Cell division in bacteria
- Bacteria have a single chromosome: a plasmid – a single round molecule
- A plasmid is a DNA molecule that is separate from, and can replicate independently of, the chromosomal DNA. They are double stranded and, in many cases, circular. Plasmids usually occur naturally in bacteria, but are sometimes found ineukaryotic organisms (e.g., the 2-micrometre-ring in Saccharomyces cerevisiae).
- The nucleoid (meaning nucleus-like) is an irregularly-shaped region within the cell of prokaryotes which has nuclear material without a nuclear membrane and where the genetic material is localized. The genome of prokaryotic organisms generally is a circular, double-stranded piece of DNA, of which multiple copies may exist at any time. The length of a genome widely varies, but generally is at least a few million base pairs. Storage of the genome within a nucleoid can be contrasted against that within eukaryotes, where the genome is packed into chromatin and sequestered within a membrane-enclosed organelle called the nucleus.
- DNA replication in bacteria begins at the origin of replication.
- Replication of the chromosome continues and each origin becomes attached separately to the plasma membrane.
- Each of the 2 new origins moves to the sides and attach to/associate with the plasma membrane.
- Need to make sure each daughter cell gets a copy of the DNA. The association of the origin of replication with the plasma membrane allows this to occur.
- Once replication is complete, the plasma membrane grows inward to separate the 2 new daughter cells and a new cell wall is deposited
Human genetic material
- In human cells, 3 metres of DNA (3 x 10^9 bp) fit into a cell that has a diameter of ~0.01 mm. This involves packaging of the DNA into chromosomes
- DNA is complexed with histone proteins to form chromatin
- The histone proteins:
- Maintain the structure of the chromosome
- Help regulate the activity of genes: if it’s less tightly packed, then it can more readily be used. This controls access and helps regulate which genes are accessed in which cell. Think of the chromatin like “packing shelves”
- The chromatin is folded and coiled.
- Condensation of the chromatin occurs in preparation for cell division
- Chromosomes contain 1) DNA and 2) histones to help package up the DNA to fit into the nucleus
- There are 23 chromosomes, ordered based on size: 1 is the largest and 23 is the smallest
- Autosomes = sex-independent chromosomes (girls and boys have the same)
- X and Y Chromosomes = sex-dependent chromosomes (depends on your sex)
- There are 2 copies of each chromosome: 1 from Mum, one from Dad. We need exactly 2 copies in order for us to function properly
- Down’s syndrome = trisomy of chromosome 21
Duplication and ditribution of chromosomes during mitosis
- Cell division requires the replication of DNA (=replication of chromosomes) and all the cellular components
- Once the DNA is replicated it condenses, making the chromosomes shorter and thicker
- Mitosis is the division of the cell nucleus
- Cytokinesis is the division of the cytoplasm
- Each chromatid is made of 1 double stranded DNA helix. The two strands are antiparallel.
Homologous chromosomes are chromosome pairs of the same length, centromere position, and staining pattern, with genes for the same characteristics at corresponding loci. One homologous chromosome is inherited from the organism's mother; the other from the organism's father
They pair (synapse) during meiosis, or cell division that occurs as part of the creation of gametes. Each chromosome pair contains genes for the same biological features, such as eye color, at the same locations (loci) on the chromosome. Each pair, however, can contain the same allele (both alleles for blue eyes) or different alleles (one allele for blue eyes and one allele for brown eyes) for each feature.
Non-homologous chromosomes representing all the biological features of an organism form a set, and the number of sets in a cell is called ploidy. In diploid organisms (most plants and animals), each member of a pair of homologous chromosomes is inherited from a different parent. But polyploid organisms have more than two homologous chromosomes. Homologous chromosomes are similar in length, except for sex chromosomes in several taxa, where the X chromosome is considerably larger than the Y chromosome. These chromosomes share only small regions ofhomology.
Humans have 22 pairs of homologous non-sex chromosomes (called autosomes), and one pair of sex chromosomes, making a total of 46 chromosomes in a genetically normal human. Each member of a pair is inherited from one of the two parents. In addition to the 22 pairs of homologous autosomes, female humans have a homologous pair of sex chromosomes (2 Xs), while males have an X and a Y chromosome.
Homologous chromosomes are two pairs of sister chromatids that have gone through the process of crossing over and meiosis. In this process the homologous chromosomes cross over (not the sister chromatids) each other and exchange genetic information. This causes each final cell of meiosis to have genetic information from both parents, a mechanism for genetic variation. The homologous chromosomes are similar in length.
A chromatid is one of the two identical copies of DNA making up a duplicated chromosome, which are joined at their centromeres, for the process of cell division (mitosis or meiosis). They are called chromatids so long as they are joined by the centromeres. When they separate (during anaphase of mitosis and anaphase 2 of meiosis), the strands are called sister chromatids.
In other words, a chromatid is "one-half of a replicated chromosome". It should not be confused with the ploidy of an organism, which is the number of homologous versions of a chromosome.
A chromosome is an organized structure of DNA and protein that is found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.
The cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication (replication). In cells without a nucleus (prokaryotic), the cell cycle occurs via a process termed binary fission. In cells with a nucleus (eukaryotes), the cell cycle can be divided in two brief periods: interphase—during which the cell grows, accumulating nutrients needed for mitosis and duplicating its DNA—and the mitosis (M) phase, during which the cell splits itself into two distinct cells, often called "daughter cells". The cell-division cycle is a vital process by which a single-celled fertilized egg develops into a mature organism, as well as the process by which hair, skin, blood cells, and some internal organs are renewed.
- Interphase: growth and replication of cellular components. Includes G1, G2, and S phases.
- The tightly packed chromosomes from mitosis are unravelled for access during DNA replication, which occurs in the S phase.
- Mitotic phase (M): nucleus divides and chromosomes are distributed to daughter cells (mitosis) and the cytoplasm divides into two daughter cells (cytokinesis)
- The chromosomes are tightly packed to allow the DNA to be separated into two new cells.
- [Interphase: The mitotic phase is a relatively short period of the cell cycle. It alternates with the much longer interphase, where the cell prepares itself for cell division. Interphase is therefore not part of mitosis. Interphase is divided into three phases, G1 (first gap), S (synthesis), and G2 (second gap). During all three phases, the cell grows by producing proteins and cytoplasmic organelles. However, chromosomes are replicated only during the S phase. Thus, a cell grows (G1), continues to grow as it duplicates its chromosomes (S), grows more and prepares for mitosis (G2), and finally it divides (M) before restarting the cycle]
- [Preprophase – in plants: In plant cells only, prophase is preceded by a pre-prophase stage. In highly vacuolated plant cells, the nucleus has to migrate into the center of the cell before mitosis can begin. This is achieved through the formation of aphragmosome, a transverse sheet of cytoplasm that bisects the cell along the future plane of cell division. In addition to phragmosome formation, preprophase is characterized by the formation of a ring of microtubules andactin filaments (called preprophase band) underneath the plasma membrane around the equatorial plane of the future mitotic spindle. This band marks the position where the cell will eventually divide. The cells of higher plants (such as the flowering plants) lack centrioles; instead, microtubules form a spindle on the surface of the nucleus and are then being organized into a spindle by the chromosomes themselves, after the nuclear membrane breaks down. The preprophase band disappears during nuclear envelope disassembly and spindle formation in prometaphase]
- Normally, the genetic material in the nucleus is in a loosely bundled coil called chromatin. At the onset of prophase, chromatin condenses together into a highly ordered structure called a chromosome. Since the genetic material has already been duplicated earlier in S phase, the replicated chromosomes have two sister chromatids, bound together at the centromere by the cohesion complex. Chromosomes are typically visible at high magnification through a light microscope.
- Close to the nucleus are structures called centrosomes, which are made of a pair of centrioles. The centrosome is the coordinating center for the cell's microtubules. A cell inherits a single centrosome at cell division, which replicates before a new mitosis begins, giving a pair of centrosomes. The two centrosomes nucleate microtubules (which may be thought of as cellular ropes or poles) to form the spindle by polymerizing soluble tubulin. Molecular motorproteins then push the centrosomes along these microtubules to opposite sides of the cell. Although centrioles help organize microtubule assembly, they are not essential for the formation of the spindle, since they are absent from plants, and centrosomes are not always used in meiosis.
- The condensed (shortened and thickened) chromosomes are now visible down the microscope.
- The nuclear envelope disassembles and microtubules invade the nuclear space. This is called open mitosis, and it occurs in most multicellular organisms. Fungi and some protists, such as algae or trichomonads, undergo a variation called closed mitosis where the spindle forms inside the nucleus, or its microtubules are able to penetrate an intact nuclear envelope.
- Each chromosome forms two kinetochores at the centromere, one attached at each chromatid. A kinetochore is a complex protein structure that is analogous to a ring for the microtubule hook; it is the point where microtubules attach themselves to the chromosome. Although the kinetochore structure and function are not fully understood, it is known that it contains some form of molecular motor.When a microtubule connects with the kinetochore, the motor activates, using energy from ATP to "crawl" up the tube toward the originating centrosome. This motor activity, coupled with polymerisation and depolymerisation of microtubules, provides the pulling force necessary to later separate the chromosome's two chromatids.
- When the spindle grows to sufficient length, kinetochore microtubules begin searching for kinetochores to attach to. A number of nonkinetochore microtubules find and interact with corresponding nonkinetochore microtubules from the opposite centrosome to form the mitotic spindle. Prometaphase is sometimes considered part of prophase.
- In the fishing pole analogy, the kinetochore would be the "hook" that catches a sister chromatid or "fish". The centrosome acts as the "reel" that draws in the spindle fibers or "fishing line". It is also one of the main phases of mitosis because without it cytokinesis would not be able to occur.
- As microtubules find and attach to kinetochores in prometaphase, the centromeres of the chromosomes convene along the metaphase plateor equatorial plane, an imaginary line that is equidistant from the two centrosome poles. This even alignment is due to the counterbalance of the pulling powers generated by the opposing kinetochores, analogous to a tug-of-war between people of equal strength. In certain types of cells, chromosomes do not line up at the metaphase plate and instead move back and forth between the poles randomly, only roughly lining up along the midline. Metaphase comes from the Greek μετα meaning "after."
- Because proper chromosome separation requires that every kinetochore be attached to a bundle of microtubules (spindle fibres), it is thought that unattached kinetochores generate a signal to prevent premature progression to anaphase without all chromosomes being aligned. The signal creates the mitotic spindle checkpoint.
- During mitosis or meiosis, the spindle checkpoint prevents anaphase onset until all chromosomes are properly attached to the spindle. To achieve proper segregation, the two kinetochores on the sister chromatids must be attached to opposite spindle poles (bipolar orientation). Only this pattern of attachment will ensure that each daughter cell receives exactly one copy of the chromosome.
- When every kinetochore is attached to a cluster of microtubules and the chromosomes have lined up along the metaphase plate, the cell proceeds to anaphase (from the Greek ανα meaning “up,” “against,” “back,” or “re-”).
- Two events then occur: first, the proteins that bind sister chromatids together are cleaved, allowing them to separate. These sister chromatids, which have now become distinct sister chromosomes, are pulled apart by shortening kinetochore microtubules and move toward the respective centrosomes to which they are attached. Next, the nonkinetochore microtubules elongate, pulling the centrosomes (and the set of chromosomes to which they are attached) apart to opposite ends of the cell. The force that causes the centrosomes to move towards the ends of the cell is still unknown, although there is a theory that suggests that the rapid assembly and breakdown of microtubules may cause this movement.
- These two stages are sometimes called early and late anaphase. Early anaphase is usually defined as the separation of the sister chromatids, while late anaphase is the elongation of the microtubules and the chromosomes being pulled farther apart. At the end of anaphase, the cell has succeeded in separating identical copies of the genetic material into two distinct populations.
- Telophase (from the Greek τελος meaning "end") is a reversal of prophase and prometaphase events. It "cleans up" the after effects of mitosis. At telophase, the nonkinetochore microtubules continue to lengthen, elongating the cell even more. Corresponding sister chromosomes attach at opposite ends of the cell. A new nuclear envelope, using fragments of the parent cell's nuclear membrane, forms around each set of separated sister chromosomes. Both sets of chromosomes, now surrounded by new nuclei, unfold back into chromatin. Mitosis is complete, but cell division is not yet complete.
- Nuclear envelope protects DNA – reactions in the cytosol could damage the DNA
- Cytokinesis is often mistakenly thought to be the final part of telophase; however, cytokinesis is a separate process that begins at the same time as telophase. Cytokinesis is technically not even a phase of mitosis, but rather a separate process, necessary for completing cell division. In animal cells, a cleavage furrow(pinch) containing a contractile ring develops where the metaphase plate used to be, pinching off the separated nuclei. In both animal and plant cells, cell division is also driven by vesicles derived from the Golgi apparatus, which move along microtubules to the middle of the cell. In plants this structure coalesces into a cell plate at the center of the phragmoplast and develops into a cell wall, separating the two nuclei. The phragmoplast is a microtubule structure typical for higher plants, whereas some green algae use a phycoplast microtubule array during cytokinesis. Each daughter cell has a complete copy of the genome of its parent cell. The end of cytokinesis marks the end of the M-phase.
- Involves membrane vesicles lining up in the middle of the cell to produce a new plasma membrane.
- Cell cycle checkpoints ensure that the cell only divides when it should:
- DNA is replicated
- DNA is not damaged (avoidance of cancer)
- Sufficient nutrients are available to support new cells
- This regulates the cell cycle
- We stop progression unless certain conditions are met.
- In humans we don’t initiate division unless certain signals are received to initiate it as well
- Many cancers arise because checkpoints don’t work
- Some viruses damage checkpoint proteins to force the cell to divide when it wouldn’t normally do so
- These checkpoints are in G1, G2 and in M (metaphase – spindle checkpoint to make sure there is only 1 copy of each chromosome in each cell)
- Programmed cell death
- Balances mitosis – we need space: birth, death of cells maintains homeostasis
- Removes unwanted cells during development
- It also removes damaged cells throughout life (e.g. cells that could potentially become cancer cells).
- If a checkpoint fails, the cell dies by apoptosis
- In utero, we all have webbed fingers and toes. Apoptosis cleans out webbing (these cells are programmed to die)
- Somatic cells (all cells except sperm and ova) have 46 chromosomes (2n). (2 copies of each chromosome).
- Gamtes (sperm and ova) have half this number (n – haploid) so that when they fuse at fertilisation, the correct chromosome number will be maintained
- The process for producing cells with half the number of chromosomes (2n => n) is called meiosis
Genetic variability and crossing over
- Explains why you and your siblings aren’t the same
- We get greater genetic diversity, which is the whole point of sexual reproduction
- Genetic variation is inherent in the process of meiosis (even without crossing over):
- Crossing over gives even more variability
- During meiosis, chromosomes can exchange genetic information – “crossing over”
- This produces recombinant chromosomes, which now combine information inherited from each parent.
- This produces even more genetic variability.