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Chromosome
Chromosome
As mentioned in the previous section, DNA carries the genetic information of a cell. Actually, DNA do not exist in the nucleus on its own. Instead, it associates with some proteins called histones. They are a group of positively charged proteins that are attached to the negatively charged DNA backbone to enable the wrapping of DNA. In addition to organizing DNA and making it more compact, histones play an important role in determining which genes are active. The complex of DNA and histones and other structural proteins is called chromosomes or chromatin. During the interphase of a cell (phase which cell is not dividing), chromosome is said to be decondensed. Chromosomes exist as chromatin, a state when DNA is a mass of uncoiled, long and thin fibres. This is because enzymes can only work on the DNA when it is uncoiled. When chromatin condenses, it’s broken up into separate, linear pieces called chromosomes. In each chromosome, there is a pair of sister chromatids. At the centre is the centromere.
Each species has its own characteristic number of chromosomes in one cell. Humans, for instance, have 46 chromosomes in a typical body cell (somatic cell), while dogs have 78. Like many species of animals and plants, humans are diploid (2n), meaning that most of their chromosomes come in matched sets known as homologous pairs. The 46 chromosomes of a human cell are organized into 23 homologous pairs. Below is a karyotype of human.
Karyotype of a human male
Gametes, which are cells involved in sexual reproduction, such as sperm and eggs, have only one homologous chromosome from each pair. Gametes are said to be haploid (1n). When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom (maternal chromosome) and the other from your dad (paternal chromosome).
The two chromosomes in a homologous pair carry the same type of genetic information. They have the same genes in the same locations. However, they may not have the same versions of genes. That's because you may have inherited two different gene versions from your mom and your dad. The inheritance will be discussed in future units.
The sex chromosomes, X and Y, determine a person's biological sex. XX specifies female and XY specifies male. These chromosomes are not true homologues and are an exception to the rule of the same genes in the same places. Aside from small regions of similarity needed during gamete production, the X and Y chromosomes are different and carry different genes. The 44 non-sex chromosomes in humans are called autosomes.
Types of cell division
Types of cell division
There are two major types of cell division, namely mitotic and meiotic cell division. The similarities and differences are illustrated in the table below.
Binary fission
(Contents from Wikipedia, Fission (biology))Binary fission
Organisms in the domains of Archaea and Bacteria reproduce with binary fission. This form of asexual reproduction and cell division is also used by some organelles within eukaryotic organisms (e.g., mitochondria). Binary fission results in the reproduction of a living prokaryotic cell (or organelle) by dividing the cell into two parts, each with the potential to grow to the size of the original.
Below are the main steps of binary fission:
- The bacterium before binary fission is when the DNA is tightly coiled.
- The DNA of the bacterium has uncoiled and duplicated.
- The DNA is pulled to the separate poles of the bacterium as it increases the size to prepare for splitting.
- The growth of a new cell wall begins to separate the bacterium.
- The new cell wall fully develops, resulting in the complete split of the bacterium.
- The new daughter cells have tightly coiled DNA rods, ribosomes, and plasmids; these are now brand new organisms.
Main steps of binary fission
(Ecoddington14 / CC-BY-SA-3.0)Mitosis
Mitosis
(Some contents from Wikipedia, Mitosis)
There are four phases of mitosis. Each phase is used to describe what kind of change the cell is going through. The phases are prophase, prometaphase, metaphase, anaphase and telophase.
Interphase
Interphase
Interphase consists of G1, S and G2 phase. Details about these phases were previously discussed in Course 0. Interphase takes up 90% of the time in a cell cycle. Chromosomes are in the state of chromatin, meaning chromosomes are invisible. Just before the division, the nuclear membrane breaks down and chromosomes appear.
Prophase
Prophase
During prophase, chromosomes in the nucleus condense. The chromatin, which is the complex of DNA and proteins, coils and becomes increasingly compact, resulting in the formation of visible chromosomes. Spindle fibers begin to form a bridge between the ends of the cell.
Metaphase
Metaphase
During metaphase, the sister chromatids are aligned along the metaphase plate (the equator) by the pushing and pulling of the attached kinetochore microtubules, similar to a game of "tug of war". Both sister chromatids stay attached to each other at the centromere. The chromosomes line up on the cell's equator, or center line, and are prepared for division.
Anaphase
Anaphase
During anaphase, the sister chromatids split apart and move from the cell's metaphase plate to the poles of the cell. The kinetochore is attached to the centromere. The microtubules hold on to kinetochore and shorten in length. Another group of microtubules, the non-kinetochore microtubules, do the opposite. They become longer. The cell begins to stretch out as the opposite ends are pushed apart.
Telophase
Telophase
Telophase is the final stage in mitosis: the cell itself is ready to divide. One set of chromosomes is now at each pole of the cell. Each set is identical. The spindle fibers begin to disappear, and a nuclear membrane forms around each set of chromosomes. Also a nucleolus appears within each new nucleus and single stranded chromosomes uncoil into invisible strands of chromatin.
Meiosis
Meiosis
Meiosis is used for the production of gametes. Its goal is to make daughter cells with exactly half as many chromosomes as the starting cell and production some variation in daughter cells at the same time.
Meiosis consists of 2 division, meiosis I and meiosis II, resulting 4 daughter cells.
Prophase I
Prophase I
As in mitosis, condensing from chromatin, chromosomes are visualised. Nucleus membrane breakdown. In meiosis, each chromosome aligns with its homologous partner so that the two match up at corresponding positions along their full lengths. Because of the pairing, crossing over may occur. Crossing over will be discussed in the next section.
Metaphase I
Metaphase I
Paired homologous pairs now move together to line up at the metaphase plate, waiting for separation. As kinetochore microtubules from both centrosomes attach to their respective kinetochores, the paired homologous chromosomes align along an equatorial plane that bisects the spindle. When the homologous pairs line up on the metaphase plate, the orientation of each pair is random. That is, paternal and maternal chromosomes can occupy positions on the left or right side, and different homologous pairs can occupy any position along the equator.
Anaphase I
Anaphase I
Kinetochore microtubules shorten, pulling homologous chromosomes (which consist of a pair of sister chromatids) to opposite poles. The cell elongates in preparation for division down the center. Unlike in mitosis, only the cohesin from the chromosome arms is degraded while the cohesin surrounding the centromere remains protected. This allows the sister chromatids to remain together while homologues are segregated.
Telophase I
Telophase I
The chromosomes arrive at opposite poles of the cell. The microtubules that make up the spindle network disappear, and a new nuclear membrane surrounds each haploid set. The chromosomes uncoil back into chromatin. Cytokinesis, the pinching of the cell membrane in animal cells or the formation of the cell wall in plant cells, occurs, completing the creation of two daughter cells. Sister chromatids remain attached during telophase I.
Prophase II
Prophase II
Nucleolus disappears. Chromosomes condense and the nuclear envelope breaks down. The centrosomes move apart, the spindle forms between them, and the spindle microtubules begin to capture chromosomes. The two sister chromatids of each chromosome are captured by microtubules from opposite spindle poles.
Metaphase II
Metaphase II
The centromeres contain two kinetochores that attach to spindle fibers from the centrosomes at opposite poles. The new equatorial metaphase plate is rotated by 90 degrees when compared to meiosis I, perpendicular to the previous plate
Anaphase II
Anaphase II
The remaining centromeric cohesin is cleaved allowing the sister chromatids to segregate. The sister chromatids by convention are now called sister chromosomes as they move toward opposing poles.
Telophase II
Telophase II
Similar to telophase I, and is marked by decondensation and lengthening of the chromosomes and the disassembly of the spindle. Nuclear envelopes reform and cleavage or cell plate formation eventually produces a total of four daughter cells, each with a haploid set of chromosomes.
Source of genetic variations
Source of genetic variations
There are a few factors contributing to the genetic variations resulted in daughter cells after meiosis.
Crossing over
Crossing over
Crossing over is a process which the two chromosomes of a homologous pair exchange equal segments with each other. Crossing over occurs in the prophase I through metaphase II of meiosis I. While the homologous chromosomes are paired, breaks occur at corresponding points in two of the non-sister chromatids, i.e., in one chromatid of each chromosome. Since the chromosomes are homologous, breaks at corresponding points mean that the segments that are broken off contain corresponding genes. The broken sections are then exchanged between the chromosomes to form complete new units, and each new recombined chromosome of the pair can go to a different daughter gamete. Under the microscope, a crossover has the appearance of an X and is called a chiasma. These non-sister chromatids remain physically connected at chiasmata, holding the homologous chromosomes together.
Crossing over involves the exchange of chromosome segments between non-sister chromatids during the production of gametes. This assorts (or shuffles) the genes on chromosomes, so that the gametes carry combinations of genes different from either parent. This has the overall effect of increasing the variety of phenotypes present in a population.
The large-scale effect of crossing over is to spread variation through a population. This is the main consequence of sexual reproduction compared to asexual reproduction. The main advantage to the parents is the greater variety in their offspring. This increases the likelihood that some of the offspring will survive and reproduce. However, this has been the subject of much investigation and discussion.
Independent assortment of chromosomes
Independent assortment of chromosomes
When two or more characteristics are inherited, the separation of one homologue does not affect the separation of another homologue into the gametes that are produced during meiosis. This phenomenon is called independent assortment. Genetic variation results from independent assortment because it results in the shuffling of chromosomes into various gametes. Again, the result is a shuffling of genetic material. Thus, genetic variation increases.
Crossing over occurs when homologous chromosomes exchange genetic information. Thus, chromosomes are formed that contain genes from both parents. In a human cell, the random orientation of homologous chromosome pairs alone allows for over 8 millions different types of possible combination of chromosomes in a gametes.
Random fertilization
Random fertilization
Random fertilization occurs in fusion of two gametes of opposing sex. Random fertilization means that the collection of genes within one gamete do not give that gamete a lesser or better chance than any of the other sperm cells of fusing with an egg cell to produce a zygote. This does not really generate new variation within the species, but it does preserve the variation that is already there. It does this by giving every version of every gene a more fair shot of being passed on to the next generation.
Cell differentiation
Cell differentiation
Different cells carry out different function in multicellular organisms. For example, neurons transmit and process nerve impulses, phagocytes engulf and digest phagocytes and hepatocytes synthesise materials for metabolism. Although all cells in one organism have the same set of full genome, not the entire genome is expressed in all cell types. This is how cells at different locations in our body develop differently from others and carry out different functions. Cell are said to specialise in certain function after differentiation.
In the early stage of an embryo (a blastocyst), all cells are undifferentiated. They do not have specific roles. These cells are called embryonic stem cells. Later while the embryo continue to develop, some body cells receive some chemical signals, promoting and suppressing the expression of certain genes in their genome. Different cells receive different signals, hence develop differently. This process is known as cell differentiation. Stem cells are said to differentiated to become somatic cells.
Stem cells do not only exist in the embryo stage. Adult humans also possess stem cells, called somatic stem cells. They are stem cells that have differentiated half-way. In other words, somatic stem cells can develop into more than one cell types upon cell signals, but cannot differentiate into any cell types. These somatic stem cells can be found in bone marrow, blood and adipose tissue, etc. Working with progenitor cells, these cells repair system in the body and replenish adult body cells.
Cancer
Cancer
(part of the content is from Cell Division and Cancer, Scitable)
In normal cells, hundreds of genes intricately control the process of cell division. Normal growth requires a balance between the activity of those genes that promote cell proliferation and those that suppress it. It also relies on the activities of genes that signal when damaged cells should undergo apoptosis.
Cells become cancerous after mutations accumulate in the various genes that control cell proliferation. Scientific comparison of multiple tumor types has revealed that certain genes are mutated in cancer cells more often than others. For instance, growth-promoting genes are commonly mutated in cancer cells, becoming super-active and producing cells that are too strongly stimulated by growth receptors.
Other cancer-related mutations inactivate the genes that suppress cell proliferation or those that signal the need for apoptosis. These genes, known as tumor suppressor genes, normally function like brakes on proliferation, and both copies within a cell must be mutated (possess two recessive alleles) in order for uncontrolled division to occur. For example, many cancer cells carry two mutant copies of the gene that code for a multifunctional protein that normally senses DNA damage and acts as a transcription factor for checkpoint control genes. The mutations in tumor suppressor genes may cause the cell to divide in an uncontrolled manner.
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