Cells, DNA and cell division
•Definition: the cell is the basic unit of life.
•Cells allow us to eat, breathe, grow, reproduce and eliminate waste.
•Cell components visible under the microscope:
•Cell membrane
•Cytoplasm
•Nucleus
Cell Component
(Organelle)
Function
Mitochondria
Produces an energy molecule (ATP)
through cellular respiration
Golgi Apparatus (Body)
Stores and transports material within and
outside of the cell
Endoplasmic Reticulum
Modifies proteins and lipids and
transports material
Ribosome
Builds proteins (chains of amino acids)
Lysosome
Breaks down and recycles molecules
Nucleus
Contains and protects DNA (genetic
material)
Cytoplasm
Gelatinous material that establishes an
environment that is hospitable for cellular
activity
Cytosol
Intracellular fluid made of dissolved
substances in water
Cellular Membrane
Protects the cell and allows movement
across its barrier
Nuclear Membrane
Barrier around the nucleus, allows
exchanges with the rest of the cell
Genetic material, DNA, (deoxyribonucleic acid) is found inside the nucleus. DNA
contains all the information needed to manufacture proteins that have specific tasks to
carry out. DNA’s shape is a double helix.
DNA is divided into segments called genes, which can be found on chromosomes. The
genes present will determine which proteins a cell will manufacture. A single DNA
molecule will contain thousands of genes.
Genes: A gene is a segment of DNA that contains the genetic information required to carry
out a particular job. Different combinations of genes make each one of us unique
(Exception: identical twins).
The number of different combinations is called the genetic diversity of a particular specie.
The nucleus of a human cell contains about 25 000 genes. This complete set of genes is
called the GENOME.
What are chromosomes? Chromosomes are long strands of genetic information located in
the nuclei of cells. Each chromosome is a very long molecule of tightly coiled DNA.
Chromosomes are most visible during cell division when they replicate.
Structure of DNA: Each step in the helix consists of a pair of building blocks called
nucleotides. A nucleotide is made up of a deoxyribose sugar, a phosphate and a nitrogencontaining base. There are 4 bases. The four bases are: Adenine, Guanine, Cytosine and
Thymine (A,G,C,T).
These bases always pair together in the same way. Base pairs hold the two strands of the
DNA helix together. The rules for base pairing are: A always pairs with T, C always pairs
with G.
DNA and cell division: A DNA molecule is able to make a copy of itself. This is how
chromosomes are copied before cell division. DNA’s ability to copy itself means that all the
genetic information is passed on to new cells. DNA can make copies of itself because it is
double stranded.
How? The DNA molecule “unzips” as the steps of the ladder separate and the molecule
splits into two single strands. New bases from the cell move in and following the pairing
rules match themselves to the bases on the single strands. Each single strand builds up into
a new double strand. The new DNA molecules are identical to each other and the original
DNA molecule.
In humans: Every cell (except for sex cells) contains 23 pairs of chromosomes. One
chromosome from each pair comes from mom and the other from dad. 22 of those pairs
are numbered 1-22 and the other pair is x and y (male) or x and x (female).
Cell Division
Mitosis and Meiosis
Cell division: process by which new cells are produced for the purpose of growth,
tissue repair and sexual reproduction.
•1.Mitosis:
-division of somatic (body) cells
For growth and tissue repair
Diploid cells (contain 2 copies of each chromosome)
•2.Meiosis
-division of gametes (sex cells)
For sexual reproduction
Haploid cells (contain 1 copy of each chromosome)
•MITosis takes the cell and Makes It Two (diploid)
•Meiosis has to do with sex
•From the cell’s point of view:
•mITosis results in Identical Twins
•mEioSis results in Egg and Sperm (haploid)
Chromosome structure
Composed of DNA and protein (histones) all tightly wrapped up in one package
Duplicated chromosomes are connected by a centromere
Phases of Mitosis
•Interphase*
•Prophase
•Metaphase
•Anaphase
•Telophase
*note: Interphase occurs just before Mitosis begins. During this stage, the cell increases in
size and DNA has replicated in the nucleus of the cell.
After Mitosis:
Cytokinesis: The actual splitting of the identical cells into two separate cells is called
cytokinesis.
Mitosis results in two diploid daughter cells, each identical to the parent cell.
Meiosis
Meiosis is only seen in the gametes (i.e. the sperm and eggs) of sexually reproducing
organisms.
The stages of meiosis can be broken down into two main stages, Meiosis I and Meiosis II
Meiosis I can be broken down into four substages: Prophase I, Metaphase I, Anaphase I and
Telophase I.
Meiosis II can be broken down into four substages: Prophase II, Metaphase II, Anaphase II
and Telophase II
It is important to note that during meiosis, the cell splits twice. At the end of Telophase II,
cytokinesis reaches completion, creating four haploid daughter cells.