BIG IDEA #3
Living Systems
store, retrieve,
transmit, and
respond to
information
essential to
life processes
DISCOVERY OF DNA
 Important Researchers
 Frederick Griffith: transformation of bacterial
cells
 Avery, McCarty, MacLeod: transforming agent is
DNA
 Hershey and Chase: confirmed DNA is
transforming agent by using phages
 Chargaff: base pairing rules
 Wilkins and Franklin: used x-ray crystallography
to photograph DNA
 Watson and Crick: discovered double helix
structure of DNA
 Meselson and Stahl: semi-conservative
replication of DNA
 Beadle and Tatum: one gene: one enzyme
STRUCTURE OF DNA
 DNA is a polymer made of nucleotide monomers
 Each nucleotide is made of a phosphate group attached to a
five carbon sugar and one of four possible nitrogen bases:
A ,T,C, or G.
Sugar in DNA is deoxyribose
Sugar in RNA is ribose
Bases are divided into purines (A,G) and pyrimidines (C,T)
A always pairs with T: C always pairs with G (Hydrogen bonds hold
them together)
 U replaces T in RNA
 RNA is single-stranded: DNA is double-stranded
 Two strands are described as antiparallel and complementary
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DNA REPLICATION
 Semiconservative replication: DNA separates and each old
strand serves as a template for a new strand.
 Enzymes involved:
 Helicase: enzyme that unzips DNA at the replication fork
 DNA polymerase: adds free DNA nucleotides to the 3’ end of the
leading strand and the lagging strand.
 Lagging strand is synthesized in sections called Okasaki fragments
 DNA ligase seals the gaps between the Okasaki fragments
CELL DIVISION
 Binary Fission: used by
bacteria
 Mitosis: Used by
eukaryotes to increase cell
number in multicellular
organisms and for
reproduction in unicellular
eukaryotes. Produces two
daughter cells with a full
set of chromosomes
 Meiosis: Used by
eukaryotes to make
reproductive cells for
sexual reproduction.
Produces four gamete cells
with half the number of
chromosomes.
CELL CYCLE
 Three phases of the cell
cycle:
 Interphase:
 G1: Cell grows and carries out
metabolic activities
 S: DNA is copied
 G2: Cell growth continues and
organelles are copied
 Mitosis
 Division of the nucleus:
Prophase, Metaphase,
Anaphase, Telophase
 Cytokinesis
 Cytoplasm divides
STAGES OF MITOSIS
STAGES OF MEIOSIS
FUNDAMENTALS OF GENETICS
 Gregor Mendel: father of modern genetics
 Worked with pea plants
 Law of Segregation: Individuals have two alleles for each trait
and these randomly separate during the formation of gametes
 Alleles can be dominant or recessive
 Heterozygous: two different alleles
 Homozygous: two identical alleles
 Genotype: the actual genetic make -up
 Phenotype: the physical expression of alleles
EXCEPTIONS TO MENDEL’S GENETICS
 Multiple alleles: more than two alleles in the population
 Incomplete Dominance: neither allele is dominant Ex: Pink
Red flowers x white flowers = pink flowers
 Codominance: both alleles expressed individually Ex: Spotted
animals
 Polygenic traits: several genes interact for one trait Ex: skin
color
 Epistasis: allele at one locus af fects allele at a dif ferent locus
Ex: Yellow, black and brown labs
 Sex-linked traits: traits are located on the X or Y
chromosome. More common in men. Ex: color blindness,
hemophilia, white-eyed fruit flies
 Linked genes: genes located on same chromosome will be
inherited together
GENE REGULATION IN PROKARYOTES
 Operons regulate gene expression in prokaryotes. An operon
is a part of a DNA strand that includes a promoter site,
operator site and the structural gene to be expressed.
 Proteins called repressor proteins can bind to the operator
site. When a repressor is attached to the operator, the gene
is turned of f. When a repressor is absent, the gene is turned
on.
 Inducible operons: usually turned off and only become active when
the repressor is lifted.
 Repressible operons: usually turned on and only become inactivated
when the repressor binds.
GENE REGULATION IN EUKARYOTES
 Many dif ferent mechanisms control which genes are
expressed in eukaryotic cells.
 Some areas of DNA are so tightly condensed around histone proteins
that they can not unwind and be expressed (heterochromatin)
 DNA methylation: methyl groups can be added to nucleotides. These
will block RNA polymerase from binding and make the genes
inactive. (plays a role in genomic imprinting)
 Histone acetylation: acetyl groups are added to the histones in the
chromosomes. This loosens the DNA, making it uncoil farther and
increasing the rate of gene activity.
MORE GENE REGULATION IN
EUKARYOTES
 Control elements include TATA boxes and CAAT boxes; which
are regions of DNA near the promotor site. Proteins can bind
to these sites and either block or increase gene activity.
 Poly A tail and a 5’cap are added to an RNA message before it
leaves the nucleus. Sometimes, these end caps can be
removed to reduce gene activity.
 Alternative splicing: occurs when dif ferent introns (noncoding
regions of DNA) are spliced out as the RNA message is being
modified before it leaves the nucleus.
 After translation of a protein, chaperone proteins can be
blocked to prevent proper folding.
 Excess polypeptides can be tagged with ubiquitin so that a
proteasome will break them up and reduce gene activity.
CHEMICAL REGULATORS OF GENE
EXPRESSION
 Chemical messages can serve as regulators of gene
expression
 Local Regulators
 Synaptic signaling: Ex: Neurotransmitters
 Paracrine signaling: Ex: Cell secretes a chemical that travels to a
nearby cell (growth factors)
 Autocrine signaling: Ex: Cell secreted a chemical that targets the
same cell
 Long Distance Regulators
 Amino acid derivative (Protein Hormones: insulin): Cannot diffuse
through lipid membranes. Must bind to surface receptors
 Steroid Hormones (Estrogen and Testosterone): Can diffuse through
lipid membranes. Bind to interior target molecules
GENETIC VARIATION
 Processing of genetic information is imperfect and a source of
genetic variation
 DNA Replication Errors (mutations)
 Point (gene) Mutations: affect a single gene or protein. Examples
include
 Substitutions
 Insertions
 Deletions
 See diagram next slide
T YPES OF GENE MUTATIONS
DNA REPLICATION ERRORS (CONTINUED)
 Chromosomal Mutations: affect a large portion of a chromosome or
an entire chromosome.
 Nondisjuctions: Chromosomes fail to separate properly during meiosis I or
meiosis II resulting in Aneuploidy (one extra or one to few chromosomes).
Ex: Down Syndrome (extra 21 st chromosome)
 Polyploidy: One or more extra sets of chromosomes
 Deletions: Large section of chromosome is broken off
 Inversion: sequence of genes are shuffled
 Translocation: piece of one chromosome becomes attached to another
OTHER MECHANISMS FOR GENETIC
DIVERSTIY
 Mutations are only one mechanisms for genetic diversity.
Others include:
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Independent assortment during meiosis
Crossing over during meiosis
Random fertilization
Transformation in bacteria cells
Conjugation in bacteria cells
Transduction in bacteria cells: involves viruses
CELL COMMUNICATION
 Cell junctions: directly connect the cytoplasm of adjacent
cells
 Gap junction (animal cells)
 Plasmodesmata (plant cells)
 Cell-to-cell recognition involves direct contact between cell
surface molecules
 Other communication involves chemical messages released by
a secretory cell and received by a target cell
 Local regulators: short distance messenger molecules
 Paracrine signaling: Ex: growth hormone
 Synaptic signaling: Ex: neurotransmitters
 Long distance regulators: Ex: Hormones
THREE STAGES OF CELL SIGNALING:
RECEPTION, TRANSDUCTION, RESPONSE
 Reception: Involves ligands binding to receptor proteins in
the cell membrane. Three main types of membrane receptor
proteins:
 G coupled: “On/Off” switch. GDP is bound = off/ GTP is bound = on.
Once the switch is turned on, Enzymes are activated to set off the
response.
 Receptor tyrosine kinases: membrane receptors that attach
phosphates to tyrosines. They can trigger multiple signal
transduction pathways at once
 Ion channel receptors: act as gates when receptors changes shape
 Intracellular receptors: located inside cells and receive
messages from molecules that can easily pass through the
cell membrane. They often act as transcription factors. Ex:
testosterone is received by an intracellular receptor that then
enters the nucleus and acts as a transcription factor to turn
on specific genes.
THREE STAGES OF CELL SIGNALING:
RECEPTION, TRANSDUCTION, RESPONSE
 Transduction: the message becomes amplified and then
travels throughout the cell
 Usually involves either phosphorylation cascades or second
messengers
 Common second messengers include cAMP and Ca+
 Response: usually involves some sort of cytoplasmic activity
or the regulation of genes (transcription)
TRANSCRIPTION FACTORS
 Transcription factors are responsible for turning genes on and
of f and therefore control their expression
 Homeotic genes: involved in the patterns and sequences of
embryonic development (i.e. they determine when, how and
where major body parts will develop) Ex: Hox gene
 In general, transcription factors bind to a DNA molecule and
allow it to open up and be transcribed into an mRNA message.
If they do not bind, then the mRNA message will not be made
and the gene will not be expressed as a protein.
INTERNAL MOLECULAR SIGNALS IN
PLANTS
 Phototropism: plants grow towards light. Involves
photosensitive receptors called phototropins. They stimulate
the production of the plant hormone, auxin. Auxin causes
cells on the dark side of the plant to grow faster than the
light. This side will thus get longer and the plant will start to
bend toward the short (light) side.
 Photoperiodism: plants have seasonal changes that cause
them to flower.
 Long day vs. short day plants
INTERNAL MOLECULAR SIGNALS IN
ANIMALS
 Circadian rhythms: 24 hr. cycles
 Hibernation (state of inactivity and decreased metabolism
during winter)
 Estivation (state of inactivity and decreased metabolism
during summer)
 Migration (physical movement from one area to another)
 Phermonal signaling