Module+5_Cell+Reproduction.pdf

UNIT 1: CELL General Biology 1 1s...

UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 CELL REPRODUCTION Topics: 1. Cell Cycle 2. Cell Division: Mitosis 3. Cell Division: Meiosis 4. Mutation 5. Cancer Cells Cell Cycle Cell division is a very important process in all living organisms. During the division of a cell, DNA replication and cell growth also take place. All these processes, i.e., cell division, DNA replication, and cell growth, hence, must take place in a coordinated way to ensure correct division and formation of progeny cells containing intact genomes. The sequence of events by which a cell duplicates its genome, synthesizes the other constituents of the cell and eventually divides into two daughter cells is termed cell cycle. Although cell growth (in terms of cytoplasmic increase) is a continuous process, DNA synthesis occurs only during one specific stage in the cell cycle. The replicated chromosomes (DNA) are then distributed to daughter nuclei by a complex series of events during cell division. These events are themselves under genetic control. The Cell Cycle Actively dividing eukaryote cells pass through a series of stages known collectively as the cell cycle: two gap phases (G1 and G2); an S (for synthesis) phase, in which the genetic material is duplicated; and an M phase, in which mitosis partitions the genetic material and the cell divides. G1 phase. Metabolic changes prepare the cell for division. At a certain point - the restriction point - the cell is committed to division and moves into the S phase. S phase. DNA synthesis replicates the genetic material. Each chromosome now consists of two sister chromatids. G2 phase. Metabolic changes assemble the cytoplasmic materials necessary for mitosis and cytokinesis. M phase. A nuclear division (mitosis) followed by a cell division (cytokinesis). The period between mitotic divisions - that is, G1, S and G2 - is known as interphase. Figure 1. Stages of Cell Cycle 1 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Figure 2. Cell Cycle Regulation Checkpoints Cell Division: Mitosis Mitosis is a form of eukaryotic cell division that produces two daughter cells with the same genetic component as the parent cell. Chromosomes replicated during the S phase are divided in such a way as to ensure that each daughter cell receives a copy of every chromosome. In actively dividing animal cells, the whole process takes about one hour. The replicated chromosomes are attached to a 'mitotic apparatus' that aligns them and then separates the sister chromatids to produce an even partitioning of the genetic material. This separation of the genetic material in a mitotic nuclear division (or karyokinesis) is followed by a separation of the cell cytoplasm in a cellular division (or cytokinesis) to produce two daughter cells. In some single-celled organisms mitosis forms the basis of asexual reproduction. In diploid multicellular organisms, sexual reproduction involves the fusion of two haploid gametes to produce a diploid zygote. Mitotic divisions of the zygote and daughter cells are then responsible for the subsequent growth and development of the organism. In the adult organism, mitosis plays a role in cell replacement, wound healing and tumor formation. Mitosis, although a continuous process, is conventionally divided into five stages: prophase, prometaphase, metaphase, anaphase and telophase. Prophase Prophase occupies over half of mitosis. The nuclear membrane breaks down to form a number of small vesicles and the nucleolus disintegrates. A structure known as the centrosome duplicates itself to form two daughter centrosomes that migrate to opposite ends of the cell. The centrosomes organize the production of microtubules that form the spindle fibers that constitute the mitotic spindle. The chromosomes condense into compact structures. Each replicated chromosome can now be seen to consist of two identical chromatids (or sister chromatids) held together by a structure known as the cohesins at the centromere. 2 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Prometaphase The chromosomes, led by their centromeres, migrate to the equatorial plane in the midline of cell - at right- angles to the axis formed by the centrosomes. This region of the mitotic spindle is known as the metaphase plate. The spindle fibers bind to a structure associated with the centromere of each chromosome called a kinetochore. Individual spindle fibers bind to a kinetochore structure on each side of the centromere. The chromosomes continue to condense. Metaphase The chromosomes align themselves along the metaphase plate of the spindle apparatus. Anaphase The shortest stage of mitosis. The centromeres divide, and the sister chromatids of each chromosome are pulled apart - or 'disjoin' - and move to the opposite ends of the cell, pulled by spindle fibers attached to the kinetochore regions. The separated sister chromatids are now referred to as daughter chromosomes. (It is the alignment and separation in metaphase and anaphase that is important in ensuring that each daughter cell receives a copy of every chromosome.) Telophase The final stage of mitosis, and a reversal of many of the processes observed during prophase. The nuclear membrane reforms around the chromosomes grouped at either pole of the cell, the chromosomes uncoil and become diffuse, and the spindle fibers disappear. Cytokinesis The final cellular division to form two new cells. In plants a cell plate forms along the line of the metaphase plate; in animals there is a constriction of the cytoplasm. The cell then enters interphase - the interval between mitotic divisions. Figure 4. Stages of Mitosis 3 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Cell Division: Meiosis Meiosis is the form of eukaryotic cell division that produces haploid sex cells or gametes (which contain a single copy of each chromosome) from diploid cells (which contain two copies of each chromosome). The process takes the form of one DNA replication followed by two successive nuclear and cellular divisions (Meiosis I and Meiosis II). As in mitosis, meiosis is preceded by a process of DNA replication that converts each chromosome into two sister chromatids. Figure 5. Overview of Meiosis Stages of Meiosis I Meiosis I separates the pairs of homologous chromosomes. In Meiosis I a special cell division reduces the cell from diploid to haploid. 4 | STEM – General Biology 1  UNIT 1 General Biology 1 1st Semester A.Y 2024-2025 Prophase I The homologous chromosomes pair and exchange DNA to form recombinant chromosomes. Crossing over is a process that involves an exchange of genetic material between non-sister chromatids of a bivalent during meiosis I. when chromosomes condense in prophase I, each is drawn closer to its homologous partner, so that non-sister chromatids align along their strength. This tight, parallel orientation facilitates crossing over, a process by which a chromosome and its homologous partner exchange corresponding genetic segments. Crossing over is a normal and frequent process in meiosis. It greatly contributes to variation among individuals that sexually reproduce by shuffling maternal and paternal genes. Thus, crossing over introduces new combinations of alleles in both members of a pair of homologous chromosomes, which results in novel combinations of traits among offspring. Spindle apparatus formed, and chromosomes attached to spindle fibers by kinetochores. Metaphase I Homologous pairs of chromosomes (bivalents) arranged as a double row along the metaphase plate. The arrangement of the paired chromosomes with respect to the poles of the spindle apparatus is random along the metaphase plate. (This is a source of genetic variation through random assortment, as the paternal and maternal chromosomes in a homologous pair are similar but not identical. The number of possible arrangements is 2n, where n is the number of chromosomes in a haploid set. Human beings have 23 different chromosomes, so the number of possible combinations is 223, which is over 8 million.) Anaphase I In anaphase I, pairs of homologous chromosomes separate. One chromosome of each pair moves toward opposite poles, guided by the spindle apparatus. Sister chromatids remain attached at the centromere and move as one unit toward the pole. Telophase I In the beginning of telophase I, each half of the cell has a haploid set of chromosomes; each chromosome still consists of two sister chromatids. The chromosomes become diffuse and the nuclear membrane reforms. Cytokinesis The final cellular division to form two new cells, followed by Meiosis II. Meiosis I is a reduction division: the original diploid cell had two copies of each chromosome; the newly formed haploid cells have one copy of each chromosome. In animal cells, a cleavage furrow forms; in plant cells, a cell plate forms. No chromosome replication occurs between the end of meiosis I and the beginning of meiosis II because the chromosomes are already replicated. Meiosis II separates each chromosome into two chromatids: 5 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Prophase II The chromosomes condense and the spindle microtubules become attached to each sister chromatids as the nuclear membrane breaks up. Metaphase II The chromosomes which are still duplicated, or with two molecules of DNA, are aligned in the middle of the cell. Anaphase II In anaphase II, the sister chromatids of each chromosome are pulled apart and move toward the opposite sides of the cell. Each chromosome is now made up one molecule of DNA. Telophase II During telophase, new nuclear membrane forms around each cluster of chromosomes as the DNA loosens. The cytoplasm often divides at this point to form four haploid (n) cells whose nuclei contain one set of (unduplicated) chromosomes. Mutation Alteration of the nucleotide sequence of the genome of an organism, virus, or extrachromosomal DNA. Changes in the genetic sequence, and they are a main cause of diversity among organisms. These changes occur at many different levels, and they can have widely differing consequences. In biological systems that are capable of reproduction, we must first focus on whether they are heritable; specifically, some mutations affect only the individual that carries them, while others affect all of the carrier organism's offspring, and further descendants. For mutations to affect an organism's descendants, they must: o Occur in cells that produce the next generation o Affect the hereditary material Ultimately, the interplay between inherited mutations and environmental pressures generates diversity among species. 6 | STEM – General Biology 1  UNIT II: BIOLOGICAL MOLECULES General Biology 1 Summer Class Types of Mutation: SMALL-SCALE MUTATIONS Affect DNA at the molecular level by changing the normal sequence of nucleotide base. pairs Occur during the process of DNA replications (either meiosis or mitosis). a. Point mutation – Change in one base in the DNA sequence. b. Substitution - One or more bases in the sequence is replaced by the same number of bases. c. Inversion – A segment of a chromosome is reversed end to end. d. Insertion – A base is added to the sequence. 7 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 e. Deletion – A base is deleted from the sequence. LARGE-SCALE MUTATIONS Affect entire portions of the chromosome. Some large-scale mutations affect only single chromosome, others occur across non- homologous pairs. Entire genes or sets of genes are altered rather than only single nucleotides of the DNA. Mutations involving multiple chromosomes are likely to occur in meiosis, during the prophase I. a. Deletion – Loss of one or more gene(s) from the parent chromosome. b. Duplication – Addition of one or more gene(s) that are already present in the chromosome. c. Inversion – Complete reversal of one or more gene(s) within a chromosome, but the order is backwards from the parent chromosome. d. Insertion – One or more gene(s) are removed from one chromosome and inserted into another non homologous chromosome. e. Translocation – Chromosomes swap one or more gene(s) with another chromosome. f. Non-disjunction – Missing or extra chromosomes. Mutations occur when chromosomes are not separated correctly into the new cells. 8 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Figure 6. Large-scale Mutations Effects of Mutation: The effects of mutations may range from nothing to the unviability of a cell. All mutations affect the proteins that are created during protein synthesis, but not all mutations have a significant impact. Effects of Small-scale Mutation: a. Silent - The nucleotide is replaced, but the codon still produces the same amino acid. b. Missense - The codon now results in a different amino acid, which may or may not significantly alter the protein’s function. c. Nonsense - The codon now results in a “stop” command, truncating the protein at the location where the mutated codon is read; this almost always leads to a loss of protein functionality. Effects of Large-scale Mutation: Effects of large-scale mutations are more obvious than those of small-scale mutations. Duplication of multiple genes causes those genes to be overexpressed while deletions result in missing or incomplete genes. Mutations that change the order of the genes on the chromosome—such as deletions, inversions, insertions and translocations—result in genes that are close together. When certain genes are positioned closely together, they may encode for a “fusion protein”. o A fusion protein is a protein that would not normally exist but is created by a mutation in which two genes were combined. The new proteins give cells a growth advantage, leading to tumors and cancer. Often, large-scale mutations lead to cells that are not viable. The cell dies due to the mutation. Influences of Mutation: 1. Exposure to certain chemicals - Carcinogenic chemicals may cause cancer. 2. Exposure to radiation 3. Retroviruses - Retroviruses such as HIV naturally experience mutations at a much higher rate than other organisms. Engineering Connection: 1. Humans have been genetically modifying plants and animals for thousands of years. Examples: Breeding watermelons to be larger and have fewer seeds. Breeding chickens to have more white meat and more breast meat. 2. Engineers can directly manipulate the genetic code of plants and animals (controversial). Examples: Disease-resistant papaya, vitamin A-rich rice, and drought-tolerant corn 3. Engineers and scientists are currently studying gene editing in the womb. May prevent the child from having diseases and disabilities. 9 | STEM – General Biology 1  UNIT 1: CELL General Biology 1 1st Semester A.Y 2024-2025 Cancer Cells Cancer is essentially a disease of mitosis - the normal 'checkpoints' regulating mitosis are ignored or overridden by the cancer cell. Cancer begins when a single cell is transformed or converted from a normal cell to a cancer cell. Often this is because of a change in function or a DNA mutation that occurs in one of several genes that normally function to control growth. Tumors The cancer cells proliferate to form mass of cancer cells called a tumor. As the tumor grows larger, it begins to release proteins from the cell to attract new blood vessel growth (this is called angiogenesis). Benign: tumor cells remain at original site. Can be removed surgically or killed by radiation, usually eliminating any further cancer development at that site. Malignant: some tumor cells send out signals that tell the body to produce a new blood vessel at the tumor site. These cells not only have their own food and oxygen supply, they also have an avenue for escape to a new part of the body – through the new blood vessel and into bloodstream. Cells that break away from the tumor begin to spread to surrounding tissues (via the bloodstream or lymph) and start new tumors = metastasis. Usually surgery is performed to remove the tumor, followed by radiation and chemotherapy. Cancer Risk Factors: Biological or internal factors, such as age, gender, inherited genetic defects and skin type. Environmental exposure, for instance to radon and UV radiation, and fine particulate matter. Occupational risk factors, including carcinogens such as many chemicals, radioactive materials and asbestos. Lifestyle-related factors