Chapter 4 DNA Biology & Cell Division Part II 2024-2025 (PDF)
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This chapter outlines DNA biology and cell division, covering the cell cycle, mitosis, meiosis, and gametogenesis. It's a detailed look into human biology that includes definitions such as homologous chromosomes and their differentiation with haploid and diploid cells.
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Chapter 4 DNA BIOLOGY AND CELL DIVISION: PART 2 1 Outlines 4.1 The Cell Cycle Interphase The Structure of Chromosomes Mitosis and Cytokinesis 4.2 Meiosis Interphase Meiosis I Meiosis I...
Chapter 4 DNA BIOLOGY AND CELL DIVISION: PART 2 1 Outlines 4.1 The Cell Cycle Interphase The Structure of Chromosomes Mitosis and Cytokinesis 4.2 Meiosis Interphase Meiosis I Meiosis II 4.3 Comparing Meiosis and Mitosis 4.4Gametogenesis (Spermatogenesis and Oogenesis) 2 Human Biology (BIOL-105), Health track Objectives After studying this chapter, students will be able to List the stages of the cell cycle in order, including Interphase and mitosis List the stages of nuclear division that forms haploid cells, which is called meiosis. Compare between mitotic and meiotic phases. Describe the process of gametogenesis 3 Human Biology (BIOL-105), Health track Lesson 1 4 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle The 46 chromosomes in human, occur in pairs called homologous chromosomes. Homologous chromosomes are two chromosomes; one from each parent, father (paternal) and one from the mother (maternal). They look identical under the microscope. They have copies of the same genes in the same location but may be different versions of the same gene (alleles). father mother Human female Karyotype 5 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Diploid Cells: All body cells (somatic cells) and some germ cells in gonads (testes and ovaries) that divide to form gametes (sperm or egg) are diploid. They have pairs of homologous chromosomes (2n); n from the mother and n from the father. A human somatic cell has 46 chromosomes (23 pairs) which are: ▪ Autosomes: 22 homologous pairs (44 chromosomes) in both males and females. sister chromatids ▪ Sex chromosomes: one pair: centromere ✓ In females: XX chromosomes (identical chromosomes). homologous autosome pair ✓ In males: XY (two different sex chromosomes). sex chromosomes in males The 46 chromosomes of a male 6 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Haploid Cells: Sperms, eggs (ova), and some germ cells in gonads. They have half number of chromosomes (n) Each cell has one of each homologous pair of chromosomes Human gametes and haploid cells have 23 chromosomes which are: ▪ Autosomes: 22 chromosomes. ▪ Sex chromosomes: one chromosome ✓ In each ovum: one X chromosome ✓ In sperm: X ch. or Y ch. 7 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle The cell cycle The cell cycle consists of two basic phases: interphase and Mitotic phase (M). Interphase : is the period of the cell cycle during which the cell is not dividing. about 95% of the cell cycle is spent in interphase. Mitotic phase (Cell division) : it includes Mitosis, the division of the genetic material during which the nucleus divides, and two new nuclei are formed. cytokinesis, divides the cytoplasm into two distinctive cells. 8 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Interphase A long period between two cell divisions. Is divided into 3 subphases; G1 (gap1 phase), S (synthesis phase), G2 (gap2 phase) 1. G1 phase: very active growth phase. The cell is metabolically active. The cell duplicates its organelles. The cell is at its smallest size at the start. Many cells enter a nondividing state, G0, either temporarily or permanently. G0 phase: acts as a holding phase during which the cell is metabolically active. Cell Cycle. The two major phases of Neurons, osteocytes remain in G0 after adolescence. the cell cycle include the mitotic phase (cell division), and interphase 9 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Interphase 2. S (synthesis): Synthesis of DNA for next cell division The cell’s DNA is duplicated Growth continues but is slower than G1 phase. 3. G2 (second gap) : The cell continues to grow slowly as it prepares for division. In late G2, strands of replicated DNA begins to condense and coil. 10 Human Biology (BIOL-105), Health track Lesson 2 © 2017 Pearson Education, Ltd. 4.1 The Cell Cycle Mitosis and Cytokinesis The mitotic phase of the cell is typically short. The cell undergoes 2 major processes, mitosis and cytokinesis Mitosis (Nuclear division) Duplicated DNA is distributed equally between the two halves of the cell. Is divided into four stages: prophase, metaphase, anaphase, and telophase Cytokinesis (Cytoplasmic division) Division of the cytoplasm and cell body into two new daughter cells. 12 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Mitosis: One nuclear division followed by cytoplasmic division (cytokinesis). During mitosis, sister chromatids of each duplicated chromosome are separated. Occurs in diploid cells (2n) to produce 2 daughter diploid cells, each with a complete set of chromosomes (2n). In human cells, the 46 chromosomes replicate and then undergo mitosis and cytokinesis producing two daughter cells, each with 46 chromosomes (2n). Daughter cells are genetically identical to each other and to the parent cell. Ensures that all cells of the complex organism have the same set of chromosomes. Is responsible for growth, cell replacement, regeneration and asexual reproduction. 13 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Prophase The loosely packed chromatin coils and condenses into visible chromosomes. Chromosome becomes visible, each is formed of two sister chromatids attached at centromere. The nucleolus disappears, and the nuclear envelope disintegrates. As the two centrosomes start to migrate to the opposite poles of the cell, microtubules of the mitotic spindle begin to form. Prometaphase The transition between prophase and metaphase. Chromosomes continue to condense. The centromeres develop into two separate kinetochores, the points of attachment between the mitotic spindles and the sister chromatids from opposite sides of the cell. The mitotic spindle is a structure composed of the centrosomes and their emerging microtubules. 14 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Metaphase The duplicated chromosomes line up on one plane at the middle of the cell A metaphase plate forms between the centrosomes which are located at the opposite poles of the cell. The chromosomes are pulled by the mitotic spindle to the opposite sides by equal forces, but without separation of the sister chromatids. 15 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Anaphase The shortest phase. The pairs of sister chromatids are separated from one another, forming individual (unduplicated) chromosomes. The microtubules shorten, pulling chromosomes to opposite poles of the cell by their kinetochores. Each end of the cell receives one partner from each pair of sister chromatids, ensuring that the two new daughter nuclei will contain identical genetic material. 16 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Telophase Is the reverse of the prophase. Starts when the two sets of chromosomes arrive at opposite poles of the cell The mitotic spindle comes apart New nuclear envelopes form around each set of chromosome. Nucleoli reappear. Chromosomes uncoil “decondense” and return back to a loosely packed chromatin. Each new cell receives its own complement of DNA, organelles, membranes, and centrioles. The cell begins to split in half as cytokinesis begins. 17 Human Biology (BIOL-105), Health track 4.1 The Cell Cycle Cytokinesis In nearly all species, cytokinesis separates the cell contents (cytoplasm and organelles) by either a cleavage furrow (in animals and some fungi), or a cell plate that leads to the formation of cell walls that separate the daughter cells (in plants). Dr. David Phillips/Visuals Cleavage furrow is a contractile band made up of microfilaments Unlimited. Inc. that forms around the midline of the cell during cytokinesis. The contractile ring squeezes the two cells apart until they separate. Two new identical cells are formed. Ring of microfilaments Aberration occurs if the cell fails to undergo cytokinesis. This results in larger cells with more than one nucleus, which can be a sign of cancerous cells. Fig. (B) The beltlike contractile ring of microfilaments inside a dividing animal cell that is undergoing Cytokinesis (Starr C. & McMillan B. : Human Biology, 9th edition, 2012 Yolanda Cossio) 18 Human Biology (BIOL-105), Health track Figure 4.13 Cell Division: Mitosis Followed by Cytokinesis 19 Human Biology (BIOL-102), Health track Lesson 3 Human Biology (BIOL-102), Health track 20 4.2 Meiosis 4.2 Meiosis Meiosis is a nuclear division that reduces the chromosome number from 2n to n (reduction division). It occurs in germ cells in gonads (ovaries and testes) to form haploid gametes (sperms, eggs). During meiosis, the human genes are mixed, reshuffled and reduced by half. The starting nucleus is diploid, and the resulting nuclei are haploid. The four daughter cells are haploid (n), genetically different germ cells (reproductive cells) that will become gametes. ثنائي الصبغه After fertilization, the zygote and all subsequent cells have diploid number of chromosomes. 21 Human Biology (BIOL-105), Health track 4.2 Meiosis 4.2 Meiosis Meiosis consists of one round of chromosome duplication and two rounds of nuclear division (Meiosis I and meiosis II). Before meiosis I, the precursor cell of the germ cells undergoes typical interphase, where DNA is duplicated in S phase. n n n 2n n n Meiosis I n Human Biology (BIOL-105), Health track Meiosis II 22 4.2 Meiosis 4.2 Meiosis Meiosis I Prophase I, prometaphase I, metaphase I, anaphase I, telophase I, and cytokinesis. Reduces the number of chromosome sets from 2n to n. The genetic information is mixed during to create unique recombinant chromosomes. Homologous chromosomes separate. Meiosis II Prophase II, prometaphase II, metaphase II, anaphase II, telophase II, and cytokinesis. Occurs in a way similar to that of mitosis. Sister chromatids separate. 23 Human Biology (BIOL-105), Health track 4.2 Meiosis Figure 4.17 Meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. 24 Human Biology (BIOL-105), Health track 4.2 Meiosis Meiosis I Prophase I Chromatin condenses into visible chromosomes. meiotic spindle start to form. Centrioles move to opposite ends of the cell. The nuclear envelope begins to break down. The homologous chromosomes pair up tightly “synapsis”. In synapsis, the genes on the chromatids of the homologous chromosomes are precisely aligned with each other. Crossing over, an exchange of chromosome segments between non-sister homologous chromatids occurs, the regions where exchange occurs are called chiasmata (singular =chiasma) 25 Human Biology (BIOL-105), Health track 4.2 Meiosis Prophase I The number of chiasmata varies with the species and the length of the chromosome. At the end of prophase I, the homologous pairs are held together only at chiasmata and are called tetrads. The crossover is the first source of genetic variation among the offspring. One crossover between homologous non-sister chromatids leads to a reciprocal exchange of equivalent DNA between a maternal chromosome and a paternal one. When that sister chromatid is moved into a gamete, it carries some DNA from one parent and some DNA from the other parent. The recombinant sister chromatid has a combination of maternal and paternal genes. 26 Human Biology (BIOL-102), Health track Figure 4.14 The effects of crossing over, the blue chromosome came from the individual’s father and the red chromosome came from the individual’s mother. Crossover occurs between non-sister chromatids of homologous chromosomes. 27 Human Biology (BIOL-102), Health track 4.2 Meiosis Prometaphase I Microtubules of mitotic spindle are attached to the kinetochore proteins at the centromeres. The microtubules assembled from centrosomes at opposite poles of the cell grow toward the middle of the cell. Each tetrad is attached to microtubules from both poles, with one homologous chromosome attached at one pole and the other is attached to the other pole. The homologous chromosomes are still held together at Figure 4.16 In prometaphase I, chiasmata. microtubules attach to the fused kinetochores of homologous The nuclear membrane has broken down entirely. chromosomes. 28 Human Biology (BIOL-105), Health track 4.2 Meiosis Metaphase I For each tetrad, there are two possibilities for orientation, the possible number of alignments equals 2n , Where n is the number of chromosomes per set. Humans have 23 chromosome pairs, which results in over eight million (223) possibilities. This number does not include the variability created in the sister chromatids by crossover. Any two haploid cells resulting from meiosis will have different genetic composition. 29 Human Biology (BIOL-105), Health track 4.2 Meiosis Figure 4.15 To demonstrate random, independent assortment at metaphase I, consider a cell with n = 2. In this case, there are two possible arrangements at the equatorial plane in metaphase I, as shown in the upper cell of each panel. These two possible orientations lead to the production of genetically different gametes. With more chromosomes, the number of possible arrangements increases dramatically. 30 4.2 Meiosis Independent alignment at metaphase I increases variability. When a parent cell has 3 pairs of homologous chromosomes, there are 8 possible chromosome alignments at the equator. (Sylvia S. Mader: Human Biology, 14th edition, 2016 McGraw Hill Education) 31 Human Biology (BIOL-105), Health track 4.2 Meiosis Anaphase I The spindle fibers pull the linked chromosomes apart. Thesister chromatids remain tightly bound together at the centromere. The chiasma connections get broken as the fibers attached to the fused kinetochores pull the homologous chromosomes apart. Telophase I The separated chromosomes arrive at opposite poles. The chromosomes decondense and nuclear envelopes form. 32 Human Biology (BIOL-105), Health track 4.2 Meiosis Cytokinesis Physical separation of the cytoplasmic components into two daughter cells At each pole, there is one member of each pair of the homologous chromosomes. The newly formed cells are haploid (n). Although the sister chromatids were once duplicates of the same chromosome, they are no longer identical. 33 Human Biology (BIOL-105), Health track 4.2 Meiosis Meiosis II ✓ Joined sister chromatids will be split to form four haploid cells. ✓ In some species, cells enter a brief interphase, interkinesis, that lacks an S phase, before entering meiosis II. ✓ Chromosomes are not duplicated during interkinesis. The two cells produced in meiosis I go through the events of meiosis II in synchrony. ✓ Meiosis II resembles the mitotic division. 34 Human Biology (BIOL-105), Health track 4.2 Meiosis Prophase II If the chromosomes are decondensed in telophase I, they condense again Nuclear envelopes fragment into vesicles. The centrosomes duplicated during interkinesis move away from each other toward opposite poles and new spindles are formed. Prometaphase II The nuclear envelopes are completely broken down, and the spindle is fully formed. Each sister chromatid forms an individual kinetochore that attaches to microtubules from opposite poles. Sister chromatids are held together at the centromere. Metaphase II The sister chromatids are maximally condensed and aligned at the cell equator as a single line. 35 Human Biology (BIOL-105), Health track Figure 4.16 In prometaphase II, microtubules attach to individual kinetochores of sister chromatids. In anaphase II, the sister chromatids are separated. 36 Human Biology (BIOL-105), Health track 4.2 Meiosis Anaphase II The sister chromatids are pulled apart by the spindle fibers and move toward opposite poles. Telophase II The chromosomes arrive at opposite poles and begin to decondense. Nuclear envelopes form around the chromosomes. Cytokinesis Separation of the two cells into four genetically unique haploid cells. 37 Human Biology (BIOL-105), Health track Lesson 4 Human Biology (BIOL-102), Health track 38 4.3 Comparing Meiosis and Mitosis Figure 4.17 Meiosis and mitosis are both preceded by one round of DNA replication; however, meiosis includes two nuclear divisions. The four daughter cells resulting from meiosis are haploid and genetically distinct. The daughter cells resulting from mitosis are diploid and identical to the parent cell. 39 Human Biology (BIOL-105), Health track 4.4 Gametogenesis 4.4 Gametogenesis (Spermatogenesis and Oogenesis) The production of sperms and eggs in the gonads (testes and ovaries) It involves the process of mitosis and meiosis. Meiosis and its associated cell divisions produces haploid cells with half of each paired chromosomes normally found in diploid cells. The production of sperm is called spermatogenesis and the production of eggs is called oogenesis. 40 Human Biology (BIOL-105), Health track 4.4 Gametogenesis Spermatogenesis It occurs in the seminiferous tubules that form the bulk of each testis. It begins at puberty, after which time sperm are produced constantly throughout a man’s life. One production cycle, from spermatogonia through formed sperm, takes about 64 days. Sperm counts slowly decline after age 35. Spermatogenesis begins with mitosis of the diploid spermatogonia (2n), they each have a complete copy of the father’s genetic material (46 chromosomes). However, mature gametes, sperms are haploid (1n), containing 23 chromosomes. The daughter cells of spermatogonia (primary spermatocyte) must undergo a second cellular division throughout meiosis. 41 Human Biology (BIOL-105), Health track Figure 4.18 Spermatogenesis (a) Mitosis of a spermatogonia stem cell involves a single cell division that results in two identical, diploid daughter cells (spermatogonia to primary spermatocyte). Meiosis has two rounds of cell division: primary spermatocyte to secondary spermatocyte, and then secondary spermatocyte to spermatid. This produces four haploid daughter cells (spermatids). (b) In this electron micrograph of a cross-section of a seminiferous tubule from a rat, the lumen is the light-shaded area in the center of the image. The location of the primary spermatocytes is near the basement membrane, and the early spermatids are approaching the lumen 42 Human Biology (BIOL-105), Health track 4.4 Gametogenesis Spermatogenesis Two identical diploid cells result from mitotic division of spermatogonia. One of these cells remains a spermatogonium, and the other becomes a primary spermatocyte (2n) Primary spermatocyte enters meiosis I producing two secondary spermatocytes, each with only half the number of chromosomes (n). Meiosis II occurs in both secondary spermatocytes, each of the 23 replicated chromosomes divides, separating the chromosome pairs, resulting in four spermatids with half of the number of chromosomes (n). Spermatids have round shape, central nucleus, and large amount of cytoplasm. Spermiogenesis transforms spermatids to spermatozoa, which will be released into the lumen and move along ducts in the testis toward the epididymis for maturation. 43 Human Biology (BIOL-105), Health track 4.4 Gametogenesis Oogenesis It begins with the ovarian stem cells called oogonia Oogonia are formed during fetal development before birth They divide via mitosis, and then are arrested in prophase I of meiosis I , to resume it years later, beginning at puberty and continuing until the woman is near menopause (the cessation of a woman’s reproductive functions). The number of primary oocytes present in the ovaries declines from one to two million in an infant, to approximately 400,000 at puberty, to zero by the end of menopause. The release of an oocyte from the ovary is called ovulation, it marks the transition from puberty into reproductive maturity in the females. Ovulation occurs approximately once every 28 days. Just prior to ovulation, a release of luteinizing hormone (LH) triggers the completion of meiosis I and cytokinesis in a primary oocyte, resulting in a large secondary oocyte, which eventually leaves the ovary during ovulation, and a small polar body, may or may not complete meiosis; in either case, it eventually disintegrates. 44 Human Biology (BIOL-105), Health track Oogenesis Figure 4.19 Oogenesis The unequal cell division of oogenesis produces one to three polar bodies that later degrade, as well as a single haploid ovum, which is produced only if there is penetration of the secondary oocyte by a sperm cell. 45 Human Biology (BIOL-105), Health track 4.4 Gametogenesis Oogenesis Although oogenesis produces up to four cells, only one survives. Meiosis of a secondary oocyte is arrested at metaphase II and is completed only if a sperm manages to penetrate its barriers, then meiosis II completes, producing one haploid ovum that becomes the first diploid cell, zygote, once it is fertilized by a sperm. The ovum can be considered as a brief, transitional, haploid stage between two diploids, the oocyte and zygote. The larger amount of cytoplasm contained in the female gamete is used to supply the developing zygote with nutrients during the period between fertilization and implantation into the uterus. At fertilization, the sperm contributes only its DNA. The cytoplasm and organelles in the developing embryo are of maternal origin. This includes mitochondria, which contain their own DNA. Mitochondrial DNA is maternally inherited. 46 Human Biology (BIOL-105), Health track Summary 4.1 The Cell Cycle Cells reproduce by a repetitive cycle called the cell cycle in which one cell grows and then divides into two. The cell cycle has two primary phases: interphase and the mitotic phase. During interphase, the cell grows, and the DNA is replicated. During the mitotic phase, first the nucleus and then the cell divide into two. Mitosis is a sequence of events in which the replicated chromosomes are separated to form two new genetically identical nuclei. Cytokinesis is the process whereby the cell divides into two new cells, each with one of the new nuclei produced by mitosis and roughly half of the cell's organelles and mass. 47 Human Biology (BIOL-105), Health track Summary 4.2 Meiosis Meiosis is a sequence of two cell divisions that produces haploid cells. Meiosis occurs only in cells destined to become sperm or egg. Crossing-over during meiosis mixes the genes of homologous chromosomes, and subsequent cell divisions reduce the number of chromosomes by half. 4.3 Comparing Meiosis and Mitosis Mitosis Meiosis Growth and repair of cells Formation of gametes Occurs in somatic cells Occurs in germ cells 1 division 2 divisions Results in 2 diploid, genetically identical cells Results in 4 haploid, genetically different cells 48 Human Biology (BIOL-105), Health track Summary 4.4 Gametogenesis (Spermatogenesis and Oogenesis) Spermatogenesis in males, produces four viable sperms. Oogenesis in females, produces one egg and polar body(ies). Oogenesis goes to completion if the sperm fertilizes the developing ovum. 49 Human Biology (BIOL-105), Health track Activity 1- The cleavage furrow is made of A. tubulin microfilaments. B. actin microtubules. C. actin microfilaments. D. myosin microfilaments. 2- A cell contains 8 pairs of chromosomes, how many chromosomes in a daughter cell produced after miosis I? A. 4 B. 8 C. 16 D. 32 50 Human Biology (BIOL-105), Health track Activity 3. Which of the following structures does not undergo division during cytokinesis? A. chromosome B. cell membrane C. cytoplasm D. endoplasmic reticulum 4. The shortest phase of mitotic division is A. prophase B. anaphase C. metaphase D. telophase 51 Human Biology (BIOL-105), Health track Activity 5. In what way spermatogenesis is like oogenesis? A. Both start continuously from puberty. B. Both result in cells with half the chromosome number. C. Both are arrested in prophase I. D. Both result in 1 or 2 polar bodies. 6. The function of the Luteinizing hormone is A. transforming spermatids into sperms. B. making zygote from an egg and a sperm. C. resuming meiosis in a primary oocyte. D. stopping meiosis in a primary oocyte. 52 Human Biology (BIOL-105), Health track Activity a 7. Name the following? d a: Chromosome a: ……………… b b: Spindle b: ……………… c: Centrosome c: ……………… c d: Polar body d: ……………… e: Nucleus e: ……………… e 53 Human Biology (BIOL-102), Health track Thank you & Good Luck ☺ 54
