8 Cellular Reproduction Cells from Cells Djs 22-23 PDF

WHAT CELL REPRODUCTION ACCOMPLISHES Early hum an em bryo Binary fission am oeba Star Fish Regeneration Chrom osome Tum or Grow th Hum an kidney cell Grow th of a clipping CELLULAR...

WHAT CELL REPRODUCTION ACCOMPLISHES Early hum an em bryo Binary fission am oeba Star Fish Regeneration Chrom osome Tum or Grow th Hum an kidney cell Grow th of a clipping CELLULAR LM LM REPRODUCTION [CELLS FROM CELL] Grow th height increase  Replaces damaged or lost cells,  Permits growth, and  Allows for reproduction. Chapter 8-TB Prof Dr Dj S  IMPORTANCE OF CELL DIVISION Fundamental characteristics of life Ability to grow Ability to reproduce In single celled organisms: Method of increasing numbers In multi celled organisms: Growth Replacement of lost cells Healing of injuries Formation of reproductive cells This in turn leads to new organisms which in turn grow by cell division When a cell undergoes reproduction / cell division, TWO “daughter” cells are produced that are genetically identical to each other and the “parent” cell. Before a parent cell splits into two, it duplicates its chromosomes. During cell division, each daughter cell receives one identical set of chromosomes from original parent cell.  CELL DIVISION Cell division- Process in which one cell becomes 2 new cells 3 types of cell division Binary fission Method used by prokaryotes Single loop of DNA replicates, the 2 loops separate and new cell membrane forms between 2 DNA molecules Ensures that the genetic information in parent and daughter cells are the same Mitosis Method employed by eukaryotic cells Similar to binary fission Results in daughter cells that are identical to parent cells Meiosis Method seen in eukaryotic cells Daughter cells possess half the genetic information of parent cells Parthenogenesis Process in which an unfertilized egg develops into a new individual; common among insects and some other arthropods. Noticed in Komodo Dragon (World’s Largest Lizard) Zookeepers at the Chester Zoo were surprised to discover with out their female partner egg hatched. Human conception without fertilization by a man ‘virgin birth’ The theological doctrine that Jesus Christ had no human father; Christians believe that Jesus's birth fulfilled Old Testament pro phecies and was attended by miracles; the Nativity is celebrated at Christmas  PACKAGING OF DNA INTO CHROMOSOMES EUKARYOTIC CHROMOSOMES Chromosomes DNA double helix “Beads on a In Eukaryotic Cells- most Genes are located on Chromosomes string” and a few genes are found in DNA of Mitochondria and Chloroplasts. Each eukaryotic chromosome contains one very long DNA molecule, typically bearing thousands of genes. Chromosomes are made of chromatin, fibers composed of roughly equal amounts of DNA and protein molecules, which help organize the chromatin and control the activity of its genes. Many a time, the chromosomes exist as thin chromatin fibers that are much longer than the nucleus they are stored in. If fully extended, the DNA in just one of your cells would be more than six feet long! As a cell prepares to divide, its chromatin fibers coil up, forming compact chromosomes that can be viewed under a light microscope. When a cell is not dividing, the chromosomes are too thin to be seen under a light microscope. Centromere The DNA in a cell is packed into an elaborate, multilevel system of coiling and folding. Duplicated  Histones are proteins used to package DNA in eukaryotes. chromosomes (sister chromatids)  Nucleosomes consist of DNA wound around histone molecules. Figure 8.4 INFORMATION FLOW: DUPLICATING CHROMOSOMES Figure 8.5 Before a cell begins the process, the DNA molecule of each chromosome is copied through the process of DNA replication & new histone protein molecules attach as needed. As a result, each chromosome produce two copies - called sister chromatids, (which contain identical genes) Chromosome duplication Two sister chromatids are joined together tightly at a narrow “waist” called the centromere. When the cell divides, the sister chromatids of a duplicated chromosome separate from each other. Once separated from its sister, each chromatid Centromere  is considered a full-fledged chromosome and Sister  is identical to the original chromosome. chromatids Chromosome distribution to daughter cells CELL CYCLE AND MITOSIS Features of cell cycle: Continuous process Consists of all stages of growth and division for a eukaryotic cell Different cells vary in the time spent in various stages of cell cycle Interphase Longest stage of cell cycle Cell engages in metabolic activities and prepares for cell division 3 distinct phases: G1, S, G2 Mitosis Stage of cell cycle where the cell divides its genetic material 4 phases: Prophase, Metaphase, Anaphase, Telophase  FIGURE 8. 6 CELL CYCLE AND MITOSIS G1- Cell gathers nutrients and S phase other (DNA synthesis; chromosome resources duplication) Cell grows in volume and carries out its metabolic roles like producing tRNA, mRNA, ribosomes, enzymes and other cell components G2- Final stage of interphase In multicellular organisms, Cell prepares the components needed metabolic functions include Interphase: metabolism and to divide like proteins to move the production of proteins for muscle growth (90% of time) chromosomes contraction, photosynthesis or Nuclear membrane is intact glandular-cell secretion G2 Chromatin has replicated however G1 individual chromosomes are not visible Mitotic The nucleolus, site of ribosome (M) phase: manufacture are still visible at this stage cell division G0- Cells may become differentiated (10% of time)or specialized in their function. E.g. muscle and nerve cells. Cells do not move forward in the cell cycle. This is also refereed as quiescent phase (dormant / motionless) S Stage where DNA synthesis occurs Cytokinesis Some cells stay in G0 phase Cell distributes the copies of genetic information as chromosomes to theofdaughter (division cells cytoplasm) permanently. DNA in chromosomes is wrapped around histone proteins to form Nucleosomes E.g. nerve cells. Nucleosomes are coiled into Chomatin. Individual chromatin strands are very thin Others move forward & enter S phase Chromatin coils to form chromosomes. As chromosomes become visible during mitosis, 2 thread like structures are formed called Chromatids Chromatid is one of two parallel parts of chromosomes. Each chromosome contains one DNA molecule Sister chromatids are the two chromatids of a chromosome that are produced after replication having the same DNA. Centromere is the point where the sister chromosomes are attached MITOSIS 2 events occur during eukaryotic cell division: Replication of genetic material equally :- karyokinesis Division of cytoplasm :- Cytokinesis 4 phases Prophase Metaphase Anaphase Telophase  PROPHASE Tangled chromatin thickens & coils, becoming visible as separate chromosomes Nucleus disassembles. Nucleolus is not visible Formation of spindle and spindle fibers towards the end of prophase (Spindle is a structure made of microtubules that reach across the cell from one side to another) The spindle fibers attach to the chromosome so that they can move the chromosomes during later stages of mitosis METAPHASE Chromosomes align on the equatorial plate No nucleus present as the nuclear membrane has completely disassembled Chromosomes are tightly coiled and are attached to the spindle fibers and move along them to arrange themselves at the equatorial plate Each chromosome still consists of 2 chromatids attached at the centromere  ANAPHASE Nuclear membrane still absent and spindles extend from pole to pole Sister chromatids separate as they move along the spindle fibers towards opposite poles. During this separation the chromatids are known as daughter chromosomes Separation of sister chromatids occur due to 2 events:  Enzymes in the cell digest the centromere  Chromatids begin to move Movement of chromatids is due to:  Movement of pole of spindle fibers to which the chromatids are attached  Proteins at the centromere pull the chromatids towards opposite poles Kinetochore is a protein attached to each chromatid at the centromere which causes the shortening of spindle fibers. Due to this, the spindle fibers pull its chromatids closer to the pole.  TELOPHASE Spindle fibers disassemble Nuclear membrane forms around 2 new sets of chromosome Chromosomes begin to uncoil into chromatin Nucleolus reforms Cell begins to make new ribosomes for protein synthesis and prepares to reenter Interphase  CONTROLLING MITOSIS Regulation of cell division is important It should not interfere with the other activities of the cell Check points- Timing during cell division to determine the readiness of cell to move ahead with cell division Cell use proteins to evaluate Genetic health, Location in body and Need for more cells Poor genetic health, Wrong location, Crowded conditions- Good genetic health, Correct location, Uncrowded conditions- Proteins gather information and assess if cell division is appropriate 2 classes of genes that code for these proteins Proto-oncogenes codes for proteins that provide signals that encourage cell division Tumor suppressor genes codes for proteins that provide signals that discourage cell division Controlled cell division requires signals from both genes to achieve balance of information. Eg: p53 gene- Tumor suppressor gene (If the DNA is healthy, p53 allows the cell to divide) Although the cells of the body are same, they do not possess identical functions. E.g. bone cells, nerve cells, muscle cells etc The difference is not in the genes they possess but in the genes they express  If p53 detects damaged DNA, it repairs the DNA with the activation of other genes If the damage is too extensive, it causes the cell to self destruct Apoptosis - P rocess w hereby the cell digests itself from inside out. It is a programmed cell death  Tumor- Mass of cells formed due to uncontrolled mitosis. Broadly two types BENIGN TUMOR- Cell mass that does not fragment and spread beyond its original area of growth Can become harmful by growing large and interfering with normal functions MALIGNANT TUMOR- They spread or invade body parts These cells metastasize - move from original size and grow new tumors in other regions of the body Metastasis is the spread of cancer cells to new areas of the body, often by way of the lymph system or bloodstream. A metastatic cancer, or metastatic tumor, is one that has spread from the primary site of origin, or where it started, into different areas of the body.  TREATMENT STRATEGIES - Three Main Types Of Cancer Treatment. 1) Surgical Removal - to remove a tumor is usually the first step. Skin and breast cancers are surgically removed Early detection is important as this increases the chance of surgical removal before it metastasizes In some cases this approach is not possible. E.g. Leukemia which is caused by uncontrolled growth of WBC. In this case the cells are spread through the body. Surgery is also not useful when the nearby healthy tissues need to be destroyed in the process. E.g. Removing certain brain cancer cells can severely damage the brain 2) Chemotherapy the use of drugs to disrupt cell division, is used to treat widespread or metastatic tumors. Some chemotherapeutic agents destroy spindle, necessary for cell division However it cannot control cancer alone. Often is coupled with radiation therapy It exposes the body’s cells to toxic ingredients and weakens the normal defense mechanisms. It also causes intestinal disorders and hair loss 3) Radiation therapy Parts of the body that have cancerous tumors are exposed to concentrated beams of high-energy radiation (uses X rays & Gamma rays to destroy DNA of cancer cells), which often harm cancer cells more than normal cells. Radiation therapy is often effective against malignant tumors that have not yet spread. Used in areas where surgery is impractical It can be applied outside the body or by implanting radioactive ‘seeds’ into tumor When radiation is applied, a beam of radiation is focused on the cancer cells Shields protect healthy cells as much as possible These have negative side effects Patients undergoing chemotherapy must be given antibiotics to help defend against dangerous bacteria.  CELL DIVISION & SEXUAL REPRODUCTION Cells of sexually reproducing organisms have 2 sets of chromosomes, (2 sets of genetic information) If the gametes contained 2 sets of chromosomes, the zygote would contain 4 sets of chromosome resulting in twice the amount of genetic information. But the organisms that reproduce sexually at the end must form only one set of chromosomes. Hence the gametes, (eggs & sperms) must be formed by a method that reduces the amount of genetic material to half Haploid cells- Cells that carry only one complete copy of genetic information Diploid cells- Cells that carry two complete copies of genetic information Meiosis is a form of cell division that aids sexual reproduction Responsible for production of eggs and sperm - Gametes Generates haploid reproductive cells from diploid cells Takes place in cells of reproductive organs- Gonads Testes in males Ovaries in females Ovarian and testicular cells divide by meiosis to form reproductive cells- Gametes/ Germ cells In algae like Spirogyra, individual cells become specialized for gamete production In flowering plants, pistil produces eggs (ova) and anther produces pollen (sperm)  HOMOLOGOUS CHROMOSOMES Resemble each other in length and centromere position and carry genes controlling the same inherited characteristics. Different individuals of a single species have the same number and types of chromosomes. A human SOMATIC cell  is a typical body cell and  has 46 chromosomes.  22 pairs of matching chromosomes, called autosomes, and  2 different sex chromosomes, X & Y, (allososme ) which fix a person’s gender / sex (male / female). In mammals, Males have one X chromosome and one Y chromosome [XY] and Females have two X chromosomes [XX]. karyotype To produce a karyotype, you have to Pair of homologous chromosomes  break open a human cell in metaphase of mitosis,  stain the chromosomes with dyes, Centromere  take a picture with the aid of a microscope, and  arrange the chromosomes in matching pairs by size. Sister chromatids One duplicated chromosome  GAMETES AND THE LIFE CYCLE OF A SEXUAL ORGANISM The life cycle of a multicellular organism is the sequence of stages leading from the adults of one generation to the adults of the next. Having two sets of chromosomes, one inherited from each parent, is a key factor in the life cycle of humans and all other species that reproduce sexually. Humans are said to be diploid organisms because all body cells contain pairs of homologous chromosomes. A haploid cell has only one member of each pair of homologous chromosomes. FI G URE 8. 12 Haploid gametes (n = 23) In the human life cycle, a haploid n Egg cell (23n) sperm cell from the father fuses with a haploid egg cell from the mother in a n Sperm cell (23N) process called fertilization. The resulting fertilized egg, called a MEIOSIS zygote, is diploid, with two sets of FERTILIZATION chromosomes, one set from each parent. All sexual life cycles involve an Diploid Multicellular alternation of diploid and haploid diploid adults 2n zygote (2n = 46) stages. (2n = 46) Producing haploid gametes by meiosis MITOSIS keeps the chromosome number from and development Key doubling in every generation. Haploid (n) Diploid (2n) FIGURE 8.13-S3 1 Chromosomes 2 Homologous 3 Sister chromatids duplicate. chromosomes separate. separate. Pair of A pair of Sister homologous homologous chromatids chromosomes chromosomes in diploid parent cell INTERPHASE BEFORE MEIOSIS MEIOSIS I MEIOSIS II  THE PROCESS OF MEIOSIS Meiosis, the process of cell division that produces haploid gametes in diploid organisms, resembles mitosis, but with two differences. 1. The 1st difference - No of chromosomes during meiosis is cut into half. (REDUCTION DIVISION) Cell which was duplicated, its chromosomes undergoes two consecutive divisions, Meiosis I & Meiosis II. As one duplication of the chromosomes is followed by two divisions, each of the four daughter cells resulting from meiosis has a haploid set of chromosomes. 2. The 2nd difference of meiosis compared with mitosis is an exchange of genetic material—pieces of chromosomes— between homologous chromosomes. Referred as crossing over, (during the prophase I). FIGURE 8.14- 1 MEIOSIS I: HOMOLOGOUS CHROMOSOMES SEPARATE INTERPHASE PROPHASE I METAPHASE I ANAPHASE I TELOPHASE I Sites of & crossing over Spindle tracks Sister chromatids CYTOKINESIS attached to remain attached Centrosomes Spindle chromosome Cleavage furrow Nuclear envelope Sister chromatids Pair of homologous Centromere Uncondensed chromosomes chromosomes Homologous Pairs of Pairs of Two haploid chromosomes homologous homologous cells form; Chromosomes duplicate. pair up and chromosomes chromosomes chromosomes exchange line up. are still doubled. segments. split up. FIGURE 8.14-3 MEIOSIS II: SISTER CHROMATIDS SEPARATE TELOPHASE II AND PROPHASE II METAPHASE II ANAPHASE II CYTOKINESIS Sister chromatids Haploid daughter separate cells forming During another round of cell division, the sister chromatids finally separate; four haploid daughter cells result, containing single chromosomes.  REVIEW: COMPARING MITOSIS AND MEIOSIS For both Mitosis & Meiosis, the chromosomes duplicate only once, in the preceding interphase. The number of cell divisions varies: Mitosis involves one division of the nucleus and cytoplasm (duplication, then division in half), producing two diploid cells. Meiosis entails two nuclear and cytoplasmic divisions (duplication, division in half, then division in half again), yielding four haploid cells. All the events unique to meiosis occur during meiosis I. Meiosis II is virtually identical to mitosis in that it separates sister chromatids. But unlike mitosis, meiosis II yields daughter cells with a haploid set of chromosomes. Parent cell 2n = 4 Parent cell 2n = 4 MITOSIS MEIOSIS Prophase MEIOSIS I Prophase- 1 Homologous Duplicated chromosomes come together. chromosome Site of crossing over Metaphase Metaphase- 1 Chromosomes Homologous FIGURE 8. 15 align. pairs align. Homologous chromosomes Sister Anaphase- 1 separate. chromatids Telophase- 1 separate. Haploid n=2 2n 2n MEIOSIS II Anaphase Sister Telophase chromatids separate. n n n n THE ORIGINS OF GENETIC VARIATION Offspring of sexual reproduction are genetically different from their parents and one another. How does meiosis produce such genetic variation? 1. INDEPENDENT ASSORTMENT OF CHROMOSOMES Figure 8.16 illustrates one way in which meiosis contributes to genetic variety. When aligned during metaphase I of meiosis, the side-by-side orientation of each homologous pair of chromosomes is a matter of chance. For a species with more than two pairs of chromosomes, such as humans, every chromosome pair orients independently of all the others at metaphase I. POSSIBILITY 1 POSSIBILITY 2 FIGURE 8. 16 Two equally probable arrangements of Factors contribute to chromosomes at metaphase of genetic variation in meiosis I For a human, n = 23, offspring so there are 223,  Mutation or about 8 million, possible chromosome  Crossing- Over combinations that can  Segregation appear in gametes. Metaphase of meiosis II  Independent Assortment A single man and a  Fertilization single woman can produce zygotes with 64 trillion combinations of Gametes chromosomes! Combination a Combination b Combination c Combination d Because possibilities 1 and 2 are equally likely, the four possible types of gametes will be made in approximately equal numbers. 2. Crossing over is the exchange of corresponding segments between nonsister chromatids of homologous chromosomes, which occurs during prophase I of meiosis. With crossing over, gametes arise with chromosomes that are partly from the mother and partly from the father. 3. Fertilization (a close-up view) Fusion of gametes to produce a new organism 4. MUTATION Point Mutation Change in nucleotide Chromosomal Mutation Rearrangement of genes Both mutations can result in creation of new proteins Both types of mutations increase genetic diversity by creating new alleles 5.SEGREGATION Process during which the alleles on homologous chromosomes separate during meiosis I Segregation increases genetic diversity allowing parents to produce off springs different from them and their siblings When Meiosis goes awry What happens when there is an error in the process of meiosis? Such a mistake can result in genetic abnormalities that range from mild to severe to fatal. How Accidents during Meiosis Can Alter Chromosome Number Nondisjunction is the failure of homologous chromosomes or sister chromatids to separate properly during cell division. In nondisjunction, the members of a chromosome pair fail to separate at anaphase, producing gametes with abnormal numbers of chromosomes. Nondisjunction can occur during meiosis I or II.  HOW ACCIDENTS DURING MEIOSIS CAN ALTER CHROMOSOME NUMBER Figure 8.21 shows what can happen when an abnormal gamete produced by nondisjunction unites with a normal gamete during fertilization. When a normal sperm fertilizes an egg cell with an extra chromosome, the result is a zygote with a total of 2n + 1 chromosomes. Because mitosis duplicates the chromosomes as they are, the abnormality will be passed to all embryonic cells. If the organism survives, it will have an abnormal karyotype and probably a syndrome of disorders caused by the abnormal number of genes. Abnormal egg cell with extra chromosome n+1 Normal sperm cell Abnormal zygote with extra chromosome 2n + 1 n (normal)  DOWN SYNDROME: AN EXTRA CHROMOSOME 21 In a condition trisomy 21, there are three number 21 chromosomes, making 47 chromosomes in total. A person with trisomy 21 has a condition called Down syndrome, which affects about 1 out of every 700 children, Down’s Syndrome symptoms is the most common chromosome number abnormality, and Slanted eyes is the most common serious birth defect in the United States. Flattened features Large tongue Short stature Faulty speech Low muscle tone Delayed motor skills usually have a life span shorter than normal Mother’s age at child birth plays an important Normal Trisomy 21 role in occurrence of trisomy. Immune system plays an important role With age, the immune system fails to identify or recognize the abnormal embryo from the normal. The fetuses of pregnant women age 35 and older are therefore candidates for chromosomal prenatal screenings. Abnormal numbers of sex chromosomes Nondisjunction in meiosis can lead to abnormal numbers of sex chromosomes, X and Y. Unusual numbers of sex chromosomes seem to upset the genetic balance less than unusual numbers of autosomes. Table 8.1 lists the most common human sex chromosome abnormalities. Turner’s Syndrome is a condition that affects females in which one of the sex chromosomes (X chromosome) is completely missing or has abnormalities. Several physical abnormalities are noted with a distinct short stature, webbed neck and low-set ears.