Chapters 12, 13, 16, 17
Limits to Cell Growth
 The larger a cell becomes, the more demands a cell places
on its DNA
 If extra copies of DNA are not made, an “information crisis”
would occur
 The cell also has more trouble moving nutrients and
wastes across the cell membrane
 Food, oxygen, water, and wastes move through the cell
membrane
 The rate at which the exchange takes place depends on the
surface area of the cell
 The rate at which food and oxygen are used up and wastes
produced depends on the cell’s volume
Ratio of Surface Area to Volume
 Volume increases much more rapidly than surface area
causing the ratio of surface area to volume to decrease
 This decrease creates serious problems for the cell
such as:
 Inability to remove wastes from the cell
 Lack of sufficient oxygen and food entering through the
cell membrane
Division of the Cell
 The process by which a cell divides into two new
daughter cells is called cell division
 Before cell division occurs, the cell replicates, or
copies, all of its DNA
 Each daughter cell gets one complete set of genetic
information
 Each daughter cell also has an increased ratio of
surface area to volume
Cell Division
 Each cell has only one set of genetic information
 must be copied before cell division begins
 The first stage, division of the cell nucleus, is called
mitosis
 The second stage, division of the cytoplasm, is called
cytokinesis
 Reproduction by mitosis is classified as asexual
 Mitosis is the source of new cells when a multicellular
organism grows and develops
Chromosomes
 Chromosomes are made of DNA (genetic information)
and proteins (histones)
 The cells of every organism have a specific number of
chromosomes
 Fruit flies = 8, human = 46, carrots = 18
 Chromosomes are not visible in most cells except during
cell division
 Each chromosome consists of two identical sister
chromatids which separate during cell division
 Each pair of chromatids is attached in an area called the
centromere
The Cell Cycle
 Interphase is the period in between periods of cell
division
 The cell cycle is the series of events that cells go through
as they grow and divide
 During the cell cycle, a cell grows, prepares for division,
and divides to form two daughter cells, each of which
then begins the cycle again
 The cell cycle consists of four phases
 M, S, G1, and G2
The Cell Cycle
The Cell Cycle
Events of the Cell Cycle
 During the normal cell cycle, interphase can be quite
long, whereas the process of cell division takes place
quickly
 The G1 phase is a period in which cells do most of their
growing
 In the S phase, chromosomes are replicated and the
synthesis of DNA molecules takes place
 During the G2 phase, many of the organelles and
molecules required for cell division are produced
Mitosis
 Prophase:
 Chromosomes become visible, centrioles begin to organize the
spindle and move to opposite ends of the cell, fibers attach to
centromeres, nucleolus and nuclear envelope disappear
 Metaphase:
 Chromosomes line up across the center of the cell
 Anaphase:
 Centromeres split and individual chromatids are separated into
two groups near the poles
 Telophase:
 Chromosomes disperse, nuclear envelope and nucleolus reform, spindle breaks apart
Mitosis
Cytokinesis
 Cytokinesis is the division of the cytoplasm itself and
usually occurs at the same time as telophase
 In most animal cells, the cytoplasm is drawn inward
until the cytoplasm is pinched into two nearly equal
parts. This is called a cleavage furrow.
 In plants, a structure known as the cell plate forms
midway between the divided nuclei
Cytokinesis
in Animal
Cells
Controls on Cell Division
 When placed on a petri dish with a thin layer of nutrient
solution, cells will grow until they form a thin layer on
the bottom of the dish
 When cells come into contact with other cells, they
respond by not growing
 If cells are removed from the center of the dish, the cells
bordering the open space will divide until they have
filled the space
 Controls on cell growth and division can be turned
off and on
Cell Cycle Regulators
 Several scientists discovered that cells in mitosis




contained a protein that when injected into a
nondividing cell, would cause a mitotic spindle to form
They called this protein cyclin because it seemed to
regulate the cell cycle
Cyclins regulate the timing of the cell cycle in eukaryotic
cells
Proteins that respond to events inside the cell are called
internal regulators
External regulators respond to events outside of the cell
Cell Cycle Regulators
Uncontrolled Cell Growth
 Cancer is a disorder in which some of the body’s own
cells lose the ability to control growth
 Cancer cells do not respond to the signals that regulate
the growth of most cells
 They divide uncontrollable and form masses of cells
called tumors that can damage the surrounding tissues
 Causes include smoking, radiation, and viral infections
 Damaged or defective p53 genes cause the cells to lose
the information needed to respond to signals that
would normally control their growth
 p53 is a protein that functions to block the cell
cycle if the DNA is damaged. If the damage is
severe, this protein can cause apoptosis (cell
death).
 p53 levels are increased in damaged cells. This
allows time to repair DNA by blocking the cell
cycle.
 A p53 mutation is the most frequent mutation
leading to cancer.
 p27 is a protein that binds to cyclin and cdk
blocking entry into S phase. Recent research
(Nature Medicine 3, 152 (1997)) suggests that
breast cancer prognosis is determined by p27
levels. Reduced levels of p27 predict a poor
outcome for breast cancer patients.
Uncontrolled
Cell Growth
CHAPTER 13: MEIOSIS AND
SEXUAL CYCLES
Meiosis - cell division that reduces the diploid
# to the haploid # in the formation of sex cells
(gametes).
Example (Humans) - 46 chromosomes is
reduced to 23.
MOST IMPORTANT - the cells produced at the
end of meiosis contain one chromosome of
each homologous (matching) pair.
TERMS:
GENE - HEREDITARY INFORMATION, IN A SECTION OF A DNA MOLECULE ON
A CHROMOSOME.
LOCUS (LOCI) - A GENE’S SPECIFIC LOCATION ON A CHROMOSOME.
CLONE - A GROUP OF GENETICALLY IDENTICAL INDIVIDUALS ( WHAT
MITOSIS PRODUCES)
ASEXUAL REPRODUCTION - REPRODUCTION W/O SEX (NO MALE/FEMALE; 1
PARENT; OFFSPRING IS A CLONE OF PARENT.
HOMOLOGOUS CHROMOSOMES - A MATCHING PAIR ALWAYS ONE FROM
EACH PARENT.(one paternal/ one maternal.)
AUTOSOMES - CHROMOSOMES NOT DIRECTLY INVOLVED IN DETERMINING
SEX. (IN HUMANS: 22 HOMOLOGOUS PAIR).
SEX CHROMOSOMES - THE CHROMOSOMES DIRECTLY INVOLVED IN
DETERMINING SEX (IN HUMANS THE LAST HOMOLOGOUS PAIR).
(a) CALLED (X) & (Y) CHROMOSOMES.
(b) XX = FEMALE & XY = MALE.
(c) In other organisms:
(1) Insects (Grasshoppers, Roaches): X-O sex chromosomes. O represents
no sex chromosome = Male
(2) Birds, Butterflies and some fish: Z-W sex chromosomes. Female
gamete determines sex. Males are ZZ, Females are ZW
(3) Parthenogenesis – wasps, bees and ants. If the egg is fertilized it
becomes a female and is diploid. If the egg is unfertilized it is male and haploid.
FERTILIZATION (or SYNGAMY) - UNION OF GAMETES.
KARYOTYPE: DISPLAY OF AN INDIVIDUAL’S CHROMOSOMES.
CHROMOSOMES ARE COLLECTED DURING METAPHASE. THIS IS DONE BY
NUMBER, SIZE & TYPE CHROMOSOME.
THE HUMAN LIFE CYCLE:
(characteristic of most animals)
Gametes are the only
haploid cells.
The diploid zygote
divides by mitosis
producing a diploid
organism.
MEIOSIS STEPS:( FIG. 13.5.)
(a) Each chromosome replicates. (This
shows 1 homologous pair). Remember sister chromatids & centromere.
(b) Meiosis I segregates the homologous
pair into 2 different cells (each new daughter
cell is in HAPLOID).
(c ) Meiosis II separates sister
chromatids into chromosomes. No
chromosome duplication)
MEIOSIS TERMS:
Synapsis - ( in prophase I ) - the duplicated chromosomes pair with their
Homologues). This is a PROCESS. Homologous chromosomes made of two sister
chromatids come together as pairs.
Homologue - one of a homologous pair.
Tetrad - the four closely associated chromatids of a homologous pair together.
This happens during synapsis.
Crossing over - (a process) reciprocal exchange of genetic material between
nonsister chromatids.
COMPARING MITOSIS & MEIOSIS.
MEIOSIS - Prophase I with -(a) Tetrad & synapsis making a synaptonemal complex (b)
Crossing over with the chiasma.
MITOSIS- No tetrads, synapsis,
or crossing over.
DAUGHTER CELL
DIFFERENCE - Mitosis has
produced 2 identical cells.
Meiosis produced daughter
cells with one of each
homologous pair.
SUMMARY: differences between Mitosis & Meiosis.
FIG. 13.8 - This shows the most important
concept of meiosis (how it produces genetic
variation in organisms).
INDEPENDENT
ASSORTMENT: At the end
of meiosis chromosome
pairs distribute themselves
independently of one
another. This causes 4
different combinations of
chromosomes with 2
homologous pair.
1st MEIOTIC DIVISION RESULTS IN INDEPENDENT
ASSORTMENT OF MATERNAL & PATERNAL
CHROMOSOMES IN DAUGHTER CELLS.
FORMULA: The number of combinations possible
when chromosomes assort independently into gametes
during meiosis is 2n, where (n) is the haploid # in the
organism.
EXAMPLE - Human haploid (n) is 23. 223 is over 8
million. A male can produce 8 million genetically
different combinations of sperm & a female 8 million
combinations of eggs.
RANDOM FERTILIZATION then would produce 8
million x 8 million(over 64 Trillion) possibly different
genetic combinations in the offspring.
Crossing Over produces individual
chromosomes that
combine genes inherited
from our two parents.
Independent
Assortment, Random
Fertilization, &
Crossing Over - result
ways that genetic
variation can be
produced.
SUMMARY:
Prophase I & Anaphase I produce the most
variation in the 4 new daughter cells.
If clones were genetically different, this would
be due to mutation (change in the code of
DNA).
Remember these !!!!
Which might be a daughter
cell of meiosis I ?
Which might be a daughter
cell of meiosis II?
CHAPTER 16 - THE MOLECULAR
BASIS OF INHERITANCE
DNA - most celebrated molecule of all time.
It is made of nucleic acids that have the
unique ability to direct their own replication.
PROBLEM: Since a chromosome is made of protein
& DNA which one is carrying the genetic material?
There can be an infinite # of proteins so it would be
a prime candidate to carry genetic material.
JAMES WATSON – CO-FOUNDER OF THE STRUCTURE OF
DNA
Watson & Crick working on the
DNA structure model. (April 1953)
Transformation of Bacteria - Frederick
Griffin.(1928)
The captions under the picture is all that is needed to explain
this experiment.
TRANSFORMATION - the change in genotype & phenotype due
to the assimilation of external DNA by a cell.
EVIDENCE THAT VIRAL DNA CAN PROGRAM
CELLS (FIG. 16.2)
Virus is made of a HERSHEY-CHASE EXPERIMENT
protein coat &
DNA core. Virus
injects DNA into
a Bacteriophage.
DNA coat has
radioactive
That the
35)
protein coat (S
bacteria are
called T2
while DNA is
Phages.
radiated with
(P32).
ADDITIONAL EVIDENCE THAT DNA IS THE
GENETIC MATERIAL OF CELLS
Erwin Chargaff - Said that the bases of DNA
(A, T, C, G) vary from one species to another.
He also found a regular ratio of bases. (A approximately
= T; and G approx. = C). This was known as Chargaff’s Rules.
NOTE: All these discoveries were before Watson & Crick discovered the double helix
structure of DNA.
Structure of a DNA strand
DNA is composed of nucleotides ( 5
carbon sugar, phosphate & a
nitrogenous base (A,T,C,G).
Phosphate of one nucleotide is
attached to the sugar of the next
nucleotide.
Fig. 16.5 (a) The Double Helix Structure of DNA.
Adenine (A) is always paired
with Thymine (T) & Guanine
(G) is always paired with
Cytosine (C).
The nitrogenous bases are held
together with Hydrogen bonds
(weak).
We even know the distances
between steps of the DNA
rungs. What’s a nm?
Notice the strands are oriented in opposite
directions.
This entire
structure was
worked out by
Watson & Crick in
1953 with help from
Rosalind Franklin’s
x- ray diffraction
photo of DNA
Base Pairing in DNA.
A & G are double ring
compounds called Purines.
T & C are single ring compounds
called Pyrimidines.
Each rung of DNA is made
of a Purine attached to a
Pyrimidine. Held together
by H bonds.
The SEMICONSERVATIVE MODEL - DNA
replication model
Meselson & Stahl tested the three hypothesis's on DNA
replication
Beginnings of how DNA Replicates.
Elongation of DNA at a
replication fork is
catalyzed by a enzyme
called DNA
polymerase.
Rate of elongation in humans is approx.50/sec.
Adding a Nucleotide:
A similar molecule to ATP (NTP) is used to link the
new nucleotide to the proper position.
The enzyme that
catalyzes the
reaction is DNA
POLYMERASE.
THE TWO STRAND OF DNA ARE
ANTIPARALLEL
Know: Where the 5’ & 3’ end
are.
PROBLEM: DNA polymerase
can ONLY add nucleotides to the
free 3’ end of a growing DNA
strand.
So..A new DNA strand can only
elongate in the 5’ to 3’ direction.
SYNTHESIS OF LEADING & LAGGING
STRANDS DURING DNA REPLICATION.
DNA polymerase is adding new
DNA fragments in a 5’ to 3’
direction continuously along a
replication fork, adding to the 3’
end.
Lagging strand is
synthesized in segments
called Okazaki fragments.
DNA ligase joins the
fragments into a single
DNA strand.
Okazaki fragments are about 100 -200
nucleotides long in eukaryotes.
PRIMING DNA SYNTHESIS
DNA polymerase cannot
initiate a polynucleotide
strand; it can only add to the
3’ end of an already-started
strand.
The primer is a short segment
of RNA synthesized by the
enzyme primase. Each
primer is eventually replaced
by DNA.
FIG. 16.15 - THE MAIN PROTEINS OF REPLICATION & THEIR FUNCTIONS.
DNA must also be able to form
complementary base pairs with
both DNA & RNA nucleotides.
The sequence of nucleotides will
be decoded into a sequence to
make amino acids into proteins.
Replication -> Transcription ->
Translation
Enzymes must proofread DNA during its Replication and
repair damage in existing DNA.
Mismatch Repair fixes mistakes made in DNA. DNA
polymerase itself carries out the mismatch repair.
Telomeres - special sequences of DNA nucleotides found at the
end of the DNA molecule. They do not contain genes. They
protect the organism’s genes from being eroded through successive
rounds of DNA replication.
Secret to aging?
http://www.youtube.com/watch?v=J9QApCHsrJk&feature=related
Image of Telomere
squeneces (yellow)
on chromosomes
Chapter 17 – From Gene to Protein
Transcription - the synthesis of mRNA (messenger RNA) under
the direction of DNA. This is a code to make a polypeptide
(protein). This is also the synthesis of any RNA from DNA.
Translation - the
actual synthesis of a
polypeptide (which
occurs at the
ribosomes.)
Gene to RNA to Protein.
The difference in Eukaryotic
& Prokaryotic cells.
Basics of the Genetic Code:
1. There is a total of 20 amino acids possible in any protein.
2. 3 Nucleotides on mRNA code for an amino acid. This is called
the triplet code.
3. Only one strand of DNA is transcribed into mRNA. This
strand is called the TEMPLATE strand. The other strand is
called the complementary strand.
4. All Translation & Transcription occur in a 3’ to 5’ direction.
5. The mRNA is in triplet bases called CODONS.
mRNA is only a single
helix & that Uracil (U) is
a substitute for Thymine
(T).
The number of nucleotides
making up a genetic
message must be 3 times
the number of amino acids
making up the protein.
EXAMPLE - 4 amino acids
= 12 nucleotides.
Amino Acids are connected
by polypeptide bonds.
Learn to read this!!!
AUG codon is a start
codon & the amino acid
Methionine (Met).
Start Codon begins the
sentence & UAA,UAG
& UGA = no amino acid
but stops the amino
acid chain (read in a 5’
to 3’ direction)= STOP
CODON, like the
period at the end of a
Fig. 17.6 The Stages of Transcription
1. RNA binds to the
promoter region of
DNA (several dozen
nucleotides
“upstream” from
the transcription
startpoint).
2. RNA moves
“downstream” from
promoter,
unwinding DNA &
elongating RNA at
the 3” end (5’ to 3’
direction).
3. RNA polymerase
transcribes a terminator
(this sequence of
nucleotides along DNA
signals the end of
transcription unit.)
4. Eventually RNA is
released & the polymerase
moves from DNA.
5. Prokaryotes - RNA
transcript immediately
used to make protein.
6. Eukaryotes - mRNA
will undergo additional
processing.
Progresses at about 60 nucleotides/sec
in Eukaryotes.
RNA Processing 1st step:
Enzymes modify 2 ends of a eukaryote pre-mRNA molecule.
Cap made of modified guanosine triphosphate added to the
5’ end of RNA.
A Poly(A) tail consiting of 200 adenine nucleotides attached to
3’ end. ( may helps export mRNA from the nucleus.)
***Role of Cap and Tail - protect RNA from degradation****
The leader, trailer & termination
signal.
Leader & trailer are not translated.
RNA processing (splicing).
Pre-mRNA - Exons (Expressed sequence) are keep & the Introns
(Intervening sequence) are removed (both by enzymes).
Exons are then spliced together. We now have the processed
RNA ready to leave the nucleus & go to the ribosome for
translation.
Translation - Basic Concept:
1) tRNA picks up amino acids &
transport them to the ribosome
2) Each tRNA has an anticodon
(3 letters) that pick up one of the
twenty amino acids.
3) When the tRNA’s deliver their
amino acid, they add them to a
growing polypeptide chain.
tRNA’s are now available to pick
up another amino acid to repeat
the process.
4) New Polypeptide chain added
in the 5’ to 3’ direction.
Ribosomes are made of 2
subunits each made of many
molecules or rRNA (ribosomal
RNA) and proteins.
The sites on the ribosome:
The Anatomy of a Ribosome:
(1) P site - holds the tRNA
attached to the growing
polypeptide
(2) A site - holds the tRNA
carrying the amino acid to be
added to the polypeptide chain
(3) Discharged tRNA leaves via the
E site.
Peptide bonding between
amino acids maintains the
shape of tRNA.
Fig. 17.15 Initiation of Translation
1. Small ribosomal subunit binds
to molecule of mRNA.
2. Initiator tRNA with anticodon
UAC base-pairs with the start
codon, AUG carrying the amino
acid Met.
3. A large ribosomal unit arrives &
completes the initiation complex.
4. Initiator tRNA is in the P site. A
site is available to tRNA carrying the
next amino acid.
5. Proteins called initiation factors
bring translation components
together. GTP provides the energy
for all this.
GTP &
proteins called
elongation
factors needed
to drive this
process.
Termination of Translation
1. When ribosome reaches a termination codon on mRNA, the (A)
site of ribosome accepts a protein called a release factor instead of
tRNA.
2. Release factor hydrolyzes the bond between tRNA in the P site &
the last amino acid of the chain. This frees the polypeptide from the
ribosome.
3. The 2
ribosomal
subunits
dissociate
Fig. 17.18 Polyribosomes
A. An mRNA molecule is generally translated together with several ribosomes in
clusters called polyribosomes.
B. This enables a single mRNA to make many copies of a polypeptide simultaneously.
Proteins can be chemically modified by attachment of sugars,
lipids, phosphate groups etc.
Example: Enzymes may remove leading amino acids from
a chain. Sometimes several proteins will join together to
allow them to function or one protein may split into several
proteins.
Proteins formed here are only the primary structure & must
develop a secondary, tertiary, or even Quaternary structure.
Transcription & Translation in Bacteria:
1. Bacteria (Prokaroytes) have no nucleus, so mRNA does
not need to move through the membrane to the
ribosome.
2. Streamlined operations here - Transcription &
Translation can be occurring at the same time.
3. There is not RNA
processing in bacteria.
(All exons).
MUTATIONS: Changes in the genetic code of DNA.
Point Mutations: Chemical changes in just one or a few base
pairs in a single gene.
If a point mutation occurs in a gamete or cells giving rise to
them, it could be transmitted to offspring & future generations.
TYPES OF MUTATIONS:
1. Base-pair substitution - replacement of one nucleotide & its
partner in the complementary DNA strand with another pair of
nucleotides.
Some substitutions are silent mutations since genetic code
is redundant, there may be no change in the amino acid
coded for.
EXAMPLE: CCG mutated to CCA would make mRNA GGC
become GGU which is still glycine.
2. Missense Mutation - altered codon still codes for an amino acid
& makes sense although not necessarily the RIGHT sense. (Make a
protein, just not the correct one)
3. Nonsense mutation - Alterations that change an amino acid
code to a stop codon. Almost always leads to a nonfunctional
protein.
4. Insertions & deletions are additions or losses of one or more
nucleotide pairs in a gene.
a. Note this can cause missense or nonsense. Where the
amino acid is incorrect in a chain can be important or not.
5. Frameshift mutation - alters “reading frame” of message (#
of nucleotides inserted or deleted is not a multiple of 3.
a. ( the big cat) remove the h = teb igc at_.) This will make
all amino acids downstream from this incorrect.
What can cause Mutations?:
Mutagens - Physical & chemical agents that cause mutations or increase the
mutation rate.
Examples - X-rays, Radiation, UV light, chemicals (pesticides, radon), Viruses &
Bacteria