Cell Cycle & Division



Cell Cycle & Division
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Cell Division:

  • All cells are derived from preexisting cells (Cell Theory)
  • Cell division is the process by which cells produce new cells
  • Cell division differs in prokaryotes (bacteria) and eukaryotes (protists, fungi, plants, & animals)
  • Some tissues must be repaired often such as the lining of gut, white blood cells, skin cells with a short lifespan 
  • Other cells do not divide at all after birth such as muscle & nerve 

Reasons for Cell Division:

  • Cell growth
  • Repair & replacement of damaged cell parts
  • Reproduction of the species

Copying DNA: 

  • Since the instructions for making cell parts are encoded in the DNA, each new cell must get a complete set of the DNA molecules
  • This requires that the DNA be copied (replicated, duplicated) before cell division

Replication process

Chromosomes & Their Structure:

  • The plans for making cells are coded in DNA
  • DNA, deoxyribose nucleic acid, is a long thin molecule that stores genetic information
  • DNA in a human cell is estimated to consist of six billion pairs of nucleotides
  • DNA is organized into giant molecules called chromosomes
  • Chromosomes are made of protein & a long, single, tightly-coiled DNA molecule visible only when the cell divides
  • When a cell is not dividing the DNA is less visible & is called chromatin
  • DNA in eukaryotic cells wraps tightly around proteins called histones to help pack the DNA during cell division
  • Nonhistone proteins help control the activity of specific DNA genes
  • Kinetochore proteins bind to centromere and attach chromosome to the spindle in mitosis
  • Centromeres hold duplicated chromosomes together before they are separated in mitosis
  • Telomeres are the ends of chromosomes which are important in cell aging
  • When DNA makes copies of itself before cell division, each half of the chromosome is called a sister chromatid

  • DNA of prokaryotes (bacteria) is one, circular chromosome attached to the inside of the cell membrane

Chromosome Numbers:

  • Humans somatic or body cells have 23 pairs of chromosomes or 46 chromosomes (diploid or 2n number)
  • The 2 chromatids of a chromosome pair are called homologues (have genes for the same trait at the same location)


  • Human reproductive cells or gametes (sperms & eggs) have one set or 23 chromosomes (haploid or n number)
  • Every organism has a specific chromosome number


OrganismChromosome Number (2n)
Fruit fly8


  • Fertilization, joining of the egg & sperm, restores the diploid chromosome number in the zygote (fertilized egg cell)
  • Sex chromosomes, either X or Y, determine the sex of the organism
  • Two X chromosomes, XX, will be female and XY will be male
  • All other chromosomes, except X & Y, are called autosomes
  • Chromosomes from a cell may be arranged in pairs by size starting with the longest pair and ending with the sex chromosomes to make a karyotype
  • A human karyotype has 22 pairs of autosomes and 1 pair of sex chromosomes (23 total)

Human Male Karyotype


  • A section of DNA which codes for a protein is called a gene
  • Each gene codes for one protein
  • Humans have approximately 50,000 genes or 2000 per chromosome
  • About 95% of the DNA in chromosome is “junk” that does not code for any proteins

Cell Cycle:

  • Cells go through phases or a cell cycle during their life before they divide to form new cells 
  • The cell cycle includes 2 main parts — interphase, and cell division
  • Cell division includes mitosis (nuclear division) and cytokinesis (division of the cytoplasm)
  • Interphase is the longest part of a cell’s life cycle and is called the “resting stage” because the cell isn’t dividing
  • Cells grow, develop, & carry on all their normal metabolic functions during interphase
  • Interphase consists of 3 parts — G1, S, & G2phases


  • G1 or 1st Growth Phase occurs after a cell has undergone cell division
  • Cells mature & increase in size by making more cytoplasm & organelles while carrying normal metabolic activities in G1 
  • S or Synthesis Phase follows  G1  and the genetic material of the cell (DNA) is copied or replicated 

  • G2 or 2nd Growth Phase occurs after S Phase and the cell makes all the structures needed to divide

Cell division in Prokaryotes:

  • Prokaryotes such as bacteria do not have a nucleus
  • Prokaryotes divide into two identical new cells by the process of binary fission
  • Binary fission is an asexual method of reproduction
  • In binary fission,  the chromosome, attached to cell membrane, makes a copy of itself and the cell grows to about twice its normal size
  • Next, a cell wall forms between the chromosomes & the parent cell splits into 2 new identical daughter cells (clones)

Cell Division in Eukaryotes:

  • Eukaryotes have a nucleus & membrane-bound organelles which must be copied exactly so the 2 new cells formed from division will be exactly alike
  • The original parent cell & 2 new daughter cells must have identical chromosomes
  • DNA is copied in the S phase of the cell cycle & organelles, found in the cytoplasm,  are copied in the Growth phases
  • Both the nucleus (mitosis) and the cytoplasm (cytokinesis) must be divided during cell division in eukaryotes

Stages of Mitosis:

  • Division of the nucleus or mitosis occurs first
  • Mitosis is an asexual method of reproduction
  • Mitosis consists of 4 stages — Prophase, Metaphase, anaphase, & Telophase

  • Prophase:
    • Chromosomes become visible when they condense into sister chromatids
    • Sister chromatids attach to each other by the centromere
    • Centrioles in animal cells move to opposite ends of cell
    • Spindle forms from centriole (animals) or microtubules (plants)
    • Kinetochore fibers of spindle attach to centromere
    • Polar fibers of spindle extend across cell from pole to pole
    • Nuclear membrane dissolves
    • Nucleolus disintegrates
  • Metaphase:
    • Chromosomes line up in center or equator of the cell attached to kinetochore fibers of the spindle
  • Anaphase:
    • Kinetochore fibers attached to the centromere pull the sister chromatids apart
    • Chromosomes move toward opposite ends of cell
  • Telophase:
    • Nuclear membrane forms at each end of the cell around the chromosomes
    • Nucleolus reform
    • Chromosomes become less tightly coiled & appear as chromatin again
    • Cytokinesis begins


  • Cytoplasm of the cell and its organelles separate into 2 new daughter cells
  • In animals, a groove called the cleavage furrow forms pinching the parent cell in two

  • In plants, a cell plate forms down the middle of the cell where the new cell wall will be

Summary of Mitosis:




  1. Cell matures & carries on normal activities
  2. DNA copied & appears as chromatin
  3. Nucleolus visible
 Early Prophase  


  1. Chromosomes condense & become visible
  2. Centrioles separate & spindle starts forming
 Late Prophase

  1. Spindle forms with aster at each pole
  2. Nuclear membrane & nucleolus disintegrate
  3. Centromere of chromosomes attaches to spindle fibers

  1. Chromosomes line up at the equator of the cell attached to kinetochore fibers of spindle

  1. Centromeres split apart
  2. Homologs move to opposite poles of the cell

  1. Nuclear membrane & nucleolus reform
  2. Cell pinches into 2 cells in animals
  3. In plants, a cell plate separates the 2 new cells


Cancer is Uncontrolled Mitosis:

  • Mitosis must be controlled, otherwise growth will occur without limit (cancer)
  • Control is by special proteins produced by oncogenes
  • Mutations in control proteins can cause cancer

Meiosis & Sexual Reproduction

  • Reduces the number of chromosomes in new cells to half the number in the original cell
  • New cells have a single copy of chromosomes (23 total) but are not identical to each other or the original parent cell
  • Used for making gametes ( sperm and eggs) with the haploid or n number
  • In meiosis, cells divide twice after a single DNA duplication
  • Meiosis I separates homologs & the Meiosis II separates sister chromatids
  • Meiosis I stages are Prophase I, Metaphase I, Anaphase I, & Telophase I
  • Meiosis II stages are Prophase II, Metaphase II, Anaphase II, & Telophase II
  • Produces 4 haploid cells or gametes
  • When a sperm fertilizes an egg to form a zygote, the diploid number of chromosomes is restored (23 + 23 = 46)
  • Egg cells or ova (ovum, singular) are larger , nonmotile cells
  • Gametoogenesis is meiosis producing eggs & occurs in the female’s ovaries



  • Sperms contain less cytoplasm so they’re smaller & have a flagellum to swim to the egg
  • Spermatogenesis is meiosis producing sperm cells & occurs in the testes



Meiosis I:

  • The cell that undergoes Meiosis I is a primary spermatocyte or oocyte
  • Prophase I:
    • Chromosomes coil tightly & are visible
    • Nuclear membrane & nucleolus disintegrate
    • Spindle forms
    • Synapsis (joining) of homologous chromosomes occurs making tetrads
    • Kinetochore fiber forms on each chromosome
    • Chromosomes in tetrad exchange fragments by a process called crossing over

  • Metaphase I:
    • Tetrads become aligned in the center of the cell attached to spindle fibers
  • Anaphase I:
    • Homologous chromosomes separate
  • Telophase I:
    • May not occur in all species
    • Cytokinesis occurs producing 2 cells
    • In females,  2nd cell in females is called the 1st Polar Body
    • 1st Polar Body dies due to uneven splitting of the cytoplasm

  • Prophase II:
    • Cells called Secondary Spermatocytes or oocytes
    • DNA is not copied before cell divides
    • Chromatids attach to spindle fiber
  • Metaphase II:
    • Chromosomes become aligned in the center of the cell attached to spindle fibers
  • Anaphase II:
    • Sister chromatids separate randomly
    • Called independent assortment
  • Telophase I:
    • Cytokinesis occurs producing 4 cells in males called spermatids
    • Spermatids mature & form flagellum to become sperm
    • Cytokinesis in females produces a 2nd Polar Body that dies and an ootid 
    • Ootids mature to become ovum or egg

Asexual & Sexual reproduction:

  • Evolution is the slow process of change in living populations over time
  • Variations are differences that occur due to crossing-over among members of a sexually reproducing population
  • Variations are important to the survival of individuals in a population (some must survive to reproduce)
  • Asexually reproducing organisms rarely show variations because the organisms have identical genes