9 chromosomes, the cell cycle, and cell division

9Chromosomes, the Cell Cycle,

and Cell Division

9 Chromosomes, the Cell Cycle, and Cell Division

• Systems of Cell Reproduction

• Interphase and the Control of Cell Division

• Eukaryotic Chromosomes

• Mitosis: Distributing Exact Copies of Genetic Information

• Cytokinesis: The Division of the Cytoplasm

• Reproduction: Asexual and Sexual

• Meiosis: A Pair of Nuclear Divisions

• Meiotic Errors

• Cell Death

9 Systems of Cell Reproduction

• Since all living cells are mortal, cell reproduction or replacement is universal among living organisms.

• This may consist of simple replacement, differentiation, or specialization.


• Unicellular organisms use cell division primarily to reproduce, whereas in multicellular organisms cell division also plays important roles in growth and in the repair of tissues.

• Two Types of Cell Division: Mitosis- produces two nuclei with the same

number of chromosomes as in the original nucleus.

Meiosis- Gives on half the number of chromosomes as in the original nucleus and is associated with reproduction.


• Four events occur before and during cell division:

1. A signal to reproduce must be received.

2. Replication of DNA and vital cell components must occur.

3. DNA must be distributed to the new cells.

4. The cell membrane or cell wall must separate the two new cells.


• Prokaryotes divide by fission.

Most prokaryotes have one circular chromosome.

As DNA replicates, each of the two resulting DNA molecules attaches to the plasma membrane.

As the cell grows, new plasma membrane is added between the attachment points, and the DNA molecules are moved apart.

Cytokinesis separates the one cell into two, each with a complete chromosome.

Figure 9.2 Prokaryotic Cell Division


• Eukaryotic cells divide by mitosis or meiosis.

• Eukaryotes usually have many chromosomes.

• Eukaryotes have a nucleus, which must replicate and, with few exceptions, divide during cell division.


• The reproduction of eukaryotic cells is typically characterized by three steps:

1. The replication of the DNA within the nucleus

2. The packaging and segregation of the replicated DNA into two new nuclei (nuclear division)

3. The division of the cytoplasm (cytokinesis)


• Meiosis is specialized cell division used for sexual reproduction. The genetic information in the chromosomes is shuffled, and the cells, called gametes, typically get one-half of the original DNA complement.

9 Interphase and the Control of Cell Division

• The cell cycle has two phases: mitosis and interphase.

• A typical eukaryotic cell will spend most of its life in interphase, the period between divisions of the cytoplasm.

• Some cells, such as human nerve and muscle cells, lose the capacity to divide altogether and stay in interphase indefinitely, while other cells divide regularly or occasionally.

9 Cell Cycle

• There are 4 periods in a cell cycle.

1. Mitosis (M)- nucleus and cytoplasm divide and form 2 new cells.

2. First Gap (G1)- new cells from birth until it begins to replicate.

3. Synthesis (S)- DNA synthesis, chromosomes are replicated.

4. Second Gap Period (G2)- end of DNA synthesis until cell division or mitosis.


• Interphase consists of three sub-phases:

G1 is Gap 1, the period just after mitosis and before the beginning of DNA synthesis.

Next is S phase (synthesis), which is the time when the cell’s DNA is replicated.

G2 is the time after S and prior to mitosis.

Figure 9.3 The Eukaryotic Cell Cycle


• Transitions from G1 to S and G2 to M depend on activation of a protein called cyclin-dependent kinase, or Cdk.

• Several different cyclins exist, which, when bound to Cdk, phosphorylate different target proteins.

Figure 9.4 Cyclin-Dependent Kinases and Cyclins Trigger Transisions in the Cell Cycle


• Cyclin-Cdk complexes act as checkpoints. When functioning properly, they allow or prevent the passage to the next cell cycle stage, depending on the extra- and intracellular conditions.

• In cancer cells, these cyclin-Cdk controls are often disrupted.


• Some cells which no longer go through the cell cycle may respond to growth factors provided by other cells.

• Examples include platelet-derived growth factor, interleukins, and erythropoietin.

• Growth factors act by binding to target cells, and triggering events within the target cell that initiate the cell cycle.

• Cancer cells cycle inappropriately because they either make their own growth factors or no longer require them to start cycling.

9 Eukaryotic Chromosomes

• The basic unit of the eukaryotic chromosome is a gigantic, linear, double-stranded molecule of DNA complexed with many proteins to form a dense material called chromatin.

• After the DNA of a chromosome replicates during S phase, each chromosome consists of two joined chromatids.

Figure 9.5 Chromosomes, Chromatids, and Chromatin

9 Eukaryotic Chromosomes

• Interphase chromosomes are wrapped around proteins called histones.

• These wraps of DNA and histone proteins are called nucleosomes and resemble beads on a string.

• During mitosis and meiosis, the chromatin becomes even more coiled and condensed.

Figure 9.6 DNA Packs into a Mitotic Chromosome

9 Mitosis: Distributing Exact Copies of Genetic Information

• When the cell enters S phase and DNA is replicated, the centrosome replicates to form two centrosomes.

• During G2-to-M transition, the two centrosomes separate from each other and move to opposite ends of the nuclear envelope.

• The orientation of the centrosomes determines the cell’s plane of division.

• Centrosomes are regions where microtubules form.

9 Phases of Mitosis

• 5 Phases

1. Prophase

2. Metaphase

3. Anaphase

4. Telophase

5. Cytokinesis


• Prophase marks the beginning of mitosis.

Chromosomes compact and coil, becoming more dense and visible, form a set of sister chromatids.

Polar microtubules form between the two centrosomes and make up the developing spindle.

The mitotic spindle serves as a “railroad track” along which chromosomes will move later in mitosis.

Mitotic spindle begins to push the ends or poles apart.

A cell will not begin to divide until it has about doubled in size.

Figure 9.8 Mitosis (Part 1)


• During metaphase:

Chromosomes are fully condensed and have distinguishable shapes.

Cohesins break down.

Mitotic spindle starts to tug and lines all the chromatids up at the equator of the spindle.

Figure 9.8 Mitosis (Part 2)


• Anaphase

Each of the sister chromatids separate, thereby becoming independent chromosomes.

They are pulled to opposite poles of the spindle.

Forms a complete set of chromosomes which is the basis for a new nucleus.

The cell is pushed farther apart so it becomes much longer.


• Telophase begins when the chromosomes finish moving.

2 nuclei are organized.

The chromosomes at each end unwind into masses of chromatin.

9 Cytokinesis

• Cytokinesis- The Division of the Cytoplasm

Usually begins in early anaphase.

A ring of microfilaments form around the cells equator.

The filaments constrict the cell to form a cleavage furrow and eventually pinches the cytoplasm in two.

9 Cytokinesis: The Division of the Cytoplasm

• Plants have cell walls and the cytoplasm divides differently.

• After the spindle breaks down, vesicles from the Golgi apparatus appear in the equatorial region.

• The vesicles fuse to form a new plasma membrane, and the contents of the vesicles combine to form the cell plate, which is the beginning of the new cell wall.

Figure 9.10 Cytokinesis Differs in Animal and Plant Cells

9 Reproduction: Asexual and Sexual

• Mitosis by repeated cell cycles can give rise to vast numbers of genetically identical cells.

• Meiosis results in just four progeny, which usually do not further duplicate. The cells can be genetically different.


• Asexual reproduction involves the generation of a new individual that is essentially genetically identical to the parent. It involves a cell or cells that were generated by mitosis.

• Variation of cells is likely due to mutations or environmental effects.


• Sexual reproduction involves meiosis.

• Two parents each contribute a set of chromosomes in a sex cell or gamete.

• Gametes fuse to produce a single cell, the zygote, or fertilized egg. This creates variety among the offspring beyond that attributed to mutations or the environment.


• In multicellular organisms, somatic cells (cells that are not specialized for reproduction) each contain two sets of chromosomes.

• In each recognizable pair of chromosomes, one comes from each of the two parents.

• The members of the pair are called homologous chromosomes and have corresponding but generally not identical genetic information.

• In humans there are 46 homologous chromosomes in most body cells.


• Haploid cells contain just one homolog of each pair. The number of chromosomes in a single set is denoted by 1n.

• When haploid gametes fuse in fertilization, they create the zygote, which is 2n, or diploid.

• Tetraploid- 4 homologous chromosomes of each type 4n.


• Haplontic organisms have a predominant life cycle in a 1n (haploid) state.

• Some organisms have an alternation of generations that includes both a 1n vegetative life stage and a 2n vegetative life stage.

• In diplontic organisms, which include animals, the organism is usually diploid.

Figure 9.12 Fertilization and Meiosis Alternate in Sexual Reproduction (Part 1)

Figure 9.12 Fertilization and Meiosis Alternate in Sexual Reproduction (Part 2)


• Cells in metaphase can be killed and prepared in a way that spreads the chromosomes around a region on a glass slide.

• A photograph of the slide can be taken, and images of each chromosome can be organized based on size, number, and shape. This spread is called a karyotype.

Figure 9.13 Human Cells Have 46 Chromosomes

9 Phases of Meiosis

• Prophase I- Finds homolog and line up together.

• Metaphase I- Tetrads line up at spindle equator.

• Anaphase I- They begin to separate.

• Telophase I- Organize into a new nucleus.

• No DNA replication between meiosis I and II.

• Prophase II- new spindle forms.

• Metaphase II- sister chromatids line up at spindle.

• Anaphase II- sister chromatids separate.

• Telophase II- organized into new haploid nuclei.

9 Meiosis: A Pair of Nuclear Divisions

• Meiosis consists of two nuclear divisions that reduce the number of chromosomes to the haploid number. The DNA is replicated only once.

• The functions of meiosis are:

To reduce the chromosome number from diploid to haploid.

To ensure each gamete gets a complete set of chromosomes.

To promote genetic diversity among products.


• Meiosis

It takes 2 divisions during meiosis to make each nucleus haploid.

The 2 divisions are split up into meiosis I and II.

Both divisions use a spindle and move chromosomes, so it looks similar to mitosis.

Meiosis can take days to complete, where mitosis usually takes only hours or minutes.

Figure 9.14 Meiosis (Part 1)


• The homologous chromosomes separate in anaphase I.

• The individual chromosomes are pulled to the poles, with one homolog of a pair going to one pole and the other homolog going to the opposite pole.


• The second meiotic division separates the chromatids.

• Meiosis II is similar to mitosis but one difference is that DNA does not replicate before meiosis II.

• The number of chromosomes in the resulting cells is therefore half that found in diploid mitotic cells.

• In meiosis II, sister chromatids are not identical and there is no crossing-over.


• During meiosis I, the chromosomes appear to repel each other except at the centromere and at points of attachments, called chiasmata, which appear x-shaped.

• These chiasmata reflect the exchange of genetic material between homologous chromosomes, a phenomenon called crossing-over.

• This crossing-over increases genetic variation by reshuffling the genes on the homologs.

9Figure 9.16 Crossing Over Forms Genetically Diverse Chromosomes


• Meiosis leads to genetic diversity.

• Synapses and crossing-over during prophase I mix genetic material of the maternal with that of the paternal homologous chromosomes.

• Which member of a homologous pair segregates or goes to which daughter cell at anaphase I is a matter of chance. This phenomenon is called independent assortment.

9 Meiotic Errors

• Nondisjunction occurs when homologous chromosomes fail to separate during anaphase I, or sister chromatids fail to separate during anaphase II.

• The result is a condition called aneuploidy.

Figure 9.18 Nondisjunction Leads to Aneuploidy

9 Meiotic Errors

• One reason for aneuploidy may be a lack of cohesions.

• Failure of chromosome 21 to separate in humans results in trisomy 21—Down syndrome.

• Translocation, a process in which part of a chromosome attaches to another, can also cause abnormality.

9 Meiotic Errors

• Polyploids have extra whole sets of chromosomes, and this abnormality in itself does not prevent mitosis.

• Triploids are 3n; tetraploids are 4n.

• Although mitosis usually is unimpaired, meiosis is problematic, especially for odd numbers of sets, as in triploidy.

9 Cell Death

• Cells die in one of two ways: necrosis and apoptosis.

• Necrosis occurs when cells either are damaged by poisons or are starved of essential nutrients. These cells swell and burst.

9 Cell Death

• Genetically programmed cell death is called apoptosis:

The cell may no longer be needed, e.g., cells of the weblike tissue between the fingers of a developing human fetus.

Cells that are old or damaged may need to be replaced.

The cell death cycle is controlled by signals.

The cell becomes isolated, chops up its own chromatin, and gets ingested by surrounding living cells.

9 Gamete Formation in Animals

• Pgs. 820-824

• Gametes- term used to define sperm or egg.

• Sperm or Spermatozoa- male gametes. Sperm are small with little cytoplasm and have

flagellum so they can move. Sperm are produced in the testes by the

process of spermatogenesis. Germ cells are cells that reproduce to form

diploid cells, in the male these cells are called spermatogonia cells.

Cells that undergo meiosis are called spermatocytes.


These divide by meiosis into 4 spermatids. To become mature sperm the spermatids must

undergo further differentiation. The mature sperm consists of a tail and head.

• Oogenesis- is the formation of the female gametes called eggs or ova (sing.) ovum (plur.)

These cells are the largest cells in the female body where as sperm are of the smallest in the male body.

Division in this process is unequal. An original cell in the female only produces one ovum and 2 polar bodies (small cells).


In the ovary the germ cells are called oogonia, which divide by mitosis.

They eventually stop dividing and become oocytes. In mammals all this occurs while the female is still in

embryo. This process is stopped at the point of prophase I in meiosis until the female reaches sexual maturity.

In meiosis I the chromosomes separate as usual but the cytoplasm is divided unequally, which forms the first polar body.

In meiosis II the same thing happens which produces the 2nd polar body and one large ovum.

The polar bodies are just a means of shedding excess chromosomes from the developing egg and they soon disintegrate.

9 chromosomes, the cell cycle, and cell division

Documents

cell cycle

cell membrane

cell wall

specialized cell division

types of cell division

typical eukaryotic cell

vital cell components

cytoplasm reproduction