Cell Division PDF

MRS ANDERSON CELL DIVISION DIPLOID is a nucleus, cell or organism that contains the full set of paired chromosomes (2n). For example, in Humans 46(23 pairs) of chromosomes. HAPLOID is a nucleus, cell or organism that contains HALF the number of chromosomes. For exam...

MRS ANDERSON CELL DIVISION DIPLOID is a nucleus, cell or organism that contains the full set of paired chromosomes (2n). For example, in Humans 46(23 pairs) of chromosomes. HAPLOID is a nucleus, cell or organism that contains HALF the number of chromosomes. For example, humans it would be 23 chromosomes. DNA- Deoxyribonucleic Acid- this is a double stranded molecule that is inside the chromosomes and contains the genetic information. CHROMOSOME is the genetic structure that is found inside the nucleus of a cell and contains the DNA. The chromosome is self- replicating (can make copies of itself). Chromosomes come in identical pairs called HOMOLOGOUS PAIRS. So humans have 23 homologous pairs (46 chromosomes), we get 23 chromosomes from our mother and 23 chromosomes from our father. GENE is a segment/part of the DNA on a chromosome that controls the characteristics of the organism. Every gene has two alternative forms called ALLELE. ALLELES are the alternative forms of a gene. Example: There is a gene for eye color, so the possible alleles could be Brown and Black ❖ Think of this: If there was a ‘CELL-PHONE’ gene, the possible alleles could be ‘flip’ and ‘slide’ BUT they are both cell phones (gene), just two different types (alleles) CHROMATID is ONE copy of a duplicated chromosome, which is also found in the nucleus and is capable of ‘division, by mitosis or meiosis’. CENTROMERE is the point where the chromatids attach. It is also the point where the spindle fibres attach during cell division. CENTRIOLE is a structure that the spindle fibers radiate/ extend from SPINDLE FIBRES are the thread-like structures that extend from the centriole and connect to the centromere of the chromosome during cell division. Page 1 of 5 MRS ANDERSON CYTOKINESIS is the division of the cytoplasm after the nucleus has divided into two, to make two separate new daughter cells. This occurs after the last stage of mitosis. CLONES are organisms that are genetically identical to the parent CELL DIVISION There are TWO types of CELL DIVISION: 1. MITOSIS 2. MEIOSIS *COPYING A type of cell division in which a cell makes copies of itself and then split into MITOSIS TWO IDENTICAL DIPLOID CELLS Mitosis is a type of cell division that results in TWO daughter cells that are identical to the original parent cell; the cells produced are called CLONES. Mitosis takes place in normal body cells. Body cells can divide to make new body cells that are identical genetically, having the same number of chromosomes. So every body cell has the same genetic information. Mitosis produces cells that are genetically identical to the parent cell (clones), this takes place during asexual reproduction. Mitosis is a continuous process that is complex but it is summarized into four stages. (TELOPHASE) Before mitosis begins, the chromosomes in the nucleus make a copy of itself which remains attached to the original copy, the two are now called chromatids. STAGES OF MITOSIS The already duplicated chromosomes become shorter and fatter in appearance and absorb the stain and are now visible under the microscope. The chromatids are connected by the centromere The nuclear membrane disappears The centrioles separate and begin to move to the opposite end of the cell. The chromatids move to the center of the cell. The spindle fibres radiate from the centrioles from the opposite ends of the cell and attach themselves to each centromere on both sides The spindle fibres begin to shorten and pull on the centromere so the chromatids separate The two chromatids split at the centromere and move towards the opposite end of the cell (towards the centriole) NB. This action ensures that each of the new cells will have an identical copy of each chromosome. A new nuclear membrane forms around each set of chromatids, now called chromosomes. The chromosomes begin to get thinner and become less visible under the microscope. The two new nucleus that is formed (the daughter cells) has chromosomes that came from the parent cell. Page 2 of 5 Cytokinesis takes place after this stage  MRS ANDERSON THE IMPORTANCE OF MITOSIS 1. Mitosis is important for growth, because it produces genetically identical cells. 2. Mitosis also allows for the replacement of damaged body cells and repair cells that are damaged and worn out. 3. Mitosis produces the cells that are needed to heal cuts(skin cells) and broken bones. 4. Mitosis is the process that replaces dead skin cells and red blood cells. 5. As food passes through the oesophagus layers of cells are scraped off, but there is a layer underneath that divides by mitosis and replaces the cells that were lost. FACTS: 300 million cells die EVERY MINUTE, BUT through mitosis they are all replaced. ONLY cells that contain nucleus can undergoes mitosis, so this means that red blood cells DO NOT undergo mitosis *SPLITTING FACT: One testis can make over 200 million A type of cell division that results in sperm cell each day by meiosis. FOUR HAPLOID CELLS. MEIOSIS Meiosis is a type of cell division that is used to make GAMETES (the sex cells- ova and sperm) Meiosis ONLY takes place in organisms that can carry out SEXUAL REPRODUCTION. Meiosis reduces the number of chromosomes by HALF, producing HAPLOID CELLS. Meiosis ensures that the sex cells have HALF the number of chromosomes so that when fertilization takes place it will result in the FULL number of chromosomes of that organism. For humans; WITH meiosis the sperm cell would have 23 chromosomes while the egg cell will also have 23 chromosomes, so when fertilization takes place it will result in the zygote having 46 chromosomes, which is normal. And ITHOUT meiosis, the sperm cell would have 46 chromosomes while the egg cell will also have 46 chromosomes, so when fertilization takes place it will result in the zygote having 92 chromosomes, which is NOT normal. In girls, the first stage of meiosis is completed just before they are born. Then at puberty just before their first menstrual cycle meiosis is completed. In boys, meiosis does not begin until puberty when the testes start producing sperm NB. Meiosis ensures that NO TWO OVA OR SPERM CELLS CROSS-OVER occurs during are genetically identical. This means that ALL OVA and meiosis; this is where similar SPERM CELLS are different. chromosomes pair up and then exchange genes. This is a random process so this is why no two sperm or egg are alike Page 3 of 5  MRS ANDERSON THE IMPORTANCE OF MEIOSIS 1. Meiosis is important in sexual reproduction because it produces gametes that contain HALF the number of chromosomes (haploid cells). 2. Meiosis mixes the parental (mother and father) chromosomes when producing the daughter cells; this causes an INCREASE IN GENTECTIC VARIATION of the offsprings (all the offsprings are different from each other and is neither identical to the mother or the father BUT contains a mix of genes from both parents). DIFFERENCES BETWEEN MITOSIS AND MEIOSIS MITOSIS MEISOSIS Occurs in normal body cells (somatic cells) Occurs in the formation of GAMETES It passes on the full/normal number of chromosomes It passes on half the number of chromosomes; the the diploid number (due to duplication/copying) haploid number (due to splitting) The nuclei have IDENTICAL GENES The nuclei have MIXED GENES, due to crossing over It produces genetically identical cells (clones) It produces variation- new genotype It is needed for body growth and development It is needed for making gametes, so that after fertilization the diploid number of chromosomes are restored It ensure GENETIC CONTINUITY (the same genes It ensures GENETIC VARIATION. are passed on from parent to offsprings) TWO DAUGHTER CELLS are produced from the FOUR DAUGHTER CELLS are produced from the parent cell parent cell. Page 4 of 5 MRS ANDERSON MEIOSIS RESULTS IN FOUR DAUGHTER CELLS, because it undergoes two cell divisions Page 5 of 5