Living Systems store, retrieve, transmit, and respond to information essential to life processes

BIG IDEA #3

Important Researchers Frederick Griffi th: 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

DISCOVERY OF DNA

DNA is a polymer made of nucleotide monomersEach 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

STRUCTURE OF DNA

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

DNA REPLICATION

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 DIVISION

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

CELL CYCLE

STAGES OF MITOSIS

STAGES OF MEIOSIS

Gregor Mendel: father of modern geneticsWorked with pea plantsLaw 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-upPhenotype: the physical expression of alleles

FUNDAMENTALS OF GENETICS

Multiple alleles: more than two alleles in the population Incomplete Dominance: neither allele is dominant Ex:

Pink Red flowers x white flowers = pink flowersCodominance: both alleles expressed individually Ex:

Spotted animalsPolygenic traits: several genes interact for one trait Ex:

skin colorEpistasis: allele at one locus aff ects allele at a diff erent

locus Ex: Yellow, black and brown labsSex-linked traits: traits are located on the X or Y

chromosome. More common in men. Ex: color blindness, hemophilia, white-eyed fruit fl ies

Linked genes: genes located on same chromosome will be inherited together

EXCEPTIONS TO MENDEL’S GENETICS

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 off. 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 PROKARYOTES

Many diff erent 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.

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 diff erent 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.

MORE GENE REGULATION IN EUKARYOTES

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 cellLong 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

CHEMICAL REGULATORS OF GENE EXPRESSION

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

GENETIC VARIATION

TYPES OF GENE MUTATIONS

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 shuffl ed Translocation: piece of one chromosome becomes attached to

another

DNA REPLICATION ERRORS (CONTINUED)

Mutations are only one mechanisms for genetic diversity. Others include: Independent assortment during meiosis Crossing over during meiosis Random fertilization Transformation in bacteria cells Conjugation in bacteria cells Transduction in bacteria cells: involves viruses

OTHER MECHANISMS FOR GENETIC DIVERSTIY

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

CELL COMMUNICATION

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)

THREE STAGES OF CELL SIGNALING: RECEPTION, TRANSDUCTION,

RESPONSE

Transcription factors are responsible for turning genes on and off 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.

TRANSCRIPTION FACTORS

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 PLANTS

Circadian rhythms: 24 hr. cyclesHibernation (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

INTERNAL MOLECULAR SIGNALS IN ANIMALS