Lecture 4 Cell Biology

8/8/2019 Lecture 4 Cell Biology

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NUCLEIC ACIDS1-They are the most important macromolecules in the cells

of all living organisms2- They are carriers of genetic information

3-There have Two forms (RNA and DNA)

Composition and structure of nucleic acids Nucleic acid molecule is a polymers of monomeric unit

called nucleotides . These nucleotide monomer unitare joined by the formation of phosphodiester bond


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Nucleotide Nucleotides composed of:

(1) P entose (C5) sugar , either ribose (RNA) or deoxyribose (DNA)

Five-carbon pentose in DNA called 2-deoxyribose where the OHgroup on the 2 prime (2) carbon of ribose is replaced with H atom

but not in RNA.


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Nucleotide( 2)Nitrogen bases which belong to two chemical classes- Purine bases (adenine and guanine): contain two fusedheterocyclic rings

- Py rimidine bases (thymine, cytosine, and uracil): containa single six-memb

ered heterocyclic ring

(3) Phosphate group


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B ondPhosphodiester - Phosphate linkage that connects two sugars by

ester linkage- Diester bond is one which involves two ester

bond

- Phosphodiester bond will be formed between anytwo adjacent nucleotides (5 phosphate of onenucleotide and the 3 hydroxyl of another)


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Nucleotide

Nucleoside : nitrogen base bonded to its C 5 sugar Nucleotide : nitrogen base attached to C 5 sugar byglycosidic linkage and bonded to a phosphate


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NucleotideF

unction: M ajor components of nucleic acids Key forms of chemical energy (e .g. , ATP) Carriers of sugars in biosynthesis of polysaccharides

Regulatory molecules for certain enzymes or metabolic events


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Nucleotide

PO 4

Nucleoside

O-

P O

O-

O-

Ribose

Nitrogen base

PentoseSugar

Deoxyribose

Purines

DNA

RNA

(A)

(G)Pyrimidines

(T)

(U)

(C)


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Primar y Structure of nucleic acidIs a Sequence of nucleotides in a DNA or RNA molecule- oligonucleotide : small polymer contains only a few

nucleotides- pol ynucleotide : very large polymer contains thousand or

millions of nucleotides- the polynucleotide chain (strand) has a sense of directionwith one end of the chain terminating in a 5 phosphategroup and the other in a 3 hydroxyl group of growing

Chain- the sequence of bases in RNA and DNA is written in the5 to 3 direction


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Two polynucleotide strands wrap around eachother to form a DNA double helixThe two strands are associated because particular

bases always hydrogen bond to one another A pairs with T , and C pairs with G , producing basepairs

Base pairing of nitrogen bases


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Types of nucleic acid1- DNADouble-stranded molecules consisting of twocomplementary polynucleotide chains running in oppositedirectionStrands held together by hydrogen bonds

Basepair


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Types of nucleic acid2- RNARNA is usually a single polynucleotide strandDemonstrates secondary structure (folding back upon itself)

Four classes: mRNAs, tRNAs, rRNAs, and small RNAs


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W hat is the Gene?A particular nucleotide sequencethat can instruct the formation of a

polypeptide is called a geneM ost DNA molecules consist of

millions of base pairs and,consequently, many genesHuman genome contain about 3

billion nucleotides and 30000genesThese genes, many of which areunique to the species, determinethe structure of proteins and, thus,lifes structures and functions


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W hat is the Gene?


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Enzymes


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EnzymesE

nzymes are specialized macromolecules that speed upchemical reactions in cellsProperties:1- They are Catalytic proteins( biological catalysts) that

accelerate the rate of biological2- enzymes can be denatured and precipitated with salts,

solvents and other reagents .3- M any enzymes require the presence of other compounds -

cofactors - before their catalytic activity can be exerted .

4- I ncrease the rate of chemical reaction without themselves being consuming or permanently altered by the reaction

5- I ncrease reaction rates without altering the chemicalequilibrium between reactants and products


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Enzymes6 - E nzymes are usually very specific as to which reactionsthey catalyze and the substrates that are involved in

these reactions .


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CofactorsC ofactors and coenz ymesSome enzymes do not need any additional componentsOthers require non-protein molecules called cofactors to be

bound for activity

C ofactors can be:- inorganic (e.g. , metal ions and iron-sulfur clusters)- organic compounds (e .g. , flavin and heme) . which can

be either:

- prosthetic groups , which are tightly bound to anenzyme and play critical function roles (e .g : heme, Zinc)- coenz ymes, which are released from the enzyme's

active site during the reaction, they called coenzymes

because they work together with enzymes.


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E nzyme classificationE C 1 Oxidoreductases : catalyze oxidation/reductionreactionsE C 2 T ransferases : transfer a functional group (e .g . amethyl or phosphate group)E C 3 H ydrolases : catalyze the hydrolysis of various

bondsE C 4 L yases : cleave various bonds by means other thanhydrolysis and oxidationE

C 5 Isomerases : catalyze isomerisation changes withina single moleculeE C 6 Ligases : join two molecules with covalent bonds


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Factors influencing enzyme functionpH: E nzymes in your stomach may prefer an acidicenvironment with a low pH while enzymes elsewhere maynot .

higher temperatures speed reactions -- to a point . Above

104 degrees Fahrenheit, enzymes become denatured andcan no longer catalyze reactions .

Higher concentrations of substrate also speed reactionsuntil the solution reaches a saturation point .Past that

point, the addition of substrate will not affect the velocity of the reaction .

Irreversible inhibition occurs when the inhibitor addeddenatures or destroys the enzyme .


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Structure of Cell Membrane Good Luck

Dr Mona

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Lecture 4 Cell Biology