3/19/17. Chromosomes. Chromosome Structure. Chromosome Structure. Chromosome Structure. Chapter 10: Cell Growth & Division

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1 Chapter 10: Cell Growth & Division Section 2: The Process of Cell Division Chromosomes DNA (deoxyribonucleic acid) a molecule that stores genetic information, which controls the development and functioning of living things double helix shape (looks like a twisted ladder) In a eukaryotic cell (cell that has a nucleus), DNA is coiled into chromosomes During cell division, chromosomes are X shaped structures made of coiled-up DNA Chromosomes are intended to organize and compact DNA DNA is formed into a double helix Imagine 2 strings twisted around each other. In order for DNA to fit within a cell the double helix is wrapped around protein balls called histones The DNA and histones are formed into coils, which are formed into supercoils The supercoils are again coiled to form a chromosome The chromosome is shaped like an X Chromatids Joined strands of duplicated genetic material The left and right halves The center where the two sister chromatids are held together is the centromere When a cell divides, the chromosome splits at the centromere The chromatids separate Each chromatid goes to one of the new cells 1

2 When cells aren t dividing, DNA is not coiled into chromosomes They are uncoiled so that the genetic information may be easily read and used This uncoiled form of DNA is called chromatin; substance made up of DNA & protein Prokaryotes usually only have 1 circular chromosome Since they don t have a real nucleus, the chromosome is often attached to the cell membrane Different species have different numbers of chromosomes The number of chromosomes has little or nothing to do with the complexity of the species Humans have 46 chromosomes Gorillas and chimpanzees have 48 Dogs have 78 Animals have 2 types of chromosomes Autosomes contain genetic information for the individual organism Sex Chromosomes determine the sex of the organism They may also contain some genetic information Humans have 2 sex chromosomes and 44 autosomes Every cell of an organism that is produced by sexual reproduction has 2 copies of each autosome One autosome came from mom, the other came from dad The twin chromosomes are called homologous chromosomes, or homologues Homologous Chromosomes code for the same traits Homologues are the same size and shape, and they carry the same genes The genes may not be coded the same!!! Mom may give you a blue-ey ed g ene, but dad may g iv e us a g reen-ey ed g ene Ø The way our bodies choose which gene to follow is complicated. We ll see how it works later. 2

3 Karyotype Chromosomes are often seen in a karyotype A karyotype is a picture of chromosomes It is taken from a cell that is getting ready to divide The homologues are paired together The chromosomes are ordered and numbered so they are easy to examine Normal cells are diploid, meaning they have the 2 sets of chromosomes They are abbreviated as 2n In humans, diploid(body or somatic) cells have 46 chromosomes Reproductive cells/gamates (sperm or egg cells) are haploid, meaning that they have 1 set of chromosomes They are abbreviated as 1n This is necessary so that, during reproduction, offspring end up with the correct amount of chromosomes In humans, haploid(sex) cells have 23 chromosomes Cell Division in Prokaryotes Binary Fission Prokaryotic cells divide to form offspring through binary fission The chromosome copies itself, forming 2 rings The cell grows until it is almost double in size The cell membrane pinches the cell in half, and 2 new cells are formed Each of the 2 new cells is identical to the parent 3

4 There are 2 types of cell division in eukaryotes Mitosis results in new cells identical to the parent cell This occurs in reproduction of unicellular organisms This results in growth of multicellular organisms Meiosis results in sex cells with half the normal number of chromosomes The Cell Cycle is a set of events that repeat in the life of a cell Cell division is a small part of the cycle The time between divisions is called Interphase The cell carries out its activities The cell grows and prepares for division Cells that seldom divide spend most of their time in this phase (interphase) Interphase 1. G 1 Phase: cells grow to mature size; normal activities are also carried out during this stage which is why G1 is the longest phase of interphase. 2. S Phase(Synthesis): cell DNA is copied/replicated 3. G 2 Phase: cells prepare for division by producing organelles and molecules needed for division Centrosome: place where spindle fibers are produced 4. G 0 Phase: some cells exit the cell cycle and never divide This usually happens after G 1 Phase. (does not always happen) Ex. nerve cells Most of the time is spent in G1 Mitosis results in new cells identical to the parent cell There are 4 parts to mitosis: 1. Prophase 2. Metaphase 3. Anaphase 4. Telophase Prophase: DNA forms into chromosomes The nucleus breaks down and disappears Dark spots (centrosomes) form at the poles; organelle that helps assemble the spindle & aids chromatid movement in a dividing cell Animal cells have additional cylinders called centrioles 4

5 Prophase continued: The mitotic spindle (made of spindle fibers) forms Kinetochore fibers attach to the chromosomes centromeres (center) at the kinetochore These are like ropes that will attach to the chromosomes and pull them apart Polar fibers extend across the dividing cell between the centrosomes (dark spots at the poles) Metaphase: The kinetochore fibers move the all the chromosomes to the cell equator This is when the chromosomes are easiest to identify, so karyotypes are usually taken during this phase Anaphase: The chromatids of each chromosome separate The chromosomes begin to move toward the poles When chromatids separate, they are considered to be individual chromosomes; thick strand made up of condensed DNA Telophase: The chromosomes reach opposite ends of the parent cell The spindle fibers disappear Chromosomes uncoil and disappear A nucleus forms around each set of chromosomes Cytokinesis The cell divides through cytokinesis Separating the organelles and the cytoplasm of the cell Animal cells form a cleavage furrow which pinches the cytoplasm in two & forms 2 cells; 2 daughter cells Plant cells form a cell plate between the 2 forming cells The Cell Cycle Cytokinesis 5

6 Cell Cycle Sequence G1àSàG2àMitosisàCytokinesis G1, S, & G2 (the first three phases) together make up Interphase Importance of Mitosis growth repair replenish cells that have died 2/animation mitosis_and_cytokinesis.html 6