Chapter6micro farid

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MICROBIOLOGYfor the Health Sciences

. العمرين ابو فريد أ

CHAPTER: 6

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Section III. Chemical and Genetic Aspects of

Microorganisms

Chapter 6. Biochemistry: The Chemistry of Life

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Chapter 6 Outline

• Introduction

•Organic Chemistry

– Carbon Bonds

– Cyclic Compounds

•Biochemistry

– Carbohydrates

– Lipids

– Proteins

– Nucleic Acids Copyright © 2011 Wolters Kluwer Health | Lippincott Williams & Wilkins

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Nucleic AcidsFunction

• DNA and RNA comprise the 4th major group of biomolecules in

living cells after (Carbohydrates, Lipids, Proteins) .

• DNA and RNA are critical to proper functioning of a cell.

1. Deoxyribonucleic Acid

2. Ribonucleic Acid

• DNA is the “hereditary molecule” – the molecule that contains the

genes and genetic code.

– Information in DNA must flow to the rest of the cell for the cell

to function properly – the flow is accomplished by RNA.

• RNA molecules participate in the conversion of the genetic code

into proteins and other gene products.

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Nucleic AcidsStructure

• In addition to the elements C, H, O, and N, DNA and

RNA also contain phosphorus, P.

•The building blocks of nucleic acid polymers are called

nucleotides.

• The building blocks of DNA are called DNA nucleotides.

• The building blocks of RNA are called RNA nucleotides.

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Nucleotides

Two nucleotides, each consisting of:

1. a nitrogenous base (A G C T) adenine (A), guanine (G), thymine (T), cytosine (C) and uracil

2. a five-carbon sugar (S) Deoxyribose, Ribose

3. a phosphate group (P)

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A and G are purines; T, C and U are pyrimidines.

Thymine is found in DNA but not in RNA. Uracil is found in RNA, but not in DNA. The other 3 bases are found in both.

The Pyrimidines and Purines Found in DNA and RNA.

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Base pairs that occur in double stranded DNA molecules.

Note that A and T are

connected by two hydrogen

bonds, whereas G and C are

connected by three hydrogen

bonds.

The arrows represent the

points at which the bases are

bonded to deoxyribose

molecules.

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Types of RNA

•There are 3 types of RNA, named for their function:

– Messenger RNA (mRNA)

– Ribosomal RNA (rRNA)

– Transfer RNA (tRNA)

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DNA Structure

•For a double-stranded DNA molecule to form, the

nitrogenous bases on the two separate strands must

bond together.

•The bonding forces of the double-stranded polymer

cause it to assume the shape of a double alpha-helix,

similar to a right-handed spiral staircase.

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A section of a nucleic acid polymer.

Within a double stranded DNA molecule, A in one strand always bonds with T in the complimentary strand, and G in one strand always bonds with C in the complimentary strand. A–T and G–C are known as base pairs.

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Double-stranded DNA molecule, also known as a double helix.

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DNA Replication

•When a cell is preparing to divide, all DNA molecules in the

chromosomes of the cell must duplicate, thereby ensuring

that the same genetic information is passed on to both

daughter cells.

– This is called DNA replication.

• DNA replication occurs by separation of the 2 DNA strands

and the building of complementary strands by the addition

of the correct DNA nucleotides.

• The most important enzyme required for DNA replication is

the DNA polymerase.

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DNA Replication.

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DNA ReplicationGene Expression

•A gene is a particular segment of a DNA molecule or

chromosome.

• It is the sequence of the four nitrogenous bases of

DNA (i.e., A, G, C, and T) that spell out the

instructions for a particular gene product.

•Although most genes code for proteins, some code

for rRNA and tRNA.

•Some genes code for more than one gene product.

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The Central Dogma

•The Central Dogma explains the flow of genetic

information within a cell.

•DNA mRNA protein.

– One gene of a DNA molecule is used to make one molecule

of mRNA by a process known as transcription.

– The genetic information in the mRNA is then used to make

one protein by a process known as translation.

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DNA ReplicationGene Expression, cont.

•All genes on a chromosome are not being expressed at any

given time. It would not be logical for a cell to produce a

particular enzyme if it was not needed.

– Genes that are only expressed when the gene products are needed

are called inducible genes.

– Genes that are expressed at all times are called constitutive genes.

• The process by which the genetic code within the DNA molecule is

transcribed to produce an mRNA molecule is called transcription.

– The primary enzyme involved is RNA polymerase.

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Transcription

• In eucaryotes, transcription occurs within the nucleus;

the newly formed mRNA molecules then travel through

the pores of the nuclear membrane into the cytoplasm,

where they are used to produce proteins.

• In procaryotes, transcription occurs in the cytoplasm;

ribosomes attach to the mRNA molecules as they are

being transcribed at the DNA – thus both transcription

and translation may occur simultaneously.

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Translation

•The process of translating the message carried by mRNA, whereby particular tRNAs bring amino acids to be bound together in the proper sequence to make a protein, is called translation.

•The base sequence of the mRNA molecule is read in groups of 3 bases, called codons.

•The 3-base sequence codon can be read by a complementary 3-base sequence (the anticodon) on a tRNA molecule.

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Chart to illustrate the sequence of 3 bases (GGC) in the DNA template that codes for a particular codon (CCG) in mRNA, which in turn, attracts a particular anticodon (GGC) on the tRNA carrying an amino acid (proline).

DNA

Template

mRNA

(Codon)

tRNA

(Anticodon)

Amino

Acid

G C G

G C G

C G C

Proline

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Translation

For example, the tRNA with the anticodon base

sequence UUU carries the amino acid lysine to

the mRNA codon AAA.

Similarly, the mRNA codon CCG codes for the

tRNA anticodon GGC, which carries the amino

acid proline.

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Translation (protein synthesis).

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Mutation

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Mutation

If one of the bases of a DNA gene is incorrect or out of sequence (known as a mutation).

Mutations (involve changes in the base sequences of genes)

The amino acid sequence of the gene product will be incorrect.

For example, some diabetics a mutation in one of their chromosomes caused a rearrangement of the bases in the gene that codes for insulin.

Such errors are the basis for most genetic and inherited diseases, such as phenylketonuria (PKU), sickle cell anemia, cystic fibrosis, cleft lip, extra fingers, albinism.

Likewise, nonpathogenic microbes may mutate to become pathogens, and pathogens may lose the ability to cause disease by mutation.