Chapter 7: Cell Division - PDF

Chapter 7: Cell Division Genetic Material: The Building Blocks of Life Genetic material is organized into chromosomes, which each contain a single DNA molecule Each DNA double helix is formed into long strands of chromatin (which make up chromosomes) Cell Division: The Abili...

Chapter 7: Cell Division Genetic Material: The Building Blocks of Life Genetic material is organized into chromosomes, which each contain a single DNA molecule Each DNA double helix is formed into long strands of chromatin (which make up chromosomes) Cell Division: The Ability to Reproduce Life depends on cell division Cell division is the generation of daughter cells from a parent cell Millions of cell divisions take place in our bodies every day to replace the cells that have died ○ The energy and matter the old cells contained are recycled into the system during this process  Why Cells Divide Cell division is necessary for reproduction in all life forms, and for growth and repair in a multicellular body ○ Asexual reproduction results in offspring that are genetically identical to the parent ○ Sexual reproduction combines genetic information from two individuals of opposite mating types to produce offspring (which are genetically similar but not identical)  Binary Fission: Prokaryotes Many prokaryotes, including many types of bacteria, propagate themselves asexually through a type of cell division known as binary fission The process has four steps: ○ The genetic material (DNA) is duplicated ○ The cell elongates and the DNA is separated ○ The cytoplasm is separated down the middle ○ Two daughter cells are created, replacing the original parent cell  Cell Life Cycles: Eukaryotes The cell cycle refers to the life cycle of a eukaryotic cell from its origin to its division by either mitosis or meiosis ○ A cell’s life cycle is broken into two phases: interphase and cell division The cell prepares itself for division by increasing in size and producing proteins needed for division  Interphase During interphase, the cell is preparing to divide ○ It takes in nutrients, grows in size, and manufactures proteins needed to divide (G1 and G2) ○ Replicates the genetic material needed for division (S) The interphase is the longest phase; most cells spend approximately 90% of their lives in the interphase stage  Mitotic Division Mitotic division is the process in which eukaryotes generate two daughter cells from a single parent (asexual reproduction) ○ Mitotic division is conventionally divided into 5 phases: prophase, metaphase, anaphase and telophase, and cytokinesis ○ Prophase, metaphase, anaphase, and telophase are considered part of mitosis  Mitotic Division The main role of mitosis is to separate sister chromatids and distribute one of each chromosome into each of the daughter cells  Early Prophase and Late Prophase During prophase, the cell prepares to divide its chromosomes ○ The chromatin (genetic material) becomes highly compacted ○ Parts of the cytoskeleton called centrosomes migrate to the two opposite ends of the cell ○ A structure called the mitotic spindle is formed in the center of the cell, connected to the two centrosomes on either side The mitotic spindle is made of microtubules ○ The nuclear envelope breaks down  Metaphase Chromosomes are positioned at the center of the cell along the mitotic spindle ○ They are ordered as evenly as possible to allow for equal distribution to the two daughter cells later in the mitotic division process  Anaphase During anaphase, sister chromatids are separated and pulled to opposite ends of the cell by the progressive shortening of the spindle microtubules Once separated, each chromatid is considered a new chromosome  Telophase During telophase, the mitotic spindle breaks down, and nuclear envelopes begin to form around the two new sets of chromosomes at each end of the cell Within each nucleus, the chromosomes decondense  Mitotic Division Cytokinesis is a separate step in the process that occurs at the end of mitotic division which refers to the separation of the cytoplasm from the parent cell to the two daughter cells The two daughter cells both have their own genetically identical nuclei and surrounding cell structures  Meiosis Division Meiosis is a specialized form of cell division that is needed to make gametes (sex cells) ○ Meiosis in female animals results in gametes that will mature into eggs ○ Meiosis in male animals produces gametes called sperm Meiosis reduces the genetic information passed to the daughter cells by half (n)  Meiosis One and Meiosis Two Meiosis occurs in two stages: ○ Meiosis I sorts each member of a homologous pair into two different daughter cells, reducing the chromosome sets from 2n to n ○ Meiosis II separates sister chromatids in each cell produced by meiosis I into two different daughter cells, forming four complete cells from the original parent  Meiosis One: Prophase One During prophase I, DNA segments are swapped between nonsister chromatids in a process called crossing-over ○ Crossing-over is a random process: the number and the location of a crossover can vary from one meiosis division to another (known as independent assortment)  Cell Fertilization Fertilization is the merging of a male and female gamete (sex cells) to produce a zygote A gamete has half the amount of genetic information as a somatic cell (non sex related cells in the body)  Diploid vs Haploid The genetic information in a gamete is called a haploid set (n) ○ Meiosis reduces the amount of genetic information transmitted by the parent cell to the daughter cells by half, so that only one set of the genetic information is inherited by each daughter cell The genetic information in a somatic cell (which is double the amount in a gamete) is called a diploid set (2n) Left: Gamete Right:  When Meiosis Begins: Females In females, meiosis begins while she is still a fetus ○ The cells begin to undergo meiosis, but then are put on hold until puberty ○ Once she hits puberty, one cell resumes the process of meiosis in the ovaries monthly ○ One of the daughter cells produced differentiates into an egg cell (ova) ○ This egg cell is either fertilized or reabsorbed into the uterine lining  When Meiosis Begins: Males In males, meiosis begins during puberty when hormones signal the testes to begin producing haploid cells (sperm cells) Male meiosis occurs daily and continues into old age ○ Female meiosis dwindles by age 50  Cell Division Functions Cells are specialized or differentiated to help the organism grow, maintain their structure and health, and reproduce within the human body ○ Differentiation allows the daughter cells produced during binary fission, mitosis, and meiosis allow for each cell to perform a specific function (think skin cells v blood cells) Cells’ Roles in the Human Life Cycle All of the genetic material present in a zygote remains present in the somatic cells of adults regardless of function ○ Germ line cells are reserved early in the growth of a zygote to be used later in life to produce gametes (sex cells) ○ Stem cells are found in adults that and are unspecialized in order to grow, regenerate, and repair tissues in the human body  Karyotypes Karyotypes display chromosomes in somatic cells (non sex cells) The 46 chromosomes in this micrograph represent the karyotype of a human male ○ The numbered chromosome pairs are non sex related chromosomes whereas the XY pair is the pair responsible for deciding sex (male or female)  Eukaryotic Chromosomes Eukaryotes carry two copies of each type of chromosome ○ This pair is known as homologous chromosomes Humans have 46 chromosomes divided into 23 homologous pairs Females typically have XX sex chromosomes whereas males typically have XY sex chromosomes  Genetic Variation Genetic variation is the natural differences in DNA sequences that exist between individuals of the same species ○ It's a key factor in evolution and the survival of species, and it can be caused by a number of factors Meiosis and fertilization contribute to differences in organisms’ genetics  Mutations and Alleles Mutations are the ultimate source of genetic variation in all types of organisms ○ A genetic mutation is a change in an organism's DNA sequence that can occur during cell division ○ Different variations of a particular gene created through mutations are called alleles  Crossing-Over Causes Mutations Crossing-over during meiosis 1 creates new sets of alleles through the exchange of DNA segments in a process known as genetic recombination ○ Ie- When sections of chromatids cross over, they are genetically recombining the DNA already available This makes mutations possible  Additional Pictures: Plant Mitotic Division