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121 Cards in this Set

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five ways that asexual division has been found to occur in bacteria

Binary Fission

-most common mode of asexual division

-inward growth of cell membrane and wall

-results in identical daughter cell


-common in yeast

-two haploid bud and create a diploid

Multiple Fission

-predator of gram negative bacteria

-preys on gram (-) and gets between cell wall and multiplies

Fragmentation of mycelia cells

How does Bdellovibrio divide and reproduce?

It is a predator to Gram (-) cells. It reproduces through multiple fission.

How does Epulopisceum reproduce?

"Development from within"

-has up to 7 daughter cells, the mother cell ruptures and dies while the daughter cells are released.

Define chemoorganotroph in terms of source of energy, source of carbon.

Chemoorganothroph = organisms which oxidize the chemical bonds in organic compounds as their energy source

Energy and carbon source both corm from reduced organic compounds (glucose)

How does a Streptomyces spore differ from a Clostridium endospore?


- prokaryotes, fungal like

- shiny, refractile round structure

- are reproductive spores


- restraint structures

Lag Phase (bacterial growth curve)

Building and adjusting phase

- production of new enzymes

- increase in cell size

- increase in cell number

- increase in # of ribosomes

Exponential Phase (bacterial growth curve)

Organisms adjusted

- nutrients plentiful

- uniform cell doubling/unit time (1-2-4-8)

- time for population to double = generation time or doubling

- rate in increase # of cells/unit time always increasing

-all cell parameters are increasing at same rate

Stationary Phase (bacterial growth curve)

- No net increase in cell number or cell mass

- Nutrients limited; toxic end products build up

- Unequal synthesis of cell materials - begin storage synthesis (ex: phosphates)

- Metabolism slows

- Cell size decreases

- Decrease in # of ribosomes

- Gm + stain pink in the Gm stain - peptidoglycan (cell wall) begins to break down

Death Phase (bacterial growth curve)

death rate exponential

- cells mass remains constant for awhile

- cells eventually lyse

Explain why a bacterium that is able to synthesize all its needed building blocks for macromolecules grow faster in a growth medium where all the building blocks are present, than in a growth medium where many building blocks are absent.

The more building blocks nutrients (amino acids, purines, pyrimidines, vitamins) provided in the growth medium the less energy the cell must expand in making them

Available energy can be channeled instead toward growth putting the building blocks together to make macromolecules such as protein, DNA, RNA, peptidoglycan, etc.

***therefore the cells will grow faster since they do not use energy to make building blocks

Generation Time (g)

aka doubling time

- time to double the number of cells/ml or cell mass in a population

- varies with type of bacteria

As generation time decreases, rate of bacterial growth ______.


What is the difference between arithmetic growth and exponential growth?

Arithmetic growth

- growth that increases at constant amount per unit time

Exponential growth

- growth at a constant rate of increase per unit time (doubles with time)

What characterizes the active growth phase of bacteria?

Exponential growth (log phase) nutrients plentiful

Disulfide Bond

- strong covalent bond

- a single covalent bond between the sulfur atoms to two amino acids (cysteine)

Phosphodiester Bond

- strong covalent bond

- joins successive sugar molecules in a polynucleotide

- the linkage between the 3' carbon atom of one sugar molecule and the 5' carbon atom of another (deoxyribose in DNA and ribose in RNA)

-creates strong covalent bond between the phosphate group and two 5-carbon ring carbohydrates over two ester bonds

Peptide Bond

formed between two molecules when the carboxyl group of one molecule reacts with the amino group of another molecule, releasing a molecule of water

Cells must have sulfur because

there are two essential amino acids that need sulfur (methionine and cysteine)C

Cells must have phosphorus because

that is what is used to make nucleotides and phospholipids

Cell must have nitrogen because

that is what is needed to make amino acids

Gram negative bacterial cells are shown to be lysing. Offer two possible explanations.

- the bacteria was treated in Penicilin or Lysozyme, which is bacteriolytic

- Bdellovibrio is infecting the cell

Bacterium D was grown in trypticase soy broth and entered stationary phase. Offer two reasons for the entry into stationary phase from log phase.

1. Bacterium D was producing a lot of toxic products.

2. Nutrients are becoming depleted

T/F During exponential growth, the rate of growth is always increasing.


How does a 70S ribosome differ from an 80S ribosome?


- 30S + 50S

- Procaryote

- float free in cytoplasm of bacteria


- 40S + 60S

- Eukaryote

- fixed to endoplasmic reticulum membrane

*both have rRNA and proteins

continuous bacterial culture


bacterial batch culture


- broth stays in vessel

- goes through all four phases


- chemostat - add constant amount of fresh media to replace spend media

- maintain exponential growth (log phase) at steady state

- rate of addition too high --> flush out

- rate of addition to slow --> stationary

Medium effect on growth of bacteria

the more building blocks the faster the growth

Temperature effect on growth of bacteria

- effects folding, stability, and activity of molecules of proteins and stability/flexibility of phospholipids

- effects protein structure (H bonds, secondary, tertiary, quaternary) --> change in structure usually leads to decreased loss of protein activity

- at very high temperatures, protein denaturation occurs. not reversible.

- effects enzyme shape --> alteration of active site occurs

Optimal temp = best temperature for bacterial/cell division

Oxygen availability effect on growth of bacteria

Obligate aerobes

- grows only in O2 rich media

Obligate anaerobes

- grows only in media w/o O2

Facultative anaerobes

- will grow in O2 rich first if present, but also grows w/o O2

- ideally wants O2

Aerotolerant aerobes

- grows in O2 and no O2 media


- grows in very low amounts of O2 (basically looks like straight line in tube - Borrelia burgdorfei - Lyme's disease)

Osmotic pressure effect on growth of bacteria


- less concentration of solute (water) outside of the cell


- concentrations are the same inside and outside, no water movement


- high concentration of solutes outside of the cell

pH effect on growth of bacteria

- alteration of ionic bonds of protein tertiary structure

- denaturation reversible

growth location in thioglycollate media

thioglycollate - reduces oxygen to water

Obligate aerobe

- requires oxygen to live

- example: humans

Obligate anaerobe

- killed by oxygen, cant get rid of hydrogen peroxide

- doesn't have catalyse

- example: clostridium

Facultative anaerobe

- grow better with oxygen that without

Aerotolerant anaerobe

- tolerate oxygen and rate of growth is the same with or without oxygen


- uses low levels of oxygen

- example: Borellia burgdoferi


can survive in up to 35% salt

is an osmophile

produces a red pigment

What is meant by membrane fluidity?

Membrane fluidity is the viscosity of the lipid bilayer of a cell membrane or a synthetic lipid membrane

What affects membrane fluidity?

Viscosity of the membrane can affect the rotation and diffusion of proteins within the membrane - thereby affecting the functions of these molecules

Temperature also affects membrane fluidity

A psychrophile cannot grow at 80C. Why?

Because they are made up of unsaturated fatty acids of phospholipids

-H bonds break and denature protein

-have lower melting points (cell membrane will melt and become non functional)

-like lower temps like the refrigerator

A thermophile cannot grow at 18C. why?

Because they are made up of saturated fatty acids of phospholipids

-cell membrane will freeze at low temps and become non functional

-have hight melting points (heat lovers)


Measures light transmitted

How does a Spectrophotometer work to measure cell mass in terms of absorbency?

As cells grow and reproduce, cell mass increases, light scattered increases, transmitted light decreases, and absorbency increases.

Of what advantage is if for the bacterial pathogens Staphylococcus aureus and Propionibacterium to produce lipase when causing a skin infection?

Lipase can break down the sebum and use the product as a source of carbon and energy

Explain how human dandruff forms by the fungus Malassezia

Malassezia is a fungus that is thought to be the cause of human dandruff

- uses lipase to break down sebum into oleic acid, a fatty acid, and increase skin cell production - causing dandruff

A bacterium is considered to be nutritionally simple if it...

can synthesize all of its building blocks (AA, purines, pyrimidines, vitamins, etc.) from basal salts and a carbon and energy source (such as glucose).

A bacterium is considered to be nutritionally complex if it....

cannot synthesize all of its building blocks from basal salts and a carbon energy source (such as glucose).

***For growth of a nutritionally complex bacterium building blocks must be provided in the growth medium.

Is E. coli nutritionally simple or complex? What growth medium does it use?

- simple

- M-9 + glucose and TSB

Is Lactobacillus casei nutritionally simple or complex? What growth medium does it use?

- complex

- TSB only

Lab 7

In addition to spectrophotometric method of measuring bacterial growth, name three other ways of measuring growth

1. Viable count

-count each individual "bacteria"

-one colony = one bacteria

2. Passing through a filter

3. Petroff-Hausser Chamber

Lab 7

Give an example of a specific bacterium (Genus) that is self-inhibited in logarithmic stage due to buildup of toxic metabolic products.

Lactic acid within Lacto bacillus which will eventually create so much acid it will inhibit more growth

What are the substrate, product, assay and benefits of producing amylase (to the bacterium?)

Substrate = starch

Product = maltose

Assay = food plate with iodine

- black/blue outline = presence of starch - no amylase

-clear/yellow outline = absence of starch - amylase present

Benefits = used for metabolism


Helicobacter pylori is found in the stomach and causes stomach ulcers. Even though the stomach juices are very acidic (pH ~2-3), Helicobacter is not an acidophile. How does it survive?

Lab 9

acidophile - doesn't like to have a low pH

Helicobacter pylori

- causative agent of stomach ulcers

-depends on the enzymatic activity of its urease, to neutralize the HCl in the stomach creating a favorable, less acidic microenvironment

-its urease also reduces the viscosity of the stomach mucus covering the stomach's epithelial cells making it easier for the bacteria to reach the desired niche, the epithelial cells

Stationary phase E. coli cells are added to fresh trypticase soy broth. Would you expect a lag period to occur?


because they need to undergo a period of adjustment to the fresh medium then will begin to grow larger and become more metabolically active

You follow their growth by performing a viable count and also by a spectrophotometer.

Which method will be the first to indicate an increase in cell size? The first to indicate cell death?

both specrophotometer

Primary Protein Structure

Linear sequence of amino acids (amino and carboxyl end)

Contains peptide (covalent) bonds

Not affected by heat

Secondary Protein Structure

Primary folded upon itself - made up of alpha helix or beta pleated sheets

Held together by hydrogen bonds

Affected by heat

Tertiary Protein Structure

Secondary structure folds upon itself

Held together by disulfide (covalent), ionic, and hydrogen bonds

Linear array of amino acids determines what it looks like

Affected by heat

Quaternary Protein Structure

Two separate polypeptides folded in tertiary structure stuck together

Held together by ionic, hydrogen, and disulfide (covalent) bonds

Affected by heat

Define active site on an enzyme

Active site is where the enzyme substrate binds to it in order to have a chemical reaction occur

The circular DNA of a bacterial cell is 1.0 mm in circumference. Explain how this DNA is packaged to fit into a coccus that is 1.0 um in width.

What is the name of the enzyme required?

DNA is supercoiled in order to fit into such a small place

Name of enzyme - gyrase

What are the reactions catalyzed by amylase and lipase? Of what advantage is it to the cell to secrete these exoenzymes into the environment?

amylase reaction

positive: a clearing of turbidity (clear/yellow in color) around the colonies against the dark brown color of the surrounding agar (starch is broken down)

negative: no clearing around colonies against the dark brown color of the surrounding agar (starch present)

Lipase reaction

positive: iridescent sheen in agar surrounding bacterial growth

negative: no iridescent sheen in agar surrounding bacterial growth

How does one determine if a bacterium is motile without a microscope?


The most common method to is to stab a semi-solid nutrient medium in a straight-line inoculation technique. Any movement of the organism away from the line of inoculation can be interpreted as the ability to move.

Medium compound:

tetrazolium chloride - determines if the bacteria carries out metabolism. If they do, the bacteria will reduce tetrazolium chloride.


In an oxidized state - colorless

In a reduced state - red

Will be able to note the location of the bacteria in the medium by the presence of a red color.

***the motile bacteria contain flagella - how bacteria is able to move

Flagella arrangements

polar - mono tail

amphi - one at both ends

lopho - multiple at one end

peri - hairy allover

Name two ways that via lab tests one can determine if an organism produces acid.

Two lab tests:

-acid fast in glucose and lactose = turns yellow if it produces acid (determins if bacterium contains mycolic acids)

-mannitol test = turns yellow if it produces acid

If an organism ferments glucose must it have gas production?


-gas production doesn't always occur if an organism ferments glucose

[gas production only occurs if formic acid is produced and the organism has the enzyme to degrate it to H2+CO2]

How would you determine if chemical X is inhibitory to the growth of E. coli? Given that X is inhibitory.

By testing the effect of heat, salt, and UV light on bacterial growth

How would you determine if X was bacteriostatic or bacteriocidal (but not bacteriolytic)?

Bacteriostatic: the cells are not growing, but are NOT dead - inhibited growth

Bacteriocidal: the cells are dead - growth killed

Bacteriolytic: the cells are dead, not growing, and are lysed

What results would you expect to support that chemical X is bacteriolytic?

No growth in the media

Design an experiment to determine if Bacterium X is a thermophile and can grow at 85C or whether it can merely survive at a 5 minute 85C exposure and is not a thermophile.

Take bacteria and incubate at 85C or put in a hot water bath at 85C for 5 minutes.

If it is a thermophile, it will grow after 5 minutes

How does pasteurization differ from sterilization? What type of bacteria are targeted in this pasteurization process?


-destruction or removal of all like

-ex: flame loop, autoclave


-many pathogens killed, but endospores are NOT killed.

Bacteria targeted: coxiella and microbacterium

Three different physical ways to control/prevent growth.





-UV light (causes thymine dimers)

-ionizing radiation


-for heat sensitive liquids that cannot be autoclaved (antibiotics, protein products, beer, pharmaceuticals)

a. What is a thymine dimer?

b. Does it affect the overall helical structure of the DNA?

c. What might be the effect of the thymine dimer?

a. is a kink in the DNA causes by UV --> causes distortion in the DNA leading to mistakes in DNA replication (mutation) or even the cessation of DNA synthesis leading to cell death

b. does not affect the helical structure of the DNA

c. causes mutation in your DNA. thymine bonds to another thymine adjacent to it on the DNA strand

Why is ionizing radiation more damaging to DNA than UV light?

Because it has a much higher energy and can penetrate surfaces.

It breaks sugar/phosphate DNA backbone - damaging the protein

Why is moist heat more effective than dry heat?

Moist heat

= water + temp + pressure

Two types: sterilization & pasteurization

Most effective because:

-coagulates and denatures protein

-melts lipids

-cell membranes destroyed

Dry heat

= temp + air

Examples: flame on glass

Less effective because:

-air is less a effective conductor of heat than water

When growth medium is autoclaved, what temperature and pressure must be reached to begin killing endospores?

121 C

What effect does high temperature moist heat have on the bacterial cell that leads to its cell death?

coagulates and denatures protein

melts lipids

cell membrane fluidity destroyed

What advantage does filtration have over high heat for bacterial growth control? Disadvantage?

Advantage: Can be used for heat sensitive chemical/liquids that cannot be autoclaved

Disadvantage: viruses, toxins pass through

Name three heavy metals and their use for controlling/killing bacteria/fungi


- fungicide


- former preservative in vaccines. now substantiated link to cause of autism


- in catheters to prevent UTIs

- before antibiotics AgNO3 used in eyedrops in newborn babies to protect against opthalmia neonatorum (blindness) caused by mothers who have chlamydia or gonorrhea

How to heavy metals act to alter the tertiary structure of protein?

Bind sulfhydryls and alter active site of proteins

Block active site by:

- preventing folding of primary structure by blocking S-S bond formation so tertiary structure cannot form causing active site not to be formed

- Bind to -SH in active site. Substrate cannot bind

A new soft drink on the market contains benzoic acid? Why?

How is benzoic acid effective?

- Because it is a preservative and is also bacteriostatic and fungistatic.

- because the bacteria lays dormant and is inactive allowing it to be a preservative and not make people sick.

- ionic bonds being affected which changes the structure of the protein

Triclosan is found in many antibacterial soups and lotions. How does in inhibit bacterial growth? What are two concerns about the safety of triclosan?

Triclosan inhibits bacterial growth because it is a phenolic compound that destroys membrane integrity, by binding to a specific enzyme found in bacteria and fungi (but not in humans) needed in fatty acid biosynthesis and inhibits it. Therefore no phospholipid synthesis.

How do detergents inhibit bacterial growth? Amylase?


Why is 100% ethanol not very effective in preventing bacterial growth

Because there needs to be water so that the alcohol can get through the cell membrane.

If it was pure alcohol it wouldn't be able to penetrate the cell membrane

Chemicals that control/prevent bacterial growth may act by what general mechanisms?

Disrupt the membrane

Destroys the cell wall

Change structure of proteins

Interact with sulfide bonds

Name four factors that influence the effectiveness of a chemical as a disinfectant

Time of exposure

Type of organism

Concentration of agent


# of organisms

Name three halogens




Needing chlorine for your pool and bromine in your hot tub to kill bacteria, you visit the local pool store. The cashier rings up your purchase and says, "these are great products but I don't know the mechanism by which they work to kill bacteria, do you?" What is the answer?

They oxidize the SH bond to prevent the disulfide covalent bonds.

-it alters SH groups at the active site of proteins

NCLEX questions

Bleach is a example of an agent used in _____.

Bleach is a example of an agent used in disinfection -- a chemical process that removes harmful products of mircoorganisms (toxins) from material, usually inanimate objects

NCLEX questions

Desirable qualities in germicide include all of the following except?

toxicity to human tissues

NCLEX questions

Which factor does not affect the germicidal activity of chemicals?

the cost of the chemical

NCLEX questions

Many antibacterial hand soaps contain the chemical agent Triclosan, which belongs to the ____ antimicrobial category


You are designing bathroom facilities for a new medical center. Will you recommend installation of refillable or non-refillable soap dispensers?

Non-refillable because bacteria can infect the soap when refilling it

Why are tanning beds thought to pose risks to those who choose to be recipients of this technology?

Emits UVA and UVB lights

-can cause thymine dimers

-can lead to higher risks of skin cancer, especially melanoma

Ophthalmia neonatorum can be caused by the two bacteria ____ and ____.

Another name for it?

How is it prevented?

Neisseria gonorrhea

Chlamydia trachomatis

Neonatal Ophthalmitis

By putting erythromycin drops in the babies eyes after to birth to prevent blindness

Bacterial (procaryotic) DNA

Nitrogenous base pair = A T C G

Sugar present = deoxyribose

Double stranded? = yes

A-T, G-C base pairing? = yes

Anti-parallel polynucleotide strands? = yes

# of chromosomes? = 2

Linear or circular chromosome = circular

Surrounded by membrane? = yes

Floats free in cytoplasm? = yes

Contains genes? = yes

Eukaryotic DNA

Nitrogenous base pair = A T C G

Sugar present = ribose

Double stranded? = yes

A-T, G-C base pairing? = yes

Anti-parallel polynucleotide strands? = yes

# of chromosomes? = 1

Linear or circular chromosome = linear

Surrounded by membrane? = no cell membrane

Floats free in cytoplasm? = no, DNA in nucleus

Contains genes? = yes

What is a nucleoside?

What is a nucleotide?

nucleoside = when a sugar is attached to a nitrogenous base

base + sugar

nucleotide = when a phosphate is added to a nucleoside

base + sugar + phosphate

How does ATP differ from ADP?

How does dATP differ from ATP?

How does AMP differ from ATP?

ATP - has 3 phosphate groups (primary energy of cells)

ADP - has 2 phosphate groups (is how energy is released from muscles

dATP - deoxyribose ATP

ATP - ribose ATP

AMP - has 1 phosphate group

ATP - has 3 phosphate groups

What holds two anti-parallel polynucleotide strands together?

Base pairs A-T, G-C

What is meant by complementary base pairing?

There are 4 nitrogenous bases (adenine, guanine, thiamine, cytosine)

Adenine and guanine are purine bases while thiamine and cytosine are pyrimidine bases

----they compliment each other

Write the complimentary DNA strand.

5' G T A G C G 3'

3' C A T C G C 5'

How does (+) supercoiling of DNA differ from

(-) supercoiling of DNA?

(+) supercoiling is turning clockwise and not at normal state -- less base pairs per turn

(-) supercoiling is counter clockwise turning and coils up on itself -- which equals more base pairs per complete turn

What enzyme is responsible for (-) supercoiling from DNA?


What enzyme is responsible for removing (+) supercoiling from DNA?

Topoisomerase are able to change DNA supercoiling

What causes (+) supercoiling of DNA?

The unwinding of the DNA by helicase causes this because as you unwind the top part, the bottom portion gets twisted tighter and tighter and will begin to curl up on itself causing knots, which is reversed through gyrase which introduces (-) supercoiling

How does the antibiotic ciprofloxacin work?

Binds to gyrase and inhibits it -- does not allow neg. supercoiling to take place because gyrase cannot release pos. supercoiling at replication fork and therefore DNA synthesis ceases causing cell death

Properties of nucleic acids

Right handed helix?

- DNA = yes

- RNA - no

Nitrogenous bases present

- DNA = A T G C

- RNA = A U G C

Sugar present

- DNA = deoxyribose

- RNA = ribose

Single or double stranded?

- DNA = double

- RNA = single

How is a plasmid different from the bacterial chromosome?

Bacterial chromosomes

- possess a single chromosome made up of double stranded DNA loop


- are looped bits of DNA, which exist and replicate independently of the chromosome

- the information of the plasmid is generally not essential to the survival of the host bacteria

What is meant by bidirectional DNA replication in bacteria?

It means that the leading strand and lagging strand are going different directions, but both reading 5' to 3'.

Leading strand is going towards helicase and lagging strand is going the other direction away from helicase


DNA synthesis

- enzyme that breaks hydrogen bonds

- unzips DNA single stranded binding proteins and keeps them open so there is no reforming of hydrogen bonds

- single stranded binding protein keeps H bonds in DNA from reforming


DNA Synthesis

- enzyme that brings complementary amino acids of RNA primer

- 10-12 base pairs long

- starts replication


DNA Synthesis

- brings nucleotides to the DNA and lays them down after the primer reads 3' to 5' BUT synthesis 5' to 3' requires free 3' OH to begin to add nucleotides

- has proof reading ability


DNA Synthesis

- removes RNA primer (and converts it to DNA) and replaces ribonucleotides with deoxyribonucleotieds

- 5' to 3' synthesis

- has proof reading ability


DNA Synthesis

connects the Okazaki fragments on the lagging strand with phosphodiester bonds


DNA Synthesis

as replication fork proceeds, pos, supercoiling occurs and gyrase removes the tension by introduction negative supercoiling

Define leading and lagging strands.

What is an Okazaki fragment?

Leading (continuous) strand

- synthesis leading toward helicase fork

Lagging (discontinuous) strand

- synthesis leading away from helicase fork

Okazaki fragment

- fragment of DNA on the lagging strand

DNA Synthesis

1. Free 3' OH must be present for lagging strand (must be provided by RNA primase)

2. RNA primase attaches to DNA and synthesizes a short RNA primer

3. DNA Polymerase III then adds deoxyribonucleoties to the 3' end of the RNA primer

4. DNA Polymerase I replaces DNA polymerase III and removes the RNA and replaces it with DNA

5. DNA ligase forms phosphodiester bond between 3' OH and the growing strand and the 5' in front of it

DNA Polymerase

Building blocks (substrate) = deoxyribonucleotide

Direction of synthesis = 5' to 3'

Direction that template is read = 3' to 5'

Ability to open DNA helix without "help" = No

Requires helicase to open helix = yes

Requires primer to start synthesis = yes (primase)

Requires a free 3' OH on which to add a nucleotide = yes

Proofreading ability = yes

RNA Polymerase

Building blocks (substrate) = ribonucleotide

Direction of synthesis = 5' to 3'

Direction that template is read = 3' to 5'

Ability to open DNA helix without "help" = yes

Requires helicase to open helix = no

Requires primer to start synthesis = no

Requires a free 3' OH on which to add a nucleotide = no

Proofreading ability = no

Promoter DNA sequence (Pribnow box)

RNA synthesis

it is the RNA polymerase binding site

it is rich in A, T nitrogenous bases

Sigma Protein

RNA synthesis

required for RNA polymerase to bind to DNA

-binding at the DNA site is called the promoter

RNA Polymerase

RNA synthesis

polymerizes ribonucleotides 5' to 3'

reads DNA template 3' to 5'

can open DNA helix with no help

does not need 3' OH primer to start

How does rifampin inhibit RNA synthesis?

Binds to the RNA polymerase and blocks its movement -- therefore rifampin blocks initiation

Given the DNA strand below, write the sequence of the mRNA.

5' A T A G G C T A A G G A G G A 3'

3' T A T C C G A T T C C T C C T 5'

5' A U A G G C U A A G G A G G A 3'

Why is the promoter region of the DNA easy for the RNA polymerase to open?

It is an A and T rich region and A-T only have 2 hydrogen bonds so they are easier to break