of two DNAs formed by replication into two new cells during cell division.

restriction point

The specific time during G1 of the cell cycle at which the cell becomes committed to undergo the rest of the cell cycle.

anaphase

The stage in cell nuclear division at which the first separation of sister chromatids (or, in the first meiotic division, of paired homologs) occurs.

Metaphase

The stage in nuclear division at which the centromeres of the highly supercoiled chromosomes are all lying on a plane (the metaphase plate) perpendicular to a line connecting the division poles.

Cell cycle

The stages through which a cell passes between one division and the next. Includes all stages of interphase and mitosis.

1. Failure of the spindle to form, so that all the chromosomes remain in the same part of the cell after mitosis and thus are not separated by cytokinesis. If this occurs during mitosis, the result will be a cell that is tetraploid (4n). 2. Failure of cytokinesis to occur, so that the two daughter nuclei remain in the same cell. If this occurs during mitosis and the daughter nuclei fuse, the cell will again be tetraploid.

The two most common errors that occur during M phase that can lead to polyploid cells or organisms are:

Cell cycle regulator + ATP →CDK→ cell cycle regulator-P + ADP

They catalyze the phosphorylation of target proteins that regulate the cell cycle:

Cyclic parthenogenesis in Daphnia

When times are good, females will give rise to genetically identical diploid daughters by asexual reproduction from unfertilized diploid eggs (i.e., no meiosis) •When times are bad, enter sexual cycle • Involves environmental sex determination

D. all of the above

Which of the following statements regarding sister chromatids and homologous chromosomes are correct? A.Homologous chromosomes contain the same genes but may have different alleles of a particular gene. B.Sister chromatids are identical copies of each other produced during DNA replication. C.For each pair of homologous chromosomes, you inherited one copy from your mother and one from your father D.All of the above. E.None of the above.

G1 (or G0), S, G2, and M

in eukaryotes cell division occurs within the context of the cell cycle. The eukaryotic cell cycle can be divided into four phases:

cell division signals, DNA replication, DNA segregation, and cytokinesis

in order for any cell to divide, the following four events must occur

Fertilization

the union of gametes

Chromosomes in a diploid cell

• Diploid cells have 2 copies of ea. chromosome type (= homologous chromosomes), one from each parent • Homologous chromosomes have the same genes on them in same order, but may have different versions (alleles) of genes • Homologous chromosomes are different from sister chromatids!

meiosis I

• Pairs of homologous chromosomes line up and separate in Meiosis I (not individual chromosomes, like in mitosis)

asexual reproduction

• Results in genetically identical offspring (clones) • In prokaryotes, for example, by binary fission • By mitosis & cytokinesis in single-celled eukaryotes • By root sprouting in aspen

The Eukaryotic cell cycle

•DNA replication and cell division are temporally distinct • Interphase (incl. DNA synthesis) alternates with the mitotic phase

overview of sexual reproduction

•Diploid adults produce reproductive cells (gametes) in a process called meiosis • These gametes are haploid (one copy of each chromosome) • Diploid offspring result from the fusion of parental gametes (fertilization) produced during meiosis • Note the alternation of meiosis and fertilization common to all sexual life cycles

DNA synthesis (S) phase

•During DNA replication, the genetic information in a cell is copied prior to cell division • Chromosomes are copied to form sister chromatids • Must be correctly distributed to each of the daughter cells

Paradox of Sex

•Males make up about half of the population in many animal species, and yet they don't reproduce themselves •A population of asexual females who produced only daughters would increase twice as quickly as a population of sexual females who produced both sons and daughters

Mitotic (M) phase

•Mitosis: distribution of daughter chromosomes to daughter nuclei • Cytokinesis: division into two cells

Sex does not necessarily require two parents

•Monoecious ("1 house") plants have flowers with both male and female parts (hermaphrodites) that can potentially give rise to offspring • Reproductive assurance in the absence of other partners

Meiosis II

•No chromosome duplication • Meiosis II resembles mitosis: sister chromatids separate

Benefits of sexual reproduction must overcome costs

•Sex generates diversity! • Potentially useful in changing environments.

meiosis

•The production of 4 haploid cells (gametes) from one diploid cell - one chromosome duplication, followed by two cell divisions

Cell Division: every cell from a cell

•There are three functions of cell division • All yield genetically identical daughter cells • Involves copying of genetic information (DNA replication) and splitting into two cells