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Chromosomes, The Cell Cycle and Mitosis. Cells. Genes. Nucleus. Organism. Chromosomes. DNA. Chromosome. A structure that forms when DNA wraps around proteins and coils up. Chromosomes only form right before the cell is going to divide. Each half of a chromosome is called a chromatid.

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chromosomes the cell cycle and mitosis
Chromosomes, The Cell Cycle and Mitosis







  • A structure that forms when DNA wraps around proteins and coils up.
  • Chromosomes only form right before the cell is going to divide.
Each half of a chromosome is called a chromatid.

Sister chromatids are identical to each other.

Chromatids are joined at a centromere.

Gene: Segment of DNA found on a chromosome.

Each gene codes for a specific traits.

Each chromosome has many genes on it.

Chromosome Structure

two types of chromosomes
Two Types of Chromosomes
  • Autosomes- chromosomes that do not help determine the gender of the individual (most of the chromosomes in your body)
  • Sex Chromosomes- in humans, they are X and Y; they determine the gender of the individual
    • XX = Female
    • XY = Male
two types of cells
Two types of Cells
  • Gametes- the reproductive cells (sperm for males, eggs for females).
  • Somatic Cells- all of the cells in the body that are not gametes.
    • Ex – skin cells, muscle cells, brain cells
chromosome number in cells
Chromosome Number in Cells
  • Different cells can have different numbers of chromosomes depending on where in the body they are located.
  • Diploid VS Haploid
    • Diploid (2N)– two sets of chromosomes (one from mom, one from dad)
      • Somatic cells are diploid
    • Haploid (N or 1N)– one set of chromosomes
      • Gamete cells are haploid

Different organisms have different #’s of chromosomes.

Ex: Drosophila melanogaster (fruit flies) : 8

Homo Sapiens (human): 46

Podocnemis uuiilis (turtle): 28


Homologous Chromosomes

Karyotype - A karyotype is a picture of all of the chromosomes in the cell of an individual, taken right before the cell divides.

Humans: 22 Pairs of Autosomes, 1 Pair of Sex Chromosomes

disorder activity with questions
Disorder Activity with Questions
  • What notation would you use to characterize each disorder?
  • Is this disorder an autosomal or sex chromosome disorder?
  • How many chromosomes does a person have with each disorder?

Point Mutations (occur in the DNA on the genes)

  • Addition – adding on of extra genes
    • A B C D  A B B C D

B. Deletion – substraction of genes

    • A B C D  A C D

C. Substitution – a different is gene is put in place

    • A B C D  A B F D

D. Translocation – genes are switched around

    • A B C D  C D A B


Chromosomal Mutations (occur in the genes on the chromosome)

  • Some genetic disorders are characterized by having too many or too few chromosomes.
trisomy 21
Trisomy 21
  • Also known as “Down’s Syndrome”
  • Characterized as having an extra chromosome on the 21st pair.
  • Affects approximately 800 in 1,000


  • Maternal Age Risk at birth
  • 15 to 24 years 1 out of 1300
  • 25 to 29 years 1 out of 1100
  • 35 years 1 out of 350
  • 40 years 1 out of 100
  • 45 (and older) 1 out of 25
klinefelter s syndrome
Klinefelter’s Syndrome
  • Characterized as a male with an extra “X” chromosome (XXY)
  • The condition exists in roughly 1 out of every 500 to 1,000 males
  • Extra “X” causes slight feminization

(including small penis, tall physique, enlarged breast tissue and


turner syndrome
Turner Syndrome
  • Characterized as a female with only one “X” chromosome (XO)
  • Turner syndrome occurs in about 1 out of 2,000 live births
  • Symptoms include webbed neck, drooping eyelids, short height

and infertility



All somatic cells undergo the cell cycle in order to prepare for cell division (mitosis)

Mitosis is a form of asexual reproduction. The cells that form as a result of mitosis are clones (identical copies) of the original cell.

Why do our cells need to divide????

1. Growth of the organism

2. Repair of cells or replacement of “dead” cells

stages of the cell cycle
Stages of the Cell Cycle
  • G1 Phase
  • S Phase
  • G2 Phase
  • Mitosis
  • Cytokinesis
g1 growth 1 phase
G1 (growth 1) Phase
  • First growth stage
  • Cell increases

in size

synthesis s phase
Synthesis (S) Phase
  • Copying of all DNA
    • DNA is replicated
  • Chromosomes duplicated

Chromosomes needs to be copied before a cell divides, so that each new cell has the correct amount of DNA and the correct number of chromosomes.

g 2 growth phase
G2 (growth) Phase
  • Cell continues growing.
interphase resting stage
Interphase – Resting Stage

Interphase is the first 3 phases of the cell cycle together

G1, S and G2

Cells carrying on normal activities

  • Chromosomes aren’t visible (not coiled up).
  • Normal cell metabolism and processes are occurring.
  • Occurs before mitosis. Includes phases of cell cycle “getting ready” for mitosis
mitosis m and cytokinesis phases
Mitosis (M) and Cytokinesis Phases
  • Cell growth & protein production have stopped.
  • The cells energy is used to make 2 daughter cells (splitting of original cell into 2).
    • Mitosis – division of the nucleus into 2 nuclei in one cell.
    • Cytokinesis – division of cytoplasm, resulting in 2 new cells.
cell cycle checkpoints
Cell Cycle Checkpoints

The cell has checkpoints to ensure that each phase was completed correctly.

G1: is the cell big enough

S/G2: did the DNA copied correctly

Mitosis (and Cytokinesis): making sure the cell divided correctly.

Checkpoints not working and the cell dividing uncontrollably can lead to cancer.


Give each student has a copy of the “Understanding Cancer” worksheet from the NIH: Cell Biology and Cancer curriculum guide. This is available online at:

(To see the entire Cell Biology and Cancer curriculum guide, go to

Use a computer and projector to show students the 5 animations available online at:

  • IPAD: Internet
  • CODE: S2013109
  • GOOGLE: Endicott research center
  • The disease caused by an uncontrolled division of abnormal cells in a part of the body.
  • A malignant growth or tumor resulting from such a division of cells.
stages of mitosis
Stages of Mitosis
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
  • DNA coils tightly & becomes visible as chromosomes.
  • Nuclearmembrane disappears
  • Centrioles migrate to poles
    • Centrioles: organelles that

produce spindle

  • Spindle fibers begins to form
  • Spindle fibers from centrioles attach to each chromosome.
  • Chromosomes line up in the middle of the cell.
  • Chromosomes are already separated into sister chromatids at the centromere.
  • Spindle fibers shorten and each chromatid is pulled to the opposite end of cell.
  • Separation of chromosomes into chromatids is completed
  • Nuclear membrane reforms.
    • Now have two nuclei in one cell
  • Chromosomes uncoil
  • Cytoplasm division
  • Occurs after chromosomes separate and two nuclear envelopes reappear.
  • Forms two, identical daughter cells (they are identical to the original cell)

Check your chromosome numbers!!!

Human Cell:

Before G1 –

46 chromatids

After S –

92 chromatids (46 chromosome pairs)

After mitosis –

2 cells each with 46 chromatids!!

Daughter cells are identical to the parent cell!!!