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Genetics
DNA
• Our genetic blueprint
(instructions)
• Used by all organisms to
store hereditary
information
• DNA segments that code
for the construction of cell
components are called genes
(hereditary units, what’s
passed down in generations)
Structure of DNA
• Double stranded helix
• Circular in prokaryotes
• Arranged in
chromosomes in
eukaryotes
• Supercoiling of the DNA
allows in to be packed
tightly in the nucleus
(conserves space)
Nucleotide
• The basic unit of DNA
• Composed of:
– A sugar (deoxyribose)
– A phosphate group
– A nitrogenous base
Nucleotide (cont)
• 4 nitrogenous bases
–
–
–
–
Adenine
Guanine
Cytosine
Thymine
Bonding in DNA
• Bonds between sugar and
phosphate group are called
phosphodiester bonds
• Complimentary base pairing
between nitrogenous bases
(hydrogen bonds)
– Purines bond with pyramidines
– Adenine and Thymine (2 Hbonds)
– Guanine and Cytosine (3 Hbonds)
Homework
•
•
•
•
Read p. 186-190
P. 198 #1, 2, 4
P. 204 #1
P. 205 #3, 9
Chromosome
Gene: Sequence of bases that code
for a particular trait. Each
chromosome has 100’s to 1000’s of
genes
Cell Division
• In unicellular organisms, cell division
is reproduction. Called binary fission.
• In multicellular organisms, cell
division is for reproduction (meiosis)
or for growth and repair (mitosis)
Cell Division
4 Major Events:
1.
Reproductive signal
•
This signal, which may come from inside or outside the
cell, initiates the cellular reproductive events
2. DNA Replication
•
Production of an identical copy of all chromatin in
nucleus
3. Mitosis
•
Separation of chromatin copies into the 2 new cells
4. Cytokinesis
•
Division of cytoplasm, fluid and organelles between the
2 new cells
DNA Replication
• Before cell division can occur, DNA must make an
exact copy of itself
• This means each cell will have an identical copy of
the DNA
1.
2.
3.
Unzipping the double helix
Complimentary base pairing
Reforming of hydrogen bonds
The Cell Cycle
Growth & metabolic
processes
Replication
Preparing for division
Mitosis
Cell division in which
the end result is 2
new cells with
nuclei identical to
the original nucleus
Interphase
• Cells only spend a small amount
of time in mitosis
• 90% of the time is spend in
interphase
• This is the time period
between phases of mitotic
divisions
• It appears that the nucleus is
inactive, but it is performing
regular metabolic activity
Interphase
• During interphase,
chromosomes exist as
chromatin (long, thin strands)
and are not visible under a
compound microscope
• Near the end of interphase,
after receiving a reproductive
signal, the chromatin
replicates and the nucleus is
now ready for mitosis
Prophase
Series of events preparing the
nucleus to distribute its
replicated chromatin into 2
separate and identical sets
Early Prophase:
• Nucleolus begins to
disintegrate
• Thin strands of chromatin
begin to shorten and thicken
Middle Prophase:
• Chromatin becomes visible
• Nucleolus is completely
disintegrated
• Nuclear membrane breaks down
• Microtubules called spindle
fibres appear at poles of the
cell
• Resulting array of fibres is
called the spindle
Late Prophase:
• Nuclear membrane is gone
• Thickened chromatin has
formed double-stranded
chromosomes
• Double-stranded chromosomes
move towards the centre of the
cell
Metaphase
• Double-stranded chromosomes line
up along the equator of the cell
• Each pair of chromatids is
attached by its centromere to a
spindle
Late Metaphase:
• Centromeres split, doublestranded chromosomes begin the
pull apart in opposite directions
• Once this happens, chromatids are
called single-stranded
chromosomes
• Each has half of a centromere
Anaphase
• Single-stranded chromosomes
form a V-shape, pointing
toward the poles of the cell,
still attached to spindle by
centromere
• Chromosomes are pulled along
by the contraction of the
protein molecules in the
spindles fibres
• At the end of anaphase, there
is a complete set of singlestranded chromosomes at each
pole
Telophase
• Nuclear membrane reforms
around each set of chromosomes
• Nucleolus reappears in each
nucleus
• Chromosomes begin to unravel
into chromatin
• Final product = 2 complete
nuclei identical to each other
and the original
• At the end of telophase cell
division is not complete
Cytokinesis
The division of the cytoplasm to form two new cells
Animals:
• As chromosomes reach opposite
poles, the nuclear membrane
begins to reform
• The cell membrane begins to
pinch together at the equator
• This groove is called the
cleavage furrow
• The cleavage furrow becomes
deeper and deeper until
eventually the cytoplasm splits
into two masses
Cytokinesis
Plants:
• A structure known as the
cell plate forms from the
fusion of tiny vesicles
produced by the golgi
apparatus
• Appears as a faint, thin line
along the equator and
gradually becomes thicker
as each new cell constructs
its own cell wall from
molecules of cellulose
Plant vs. Animal Cells
• Plant cells do not have centrioles or astral rays (star
arrangement around centriole)
• Metaphase and anaphase are the same in plants and
animals
• During late telophase, centrioles in animal cells
replicate so each new cell has a pair
Quickie Quiz
1.
The 4 major events of cell division (in order) are:
2.
Draw and label the following: centomere, chromatin,
chromosome, sister chromatids
3.
Cytokinesis is marked by the formation of a(n) _____________
in animal cells and a(n) _______________ in plant cells.
4.
For the following statements identify the mitotic stage:
a)
Nuclear membrane disintegrates
b)
Chromosomes are pulled by spindles to the poles
c)
Chromosomes begin to unravel into chromatin
d)
Chromatin becomes visible
e)
Chromosomes line up along the equator.
Meiosis
• In sexual reproduction, 2 gametes (egg & sperm) unite
to form the first cell (zygote) of the new individual
• If the gametes were produced by normal mitosis, the
zygote produced would have double the number of
chromosomes
– E.g. 46 + 46 = 92 Not Viable!
• Gametes are formed by meiosis, a form of cell division
in which the number of chromosomes in the gametes
are reduced by half
Meiosis
Haploid (n): Single set of chromosomes (23 in humans)
Diploid (2n): Paired set of chromosomes in humans (46
in humans)
Homologous Chromosomes:
- Paired chromosomes are called homologous
chromosomes
- Homologous chromosomes pair during meiosis
- One from mom, one from dad
- Have the same sequence of genes, but a given gene
may exist in a slightly different form (allele). This
allows for genetic recombination.
Meiosis involves DNA replication followed by two
nuclear divisions: Meiosis I & II
Meiosis I:
• Reductional division - 1
Diploid (2n) cell to 2 haploid
(n) daughter cells
• During prophase I, synapsis
and crossing over occur
• Crossing over allows for the
exchange of genetic material
Tetrad
- During metaphase I, homologous
chromosome pairs (tetrads) line up at
equatorial plate
- During anaphase I, each chromosome
separates from its homologue
- Two non-identical nuclei result from telophase I
- Cells are considered haploid, the homologous pair has
been split
Meiosis II:
• No replication!
• 2 haploid (n) cells, from Meiosis I, give rise to 4
haploid (n) daughter cells)
• Similar to mitosis, sister chromatids separate
• Because of crossing over, each cell has a slightly
different genetic make-up
Quickie Quiz
1.
Gametes are (haploid/diploid) and contain ___
chromosomes in humans.
2.
Somatic cells (body cells) are (haploid/diploid) and
contain ____ chromosomes in humans
3.
Synapsis, the pairing of ___________ to form a(n)
_______ occurs during __________. Here, the
chromatids often intertwine and sometimes break off
and exchange genetic material in a process known as
______________.
4.
What is the end result of meiosis? Why is this
necessary?