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Cell cycle and cell division. Types of sexual reproduction





 

Plan

 

1. Chromosomes morphology

2. Cell cycle and cell division

3. Mitosis

4. Meiosis

5. Comparison of mitosis and meiosis

6. Types of sexual reproduction

7. Apoptosis

 

 

1. Chromosomes morphology

 

A chromosome is an organized structure of DNA and protein that is found in cells. Each chromosome is made up of DNA tightly coiled many times around proteins called histones that support its structure. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. DNA contains many genes, regulatory elements and other nucleotide sequences. The main functions are storage and transfer of genetic information.

Chromosomes are not visible in the cell’s nucleus–not even under a microscope – when the cell is not dividing. However, the DNA that makes up chromosomes becomes more tightly packed during cell division and is then visible under a microscope. During mitotic cell division, when there is condensation of chromatin, chromosomes are visualized well, so the morphology of eukaryotic chromosomes is studied at metaphase of mitosis. Chromosomes can be in two structural and functional states: in the condensed (spiral) and de-condensed (de-spiral). In interphase chromosomes are partially or fully de-condensed (in this state synthetic processes are active); only chromatin granules can be observed. Chromatin is chromosomal material formed by DNA double helix and associated with histone and non-histone proteins. Most of what researchers know about chromosomes was learned by observing chromosomes during cell division.

Each chromosome has a constriction point called the centromere, which divides the chromosome into two sections, or “arms.” The short arm of the chromosome is labeled the “p arm.” The long arm of the chromosome is labeled the “q arm.” The location of the centromere on each chromosome gives the chromosome its characteristic shape, and can be used to help describe the location of specific genes.

 

Centromere is responsible for contact with the spindle microtubules and for chromosome movement during cell division. In the centromeric regions of chromosomes satellite DNA is localized, which is represented by 171 base pairs (bp) that are tandem repeated. Strands include up to 3 million bp. Kinetochore is protein structure on the centromere of eukaryotic chromosome to which the spindle fibers attach during cell division. Internal region of kinetochore tightly associated with centromeric DNA and external region interact with microtubule spindle.

At the beginning of mitosis rapidly growing and immediately disintegrating microtubules actively "groping" cytoplasm in search of the kinetochore. Sheaf of microtubules attached to a single kinetochore is called K-bundles. With the progress of evolution and complexity of organisms, the number of microtubule-curing unit for kinetochore apparently increased: in yeast species Saccharomyces cerevisiae one microtubule is attached to one kinetochore, in Schizosaccharomyces pombe – from two to four, in flies Drosophila melanogaster – from four to six, in mammals – 20 or more.

 

Fig. Fluorescent electron micrograph of the cell division with mitotic spindle

 

Telomeres are the distal parts of linear chromosomes, which are composed of DNA sequences that are repeated. In humans, telomeres of chromosome are formed by thousands of TTAGGG repeated sequences (more than 3–20 kb). These chromosome areas have the same properties as heterochromatin, namely the presence of highly repeated sequences, the ability to form associates, inactivation of genes in this zone.

Telomeres play an important role in maintaining the stability of chromosomes. Back in the 30th of the XX century Barbara McClintock demonstrated that telomeres protect chromosomes from degradation, preventing telomere-telomere adhesion of chromosome and dicentrics formation.

Specific telomere elements required for sister chromatids division in mitosis and start of DNA replication. It is believed that in certain shortening of telomeres the cell cycle stops and cell aging.

Classification of chromosome on the basis of their relative length and centromere placement is offered by Patau. According to it distinguish 3 groups of chromosomes: metacentric (centromere is located in the center of the chromosome) submetacentric (centromere slightly shifted towards the end of the chromosome) and acrocentric (centromere is located in the distal part of the chromosome).

acrocentricmetacentricsub metacentric

 

Fig. Schematic representation of metacentric, sub metacentric and acrocentric chromosomes

 

The satellites of human acrocentric chromosomes contain nucleolar organizer regions. In these regions, a large number of ribosomal RNA (rRNA) is located. rRNA genes often form tandem pairs that are organized in clusters. Each cluster corresponds to a nucleolar organizer, there are about 10 nucleolar organizer in humans.

Centromeres, telomeres and nucleolar organizer are functionally different parts of chromosome.

 

2. Cell cycle and cell division

The cell cycle is a period of the cell existence from the time it is generated by the division of a progenitor cell to the time it divides or dies is called the cell division cycle or just the cell cycle.

The prokaryotic growth cycle is a relatively simple process. Prokaryotic organisms replicate genetic material, grow until reaching a critical size for some time and then divide, only to repeat the process over and over again. The frequency of replication cycles is determined by the rate of cell growth. Completion of the replication cycle is coordinated with division of the cell into two, it is called binary fission, during which daughter DNA molecules are separated; this is ending of cell cycle.

Throughout the cell cycle bacterial chromosome never changes its length and thickness by spiralization, and microtubule system does not involved in daughter chromosomes separation (microtubules are not found in prokaryotes).

The rate of growth and division in bacterial cells is greatly dependent on environmental factors, such as nutrient availability and temperature. Under optimal environmental conditions, bacterial populations can grow exponentially in size at very rapid rates, increasing from several individuals to several million (106) or billion (109) individuals in hours or days. Duplication rate for E. coli vary from 18 to 180 minutes.

The eukaryotic cell cycle

The duration of the cell cycle varies from 2 to 3 h in a single-celled organism like yeast Saccharomyces cerevisiae to around 24 h in a human cell grown in a culture dish. During this period the cell doubles in mass, duplicates its genome and organelles, and divides these between two new progeny cells.

Interphase occupies about 90 % of the cell cycle and is a period of synthesis and growth, during which the cell doubles in mass but without displaying obvious morphological changes. The process of DNA replication takes place in the synthetic, or S, phase, of the cell cycle. The chromosomes are actually separated during mitosis or M phase. The S and M phases are separated by two gap phases, or G-phases, G1 and G2, in which normal processes of cellular activity and metabolism is. Together, G1, S and G2 constitute interphase. Once interphase is complete, the cell enters mitosis (M phases), which is a brief period of profound structural changes. The focal point of mitosis is the behavior of the chromosomes.

 

 

Fig. The eukaryotic cell cycle

 

 

3. Mitosis

Mitosis itself is only one of several phases of the eukaryotic cell cycle. Mitosis is designed to produce two progeny cells each containing an identical set of the progenitor cell’s chromosomes. Mitosis is divided into five stages, each of which is characterized by changes in the appearance of the chromosomes and their organization with respect to a cellular structure, called the mitotic spindle, which is responsible for their segregation. The stages of mitosis are shown diagrammatically in Figure. One pair of chromosomes is shown, one chromosome originating from the father and one from the mother.

 

 

Fig. Stages of mitosis

 

Prophase. The first evidence of mitosis in most cells is the compaction of the threads of chromatin that existed through interphase into compact chromosomes that are visible in the light microscope. As the chromosomes compact, each can be seen to be paired structures composed of two chromatids. This is the visible effect of the DNA molecules having been replicated in interphase. Chromosome condensation reduces the chance of long DNA molecules becoming tangled (запутанный) and broken. Each chromosome has a constriction called the kinetochore, a structure that forms around a region rich in satellite DNA called the centromere. The kinetochore is the point of attachment of the chromosome to the spindle. At the same time as the chromosomes are condensing within the nucleus, the centrosomes, which lie on the cytoplasmic side of the nuclear envelope, begin to separate to establish the mitotic spindle.

Prometaphase. At the breakdown of the nuclear envelope, the chromosomes become free to interact with the forming spindle. Microtubule assembly from the centrosomes is random and dynamic. The growing ends of individual microtubules make chance contact with and are captured by the kinetochores. Because of the random nature of these events, the kinetochores of chromatid pairs are initially associated with different numbers of microtubules, and the forces acting upon each chromosome are unbalanced. Initially, therefore, the spindle is highly unstable and chromosomes make frequent excursions toward and away from the poles. Gradually, a balance of forces is established and the chromosomes become aligned at the equator, with the kinetochores of each member of a chromatid pair oriented toward opposite poles.

Metaphase. Metaphase is the most stable period of mitosis. The system is at steady state with the chromosomes lined up half facing in one direction and half the other. The metaphase spindle consists of two major groups of microtubules: those connecting the chromatids to the poles and a second group extending from each pole toward the other. Others overlap at the spindle equator. All chromatids become attached to the spindle, all of the chromosomes enter anaphase together.

Anaphase. The trigger for the separation of the paired chromatids and the start of their journey to the spindle poles is the degradation of the protein cohesin, which acts as the glue holding the pairs of chromatids together. In anaphase A the microtubules holding the chromosomes shorten, pulling the chromosomes to the spindle poles. The chromosomes move as a “V” with the kinetochores, at which the force for chromosome movement is applied, leading the way. In contrast, in anaphase B the microtubules that overlap at the spindle equator lengthen, extending the distance between the poles. Compared to other forms of cell motility, the movement of chromosomes at anaphase is extremely slow, less than 1 μ m per minute.

Fig. Anaphase only begins when all chromatids are correctly attached to spindle microtubules.

 

Telophase. This stage is the reversal for many of prophase events; the chromosomes de-condense, the spindle disassembles, the nuclear envelope reforms, Golgi apparatus and endoplasmic reticulum reform, and the nucleolus reappears. Each progeny nucleus contains one complete copy of the genome from the father and one copy from the mother.

Cytokinesis. During the last stages of telophase, the cell itself divides. In animal cells, a cleavage furrow (борозда) made of actin and myosin constricts the middle of the cell; in plants, a structure called the phragmoplast forms at the equator of the spindles where it directs the formation of a new cell wall.

 

Duration of mitosis. Actually mitosis often occurs relatively quickly. The average duration is 1–2 hours; this is only about 10 % of the cell cycle time. In animal cells mitosis usually occurs quickly and lasts in average 30–60 minutes, while in plant cells the average duration of mitosis is 2–3 hours. The most intensive mitosis occurs in embryonic cells (10–40 minutes in cleaving eggs).

Duration of mitosis depends on several factors: the size of a dividing cell, its ploidy, number of nucleus. The frequency of cell division is also dependent on the degree of cell differentiation and its specific functionality. Thus, the neurons cells or of human skeletal muscle cells are not divided at all, the liver cells are usually divided every one or two years, and some of the intestinal epithelial cells are divided more than two times a day.

Rate of cell division depends on the environmental conditions, particularly temperature. Higher ambient temperature within physiological limits increases the rate of mitosis; it may be explained by the laws of chemical reactions kinetics.

 

Fig. Events during mitotic stages

4. Meiosis

Meiosis, maturation or reduction division, makes sexual reproduction possible. Meiosis produces haploid cells, which contain just one member of every chromosome pair, a single round of DNA synthesis is followed by two nuclear divisions. In all animals, specialized germ cells in the reproductive organs undergo meiosis to produce haploid gametes (sperm and egg), which then fuse during sexual reproduction to create new diploid embryos. Plants, fungi, and some protists also perform meiosis. In plants, meiosis creates a multicellular haploid organism, called a gametophyte. Gametes are produced by mitosis of the gametophyte, which then fuse to form the embryo.

Two meiotic divisions are separated by interkinesis (it may be absent). The first division has very long, differentiated for stages prophase, prophase II and metaphase II may be absent, doubling the DNA occurs just before the first division.

Meiosis mixes maternal and paternal genes to give new combinations of traits through two mechanisms: (1) independent assortment of chromosomes at both of the meiotic divisions; and (2) exchange of chromosomal regions by crossing over.

There are three types of meiosis:

zygote, or initial (many fungi and algae) occurs immediately after fertilization and results in the formation of haploid thallus or mycelium,

gamete, or terminal (all multicellular animals and some lower plants) occurs in the genital organs and leads to gametes formation,

spore, or intermediate (higher plants) occurs before flowering and results in haploid gametophyte formation

Walther Flemming (1843 – 1905) was a German biologist and a founder of cytogenetics. Flemming investigated the process of cell division and the distribution of chromosomes to the daughter nuclei, a process he called mitosis. Together with August Weismann's (1834–1914) he discovered meiosis in animals (1882), suggested cell theory with Theodor Schwann (1808–1890) and Matthias Schleiden's (1804–1881), developed the first genetic maps with Alfred Sturtevant's (1866–1945).

Eduard Adolf Strasburger (1844 – 1912) was a Polish-German professor, one of the most famous botanists of the 19th century. He came up with one of the modern laws of plant cytology: "New cell nuclei can only arise from the division of other nuclei" and originated the terms cytoplasm and nucleoplasm. Together with Walther Flemming and Edouard van Beneden he elucidated chromosome distribution during cell division, in 1888 discovered meiosis in plants.

Date: 2015-09-02; view: 573; Нарушение авторских прав; Помощь в написании работы --> СЮДА...



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