Chapter 6 Reproduction at the Cellular Level The

Chapter 6 Reproduction at the Cellular Level

The Cell Cycle • 2 processes? • • • Mitosis for…? Somatic (non-reproductive cells) Meiosis for…? Germ (reproductive) cells…called? Both cases involve duplicating… The DNA molecule that contains the genetic info

Cellular Reproduction Basics Characteristic of life In the beginning… 1 cell Can remain 1 trillions Ex. human brain = billion nerve cells • ________ lifelong • Exception? • Why do you suppose that is? • • ~35

Chromosomes? • Histones allow tight DNA coiling • ____ forms • Chromosomes are loops within loops of chromatin • # doesn’t affect organism complexity • Highly compacted (~10, 000 x) DNA (−) + + 8 histones, 146 bp of DNA in 1. 65 turns

Chromosomes • Homologues (homologous pairs): your inherited chromosome pairs • Carry same information in same places • _____: constricted region, holds chromosomes in place (a “waist”) • Sister chromatids: 2 identical copies of homologues (pre -cell division) • Cell content duplication followed by cell division

Prokaryotic Cells • Cell division results in reproduction of the entire organism • Called _______ • Often a response to external factors (environ or nutrient conc) • Ex. E. coli divides continuously (~40 min @ 37°C)…why 37°C? • With enough resources…. every 20 min!

Eukaryotic Cells • Eukaryotes have 2 problems… • Chromosomes save space (DNA strand=2 m, nucleus=5 -8µm) • Each roughly 3 billion base pairs • About how much is this? • Eukaryotic cells do not constantly divide • _____ divide only when necessary

Interphase • Cell cycle ordered • NOT a phase of mitosis • Performs normal functions • 90% interphase • 3 stages?

Interphase • G(rowth)1: doubles organelles • Gathers material for DNA replication • S(ynthesis): DNA replication occurs, chromosomes form (______) ______ • G 2: cell growth, protein synthesis • Nervous/cardiac cells only interphase after growth (__)

Mitosis • Several steps: • Each sister chromatid is genetically identical • Goal: 2 genetically identical ____ should result

centrosomes The Spindle • Part of cytoskeleton, pulls chromatids apart • Spindle formed in prophase • Centrosomes organize microtubules • M(itosis) phase: centrioles move to opposite poles centrosomes

Individual chromosomes visible Spindle forms & moves to poles Chromatids repulsed by poles & stuck together Move “aimlessly”

The pulling & pushing aligns chromatids on equator (metaphase plate) Chromosomes maximally condensed Chromatids separate Become daughter chromosomes Nuclear envelope reforms Chromosomes uncoil Furrow forms Cytokinesis begins at end of telophase

Watch mitosis video here

Animal Cytokinesis • Mitosis & cytokinesis take about 1 hour in most mammals • Division of cell contents • Animal cells…cleavage • ______ forms (membrane indentation) • Actin & myosin filaments form band (contractile ring) • Think of a drawstring on a trash bag

Plant Cytokinesis • Form _____ • Rigid wall can't furrow • Builds new wall & plasma membrane • Builds up (match old membrane) • Strengthened with cellulose

Cell Cycle Control System • • Must be coordinated Think of a washing machine Ensures proper order Checkpoints? • 3 points: G 1, G 2, M

Cell Cycle Checkpoints • G 1: most important • Past this point…must divide! • G 2: ensures DNA/ chromosome replicated • M: ensures proper chromosome alignment • Can enter G 0 if fixing problem • What can damage DNA? • DNA damage not repairable, cells enter _____

Apoptosis • • So what if something goes wrong? Programmed cell death Cell shrivels, loses contact inhibition Nucleus fragments Plasma membrane blisters Cell breaks up, WBC consumes Exs. frog's tail & finger webbing

Senescence • _____: aging of cells due to telomere shortening • Telomere? • About 15, 000 bp, lose 50 -200 bp each time • Stops chromosome fusing • When the last primer is removed, no DNA can attach to replace it • If telomere becomes too short, cell stops reproduction because chromosomes fuse