Chapter 7 – The Cell Cycle and Cell Division
Cells – source of continuity and diversity. The continuity of life depends on the reproduction of cells, called cell
division.
Cell division is one part of the life cycle of a cell.
This life cycle from origin to division is called the cell cycle
here’s three reasons why cell division is so important:
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Unicellular organisms use cell division to make an entire organism (called cloning)
Sexually reproducing organisms use cell division to develop from a zygote.
In many cases, after an organism is fully grown it uses cell division for renewal and repair.
Let’s talk chromosomes!!
A cell’s DNA is called its genome.
A typical eukaryotic cell has a tremendous amount of DNA
A great example of this is a typical human cell which has about 3 meters of DNA…a length about 300,000 times the
diameter of the cell itself.
Yet all this DNA must be copied, separated and divided evenly between the 2 daughter cells.
This task is manageable because the DNA is packaged into chromosomes. Let’s look at a picture of chromosomes in a
cell.
Every eukaryotic cell has a characteristic number of chromosomes in each cell.
Human somatic cells (body cells, not reproductive cells) have 46 chromosomes. The term used to refer to the
complete number chromosomes is called diploid number or 2n
Human gametes – (sperm and egg) have 23 chromosomes. The term used to refer to half the complete number of
chromosomes is called haploid number or n.
Chromosomes, chromatin, & chromatids
This is a little confusing…
Each chromosome is made up of DNA wound around associated proteins. This DNA-protein complex is called
chromatin. It condenses after the DNA duplicates in preparation for cell division.
Each duplicated chromosome is made up of sister chromatids that are attached at a centromere. Even duplicated
and attached, it’s still called a chromosome
Sister Chromatids separate during mitosis (division of the cell’s nucleus).
One half of each pair travels to one end of the cell with the other half traveling to the other end of the cell.
After cytokinesis (division of the cell’s cytoplasm), there are 2 cells that result. Each one has a full complement of the
DNA found in cells.
Chromosome number
We start with your parents, who each have 46 chromosomes in somatic cells.
They (your parents) each produced gametes (sperm and egg) with 23 chromosomes in a process called meiosis
(production of gametes)
The gametes fused to make a zygote (46 chromosomes) (I know, I know…let’s try not to think about that too much!)
Mitosis and cytokinesis produced trillions of cells that make up you.
Gametes produced by you will one day (many years from now…right???) fuse with another gamete to make a zygote.
Both prokaryotes and eukaryotes produces clone cells through a process that includes:
 Receiving a reproductive signal
 Replicating the DNA
 Segregating the DNA
 cytokinesis
Prokaryotes achieve this through binary fission; Eukaryotes achieve this through mitosis
Binary fission – best summed up by a diagram on page 128 (Fig. 7.4)
Eukaryotes - Cell Cycle
Mitosis is just one very small part of the cell cycle.
The longest part of the cell cycle (about 90% of the life of a cell) is spent in interphase.
Figure 7.8 on page 133 shows the cell cycle with the relative time spend in each stage of cell cycle.
Interphase
During interphase the cell grow and copies its chromosomes.
Interphase can be divided into three subphases:
G1 phase (“first gap”) – growth of cell
S phase – (“synthesis”) – growth of cell and replication of the DNA
G2 phase – (“second gap”) – growth of cell and further preparation for cell division
Mitosis – division of a cell’s nucleus
Mitosis is a continuum of changes, but for the sake of study it is broken down into the following steps: prophase,
metaphase, anaphase, telophase
Late interphase
Nucleus is well defined
2 centrosomes (each with a pair of centrioles) appear outside of the nucleus
Microtubules extend from the centrosomes in radial arrays called asters
Chromosomes have already duplicated but are not tightly coiled (they look like a plate of spaghetti)
Prophase
In the nucleus, the chromatin condenses and can be see with a light microscope
The nucleolus (nucleoli) disappear.
Each chromosome appears as two sister chromatids
Mitotic spindle (microtubules extending from the two centrosomes) begins to form.
Centrosomes move away from each other
Later in prophase
The nuclear envelope dissolves
Microtubules of the spindle move across the whole cell and are allowed to interact with the chromosomes
(previously protected by the nuclear membrane)
Chromosomes are very condensed and attach to a microtubule by a region of the chromosome called a kinetochore
Metaphase
Centrosomes are at opposite poles.
Chromosomes line up on metaphase plate (cell’s equator)
Spindle fibers stretch from one end of the cell to the other with the kinetochore attached to the fibers in the middle
Anaphase
Pair centromeres of each chromosome separate and thus pull apart sister chromatids
Chromosomes begin moving to opposite ends of the cell because spindle fibers (microtubules) are shortening.
Microtubules without kinetochores begin to lengthen and stretch the cell out
By the end of anaphase the chromosomes are at either end of the cell.
Telophase and cytokinesis
Cell is further elongated
Nuclear envelope reappears around the set of chromosomes in each end of the cell
Chromatin fiber begins to unwind
In animals the cytoplasm is divided in two in a process called cleavage.
Plants vs. Animals
Cytokinesis in animal cells occurs when a cleavage furrow appears and then the cell pinches in half
Cytokinesis in a plant cell occurs when a cell plate forms between the 2 new nuclei.
Heredity, Variation and Genetics
Heredity is the transmission of traits from one generation to the next.
With heredity, in sexually reproducing organisms, comes variation; that is, offspring differ somewhat from parents
and siblings
Genetics is the study of heredity and variation
Genes are heredity units that contain coded information; Genes are segments of DNA; We inherit thousands of
them from our parents.
Most genes program cells to synthesize specific enzymes and other proteins that produces an organism’s inherited
traits. The programming of these traits in the form of DNA is one of the unifying themes of biology
A gene’s locus (pl: loci) is the specific location of the gene along the length of a chromosome.
Sexual reproduction
Results in greater variation
Two parents give rise to offspring that have unique combinations of genes inherited from the two parents.
Genetic variation is the important consequence
Human Life Cycle
Somatic cell – any cell other than sperm or ovum has 46 chromosomes
Each of the 46 chromosomes has a “match”. That is, another chromosome that is similar (not identical) in length,
centromere position, and staining pattern.
There are 22 of these pairs and they are called homologous chromosomes (also called homologues).
The two chromosomes of each pair of homologues carry genes controlling the same inherited characteristics
Autosomes, sex cells and karyotypes
Exception to the rule of homologues. There are two chromosomes called X and Y that are sex chromosomes.
Human females have a homologous pair X,X.
Human males have one X and one Y
Any chromosome (the other 44) that is not a sex chromosome is called an autosome
The homologues and sex chromosomes are clearly pictured on a karyotype, which is a picture of the chromosomes
arranged in pairs from longest to shortest and ending with the sex chromosomes
Human male karyotype shown by bright field G-banding of chromosomes:
Remember
Cells with a full complement of the chromosomes are called diploid (2n).
Cells with half of the chromosomes are called haploid (n).
Gametes are haploid, somatic cells are diploid.
The union of gametes (called fertilization) will result in a zygote with a diploid number
Meiosis
An overview:
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Chromosomes duplicate
Meiosis I – Homologous chromosome pairs separate
Meiosis II – Sister chromatids separate
Begin with a diploid cell
End with 4 haploid cells
Meiosis I - Interphase
Interphase – Like mitosis, the chromosomes duplicate, attach together at a centromere and are called sister
chromatids. Centrosome also duplicates
Prophase I
lasts longer and is more complex than prophase in mitosis. The chromosomes condense and homologues, each
consisting of two sister chromatids, pair up.
Synapsis – a process that uses a protein called synaptonemal complex to attach the homologous chromosomes
together.
At that point the homologous chromosomes are are together as a tetrad (cluster of four)
At various places along their length, the homologous chromosomes are crossed. These crossings are called chiasmata
(sing. chiasma). The synaptonemal complex breaks down and the chiasma is what holds the chromosomes together
until anaphase I
At the chiasma crossing over occurs which increases the genetic diversity of the offspring
Like prophase in mitosis, the nuclear membrane begins to break down and the spindle appears.
Prophase I can last for days. Occupies 90% of the time required for meiosis
Metaphase I
The tetrads line up on the metaphase plate
Anaphase I
Sister chromatids remain attached, but tetrads pull apart and move to opposite poles separating homologous
chromosomes
Telophase I and Cytokinesis
Each pole has a haploid number of chromosomes, but each chromosome has an identical twin because sister
chromatids are still attached.
Two daughter cells form.
Meiosis II
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Prophase II – a spindle appears
Metaphase II – the chromosomes are attached to the spindle like mitosis metaphase
Anaphase II – the sister chromatids separate
Telophase II and cytokinesis – nuclei form at opposite poles. At the completion of cytokinesis there are four
daughter cells each with the haploid number
Genetic variation
Independent assortment – the tetrad in metaphase I can line up with the maternal homologue on one side or the
maternal homologue on the other. It’s a 50/50 chance and they sort randomly and independent of all the other
tetrads.
The number of combinations possible is represented as 2n with n being the haploid number.
Human possibilities 223 or 8 million!!
The results of alternative arrangements of two homologous chromosome pairs on the metaphase plate in meiosis I
Crossing over
Tetrads allow for crossing over which produces recominant chromosomes
The results of crossing over during meiosis
Random Fertilization
Random nature of fertilization adds to genetic variety.
8 million possible female gametes X 8 million possible male gametes = 64 trillion possible offspring from one mating.
Regulation of cell cycle
Timing and rate of cell division in different parts of a plant or animal are crucial to normal growth, development and
maintenance.
Skin cells divide frequently; Liver cells only divide for repair; Nerve cells never divide.
Cell cycle control is important for us to understand how cells regenerate and how cells lose control of the cell cycle
(cancer).
Cell cycle control system
The cell cycle is regulated using the a set of molecules in the cell that both trigger and coordinate key events in the
cell cycle.
A key point in the process is the restriction point (R) in G1.
If the cell receives the go ahead at that checkpoint it will most likely complete the cell cycle and divide. If not, it will
move to G0 state which is a non dividing state.
Kinases and cyclin
Kinases are proteins that drive the cell cycle.
They are present at all times, but only active when attached to a cyclin.
A cyclin is a protein that is available in a fluctuating concentration
Because of this, the kinases that attach to cyclin are called cyclin-dependent kinases or Cdks.
Other regulators
Growth factors are proteins that are released by the body’s cells that promote cell division.
Ex: PDGF – Platlet –derived growth factors…produced in large amounts at locations of tissue damage, allowing cells
to replicate quickly and repair the wound.
Preventing over-crowding of cells
Lack of growth factors will cause cells to stop dividing. Cells usually do not continue to divide when they become
crowded,
This is called density-dependent inhibition and results when cells are crowded and stop producing growth factors
(perhaps because there is a lack of nutrients)
Anchorage Dependence
In order to continue dividing cells have to be attached to something such as the inside of a container or the extra
cellular matric of a tissue.
This is called anchorage dependence.
Anchorage dependence and density dependent inhibition help you maintain cells at an optimal density and location.
Cancer cells do not respond to control mechanisms.
Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence.
Scientists believe that cancer cells lack the normal checkpoints in their cell cycle.
They will continue to divide as long as nutrients are available. Normal cells will divide 20 to 50 times before they stop
dividing, age and die.
HeLa cells – cells from a cancer patient (Henrietta Lacks) in culture since 1951 continue to divide today.
Henrietta Lacks movie clip
Steps in cancer invasion
1. Transformation – normal cell becomes cancer cell. Immune system will usually find these cells and destroy
them.
2. If it evades the immune system it can proliferate to become a tumor
3. If the tumor remains at its original site it is called benign and can be removed surgically
4. If the tumor becomes invasive enough to to impair the functions of one or more organs, it is a malignant
tumor
Malignant tumors are unusual in many ways
 Excessive proliferation
 Unusual number of chromosomes
 Malfunctions in metabolism
 They lose their attachment to neighboring cells and can easily spread to nearby tissue
 Can break off and enter the blood and lymph vessels and be carried to other parts of the body
This spread of malignant cancer cells is called metastasis (ma-tas-ta-sis).