Introduction to Introduction to

Molecular Biology IMolecular Biology I

Cell BiologyCell Biology

TBB 1013TBB 1013

Sem1 - 2009/2010Sem1 - 2009/2010

ObjectivesObjectives::

1 DNA and RNA structure and function1 DNA and RNA structure and function

1.1 DNA replication1.1 DNA replication

2 Gene Expression2 Gene Expression

2.1 Central Dogma2.1 Central Dogma

2.2 Transcription2.2 Transcription

2.3 Translation2.3 Translation

2.4 Mutations2.4 Mutations

3 DNA Technology3 DNA Technology

3.1 Recombinant DNA3.1 Recombinant DNA TechnologyTechnology

Polymerase ChainPolymerase Chain ReactionReaction

Genetic engineering applicationsGenetic engineering applications

4 Gene Regulation4 Gene Regulation

4.1 Control of gene expression in prokaryotes and 4.1 Control of gene expression in prokaryotes and eukaryoteseukaryotes

Signalling between cells in eukaryotesSignalling between cells in eukaryotes

Company NameCompany Name

Introduction To Molecular Biology: Introduction To Molecular Biology: Key termsKey terms

GenomeGenome = complete set of genes. = complete set of genes. • sequences in the genetic material of an organ. sequences in the genetic material of an organ.

ChromosomesChromosomes = Discrete unit of the genome carrying = Discrete unit of the genome carrying many genes. composed of 2 Macromolecules:many genes. composed of 2 Macromolecules:

• i.i. Protein.Protein.

• ii.ii. Nucleic acids \ DNA(d) and RNA(s)Nucleic acids \ DNA(d) and RNA(s)

GeneGene = Segment of DNA that codes for a protein. = Segment of DNA that codes for a protein. • It codes for a RNA Protein.It codes for a RNA Protein.

DNA(Deoxyribonucleic Acid)DNA(Deoxyribonucleic Acid)=The genetic material of =The genetic material of all cellular organisms and most viruses; which is used all cellular organisms and most viruses; which is used to encode genetic information for all life on Earth.to encode genetic information for all life on Earth.

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Introduction To Molecular Biology: Introduction To Molecular Biology: Key termsKey terms

Viruses contain either RNA or DNA genomesViruses contain either RNA or DNA genomes Can be linear or circularCan be linear or circular Can be single or double strandedCan be single or double stranded

PlasmidsPlasmids: replicate separately from chromosome: replicate separately from chromosome Great majority are double strandedGreat majority are double stranded Circular (most)Circular (most) Generally beneficial for the cell (i.e., antibiotic Generally beneficial for the cell (i.e., antibiotic

resistance)resistance) NOT extracellular, unlike virusesNOT extracellular, unlike viruses

PlasmidPlasmid : is a genetic element that is expendable and rarely contains genes : is a genetic element that is expendable and rarely contains genes

needed for growth under all conditions.needed for growth under all conditions.

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Kinds of Genetic ElementsKinds of Genetic Elements

Molecular BiologyMolecular Biology Molecular biologyMolecular biology; ; the study of the study of gene gene structurestructure and and

functionsfunctions at the at the molecular levelmolecular level to understand the to understand the molecular basis of hereditary, genetic variation, and molecular basis of hereditary, genetic variation, and the expression patterns of genes. the expression patterns of genes.

The The Molecular biologyMolecular biology field overlaps with other field overlaps with other areas, particularly genetics and biochemistry.areas, particularly genetics and biochemistry.

The genome of an organism The genome of an organism is the totality of is the totality of genetic information and is encoded in the genetic information and is encoded in the DNADNA (or, for some viruses, (or, for some viruses, RNARNA).).

The DNAThe DNA Deoxyribonucleic Acid (DNA)Deoxyribonucleic Acid (DNA), , the genetic material of all the genetic material of all

cellular organisms and most viruses, the gigantic molecule cellular organisms and most viruses, the gigantic molecule which is which is used to encode genetic informationused to encode genetic information for all life on for all life on Earth.Earth.

The ChromosomeThe Chromosome, the , the storage place storage place for all genetic for all genetic information, the number of chromosomes varies from one information, the number of chromosomes varies from one species to another.species to another.

In In normal human cell DNA normal human cell DNA contained in the nucleus, arranged contained in the nucleus, arranged in in 23 pairs of chromosomes23 pairs of chromosomes ..

The geneThe gene,, the basic units of inheritance; it is a segment the basic units of inheritance; it is a segment within a very long strand of within a very long strand of DNADNA with specific instruction with specific instruction for for the production of one specific proteinthe production of one specific protein. . GenesGenes located on located on chromosome on it's place or chromosome on it's place or locuslocus. .

Within cells, DNA is organized into structures called Within cells, DNA is organized into structures called chromosomes.chromosomes.

Eukaryotic organisms: Eukaryotic organisms:

storestore their DNA inside the their DNA inside the cell nucleus Prokaryotic organisms : Prokaryotic organisms :

it is found in the cell's it is found in the cell's cytoplasm.. In In normalnormal human cell human cell DNADNA contained in the nucleus, arranged in contained in the nucleus, arranged in

23 pairs of chromosomes23 pairs of chromosomes..

What is a DNA?What is a DNA? A nucleic acid that A nucleic acid that carries the genetic information carries the genetic information

in the cell and is capable of self-replication and in the cell and is capable of self-replication and synthesis of RNA. synthesis of RNA.

DNA consists of two long chains of nucleotides DNA consists of two long chains of nucleotides twisted into a double helix and joined by hydrogen twisted into a double helix and joined by hydrogen bonds.bonds.

The sequence of nucleotides determines individual The sequence of nucleotides determines individual hereditary characteristics.hereditary characteristics.

Functions of DNAFunctions of DNA

Storage of genetic informationStorage of genetic information

Transmission of genetic informationTransmission of genetic information

General Structure of Nucleic AcidGeneral Structure of Nucleic Acid DNADNA & & RNARNA are long chain polymers of small compound are long chain polymers of small compound

called called nucleotidesnucleotides. .

Each nucleotide is composed of a Each nucleotide is composed of a nitrogen-containing nitrogen-containing basebase; ; pentose sugarpentose sugar (5 carbon) (5 carbon) “ “riboseribose in RNA or in RNA or deoxyribosedeoxyribose in DNA in DNA) and a ) and a phosphate groupphosphate group. .

The phosphate joins the sugars in a The phosphate joins the sugars in a DNADNA or or RNARNA chain chain through their 5through their 5 and 3′ hydroxyl group by phosphodiester and 3′ hydroxyl group by phosphodiester bonds.bonds.

Phosphates connect 3′- carbon of one sugar to 5Phosphates connect 3′- carbon of one sugar to 5 of the of the adjacent sugaradjacent sugar

There are four different types of There are four different types of nucleotidesnucleotides found infound in DNADNA,, differing only in the nitrogenous basediffering only in the nitrogenous base: : AA is for is for adenineadenine; ; GG is for is for guanineguanine;; CC is for is for cytosinecytosine and and TT is for is for thyminethymine..

These bases are classified These bases are classified based on their chemical structures based on their chemical structures into two groups: into two groups:

adenine and guanineadenine and guanine are double ringed structure termed are double ringed structure termed purinepurine thymine and cytosinethymine and cytosine are single ring structures are single ring structures termedtermed pyrimidinepyrimidine

The bases pair in a specific way:The bases pair in a specific way: Adenine Adenine AA withwith thymine thymine TT (two hydrogen bonds)(two hydrogen bonds) and and guanine guanine GG with with cytosinecytosine C C (three (three hydrogen bonds).hydrogen bonds).

Within the structure ofWithin the structure of DNA,DNA, the number of the number of thyminethymine is always is always equalequal to the number of to the number of adenineadenine and the number of and the number of cytosinecytosine is is always always equalequal to to guanineguanine..

General Structure of Nucleic AcidGeneral Structure of Nucleic Acid

The structure of DNAThe structure of DNA was described by British Scientists was described by British Scientists ((Watson and CrickWatson and Crick)) as long as long double helix shaped, double helix shaped, with its with its sugar phosphate backbone on the outside and its nitrogenous sugar phosphate backbone on the outside and its nitrogenous bases on inside; bases on inside;

The two strands of the helix The two strands of the helix run in opposite directionrun in opposite direction and are and are anti-parallel to each other. The DNA double helix is anti-parallel to each other. The DNA double helix is stabilized stabilized by hydrogen bonds between the basesby hydrogen bonds between the bases..

This structure explains how genes engage in This structure explains how genes engage in replicationreplication, , carrying information carrying information and and acquiring mutationacquiring mutation..

The G+C content of a natural DNA can The G+C content of a natural DNA can vary from 22-73% vary from 22-73% and and this can have a strong effect on the physical properties ofthis can have a strong effect on the physical properties of DNA, particularly its melting temperature. DNA, particularly its melting temperature.

General Structure of Nucleic AcidGeneral Structure of Nucleic Acid

DNA: Watson and Crick’s modelDNA: Watson and Crick’s model

Watson and Crick’s modelWatson and Crick’s model

Double HelixDouble Helix Sugar-PhosphateSugar-Phosphate backbone = backbone =

sugar is deoxyribose sugar is deoxyribose 4 types of nucleotide base = 4 types of nucleotide base =

A, C, G, T A, C, G, T 2 complementary strands 2 complementary strands

where A = T, C = G where A = T, C = G

"complementary“ = fitting "complementary“ = fitting together of 2 molecules together of 2 molecules with hydrogen bondswith hydrogen bonds

DNA has 4 BasesDNA has 4 Bases

Ribbon model

The Double Helix of DNA

DNA: Four DNA BasesDNA: Four DNA Bases

N

N

NH

N

NH2

NH

NH

O

O

CH3

N

NH

NH

N

NH2

O

N

NH

NH2

O

AdenineAdenine ThymineThymine

GuanineGuanineCytosineCytosine

DNA: The SugarDNA: The Sugar

OHO

OH

OH

DeoxyriboseDeoxyribose

DNA: The NucleotidesDNA: The Nucleotides

AA

GG TT

CC

NO

O

OH

OH

OH

O

P

NH

O

O

CH3

NO

O

OH

OH

OH

O

P N

NN

NH2

NO

O

OH

OH

OH

O

P N

NHN

NH2

O

NO

O

OH

OH

OH

O

P

N

NH2

O

AdenineAdenine pairs with pairs with ThymineThymine

HydrogenBonding

Guanine Guanine pairs with pairs with CytosineCytosine

HydrogenBonding

DNADNA

A) Sugar ; A) Sugar ;

Deoxyribose.Deoxyribose.

B) Nitrogen base; B) Nitrogen base;

(Adenine, Guanine, (Adenine, Guanine, Cytosine, Thymine)Cytosine, Thymine)

C) A phosphate groupC) A phosphate group

Genes and Base Pairs:Genes and Base Pairs:A ComparisonA Comparison

SpeciesGenes

(estimated)Base Pairs

Yeast(Saccharomyces cerevisiae)

6,000 12 million

Round Worm (Caenorhabditis elegans)

19,000 99 million

Mouse (Mus musculus)

40,000 3 billion

Fruit Fly (Drosophila melanogaster)

13,600 165 million

Human (Homo sapiens)

40,000 3 billion

In contrast to DNA; In contrast to DNA;

RNA is a RNA is a singlesingle stranded stranded

the pyrimidine base the pyrimidine base Uracil (U) Uracil (U) replaces replaces thyminethymine

RiboseRibose sugar replaces sugar replaces deoxyribose.deoxyribose.

RNARNA

DNA vs. RNADNA vs. RNA

Double-strandedDouble-stranded

Deoxyribose sugarDeoxyribose sugar

Thymine (T) baseThymine (T) base

Single-strandedSingle-stranded

Ribose sugarRibose sugar

Uracil (U) baseUracil (U) base

DNADNA RNARNA

The RNAThe RNA Three major classes of RNAThree major classes of RNA: :

messenger (mRNA) : encodes polypeptidesmessenger (mRNA) : encodes polypeptides

transfer (tRNA) : plays role in protein synthesistransfer (tRNA) : plays role in protein synthesis

ribosomal (rRNA) : plays role in protein synthesisribosomal (rRNA) : plays role in protein synthesis

The RNAThe RNA mRNAmRNA molecules represent transcripts of structural molecules represent transcripts of structural

genes that encode all the information necessary for the genes that encode all the information necessary for the synthesis of a single type polypeptide of protein.synthesis of a single type polypeptide of protein.

mRNAmRNA;; intermediate carrier of genetic information; intermediate carrier of genetic information; deliver genetic information to the cytoplasm where deliver genetic information to the cytoplasm where protein synthesis take place.protein synthesis take place.

The The mRNAmRNA also contains regions that are not translated: also contains regions that are not translated: in eukaryotes this includes the in eukaryotes this includes the 5' untranslated region, 3' 5' untranslated region, 3' untranslated region, 5' capand and poly-A tailuntranslated region, 5' capand and poly-A tail..

The RNAThe RNA All All tRNAstRNAs share a common secondary structure represented share a common secondary structure represented

by a coverleaf. by a coverleaf.

They have They have four- base paired stems four- base paired stems defining three stem loops defining three stem loops (the D loop, anticodon loop, and T loop) and the acceptor (the D loop, anticodon loop, and T loop) and the acceptor stem stem to which amino acids are addedto which amino acids are added in the charging step. in the charging step.

RNA molecules that carry amino acids to the growing RNA molecules that carry amino acids to the growing polypeptide. polypeptide.

The RNAThe RNA Ribosomal RNA (rRNA)Ribosomal RNA (rRNA) is the central component of the is the central component of the

ribosome, the function of the rRNA is ribosome, the function of the rRNA is to provide a mechanism to provide a mechanism for decoding mRNA into amino acids for decoding mRNA into amino acids and to and to interact with the interact with the tRNAs during translationtRNAs during translation by providing peptidyl transferase by providing peptidyl transferase activity. activity.

The concentration of purine and pyrimidine bases The concentration of purine and pyrimidine bases do not do not necessarily equal one another in RNA necessarily equal one another in RNA because RNA is single because RNA is single stranded. stranded.

However, the single strand of RNA is However, the single strand of RNA is capable of folding back capable of folding back on itself on itself like a hairpin and acquiring double strand structure.like a hairpin and acquiring double strand structure.

The RNAThe RNA

tRNA mRNA

Chromosomes Chromosomes and DNAand DNA

DNA: General RulesDNA: General Rules

Two nucleotide chainsTwo nucleotide chains Hydrogen BondsHydrogen Bonds Nitrogenous Bases are located togetherNitrogenous Bases are located together Bases pair specifically (A-T, G-C)Bases pair specifically (A-T, G-C) Forms Double HelixForms Double Helix DNA is wrapped around DNA is wrapped around histoneshistones to form chromosomes to form chromosomes

Genomic DNA OrganizationGenomic DNA Organization Eukaryotic genes: Eukaryotic genes: DNADNA molecules complexed with other proteins molecules complexed with other proteins

especially basic proteins called especially basic proteins called histoneshistones to form a substance to form a substance known as known as chromatinchromatin. .

AA human cell contains about human cell contains about 2 meters of DNA2 meters of DNA. DNA in body . DNA in body could stretch to the sun and back almost 100 times. So it is could stretch to the sun and back almost 100 times. So it is tightly packed.tightly packed.

Eukaryotic chromatin Eukaryotic chromatin is folded in several ways. is folded in several ways. The first order of folding involves structures calledThe first order of folding involves structures called nucleosomesnucleosomes,,

which have a core of histones, around which thewhich have a core of histones, around which the DNA winds ( four DNA winds ( four pairs of core histones H2A, H2B, H3 and H4 in a wedge shaped pairs of core histones H2A, H2B, H3 and H4 in a wedge shaped disc).disc).

The second order of chromatin folding is the 30 nm fiber. The second order of chromatin folding is the 30 nm fiber. The third order of chromatin condensation appears to involve The third order of chromatin condensation appears to involve

formation of a radial loop structure in eukaryotic chromosomes.formation of a radial loop structure in eukaryotic chromosomes.

Genomic DNA OrganizationGenomic DNA Organization

ChromosomesChromosomes are comprised of: are comprised of:

- 2 Chromatids (2C) - 2 Chromatids (2C) - Centromere- Centromere

ChromosomesChromosomes

Ecology Ecology Physiology Physiology Molecular Molecular BiologyBiology

Ecology Ecology Physiology Physiology Molecular Molecular BiologyBiology

DNA ReplicationDNA Replication DNA Replication:DNA Replication:

all gene duplication.all gene duplication.

the transfer the genetic information from a parent to a the transfer the genetic information from a parent to a daughter cell. daughter cell.

the DNA base sequence are precisely copied.the DNA base sequence are precisely copied.

Semiconservative Semiconservative replication would produce two copies that replication would produce two copies that each contained one of the original strands and one entirely each contained one of the original strands and one entirely new strand. new strand.

Conservative replication Conservative replication would leave the two original template would leave the two original template DNA strands together in a double helix and would DNA strands together in a double helix and would produce a produce a copy composed of two new strandscopy composed of two new strands containing all of the new containing all of the new DNA base pairs. DNA base pairs.

Dispersive replication Dispersive replication would produce two copies of the DNA, would produce two copies of the DNA, both containing distinct regions of DNA composed of either both containing distinct regions of DNA composed of either both original strands or both new strandsboth original strands or both new strands. .

DNA ReplicationDNA Replication

Possible Modes of DNA Possible Modes of DNA ReplicationReplication

Replication: 5’ to 3’ Direction ONLYReplication: 5’ to 3’ Direction ONLYContinuous replication on one strand

Discontinuous replication on one strand

Semiconservative DNA Replication

DNA Replication is DNA Replication is Catalyzed by EnzymesCatalyzed by Enzymes

* Okazaki fragments are on lagging strand.

Templates and Enzymes Templates and Enzymes

DNA polymerasesDNA polymerases catalyze the addition of catalyze the addition of

nucleotides.nucleotides.

DNA polymerases require a primerDNA polymerases require a primer

Primer made from RNA by an enzyme Primer made from RNA by an enzyme PrimasePrimase

Events at the DNA Replication Fork

Proofreading Proofreading DNADNA replication is extremely accuratereplication is extremely accurate

ProofreadingProofreading helps to ensure high fidelity helps to ensure high fidelity

Mutation rates in cells are 10Mutation rates in cells are 10-8-8–10–10-11-11 errors errors

per base inserted.per base inserted.

Polymerase can detect mismatch Polymerase can detect mismatch through through

incorrect hydrogen bondingincorrect hydrogen bonding

DNA Polymerase Adds Complementary BasesDNA Polymerase Adds Complementary Bases

DNA Polymerase Sees ErrorsDNA Polymerase Sees Errors

DNA Polymerase Repairs ErrorsDNA Polymerase Repairs Errors

Gene ExpressionGene Expression Gene expressionGene expression process by which a gene product process by which a gene product

(an RNA or polypeptide) is made.(an RNA or polypeptide) is made.

In In transcriptiontranscription steps, RNA polymerase make a copy steps, RNA polymerase make a copy of information in the gene (complementary RNA) of information in the gene (complementary RNA) (mRNA) complementary to one strands of DNA (mRNA) complementary to one strands of DNA

In In translationtranslation step step, , ribosomes read a messenger ribosomes read a messenger RNA and make protein RNA and make protein according to its instructionaccording to its instruction. . Thus Thus any change in gene sequence may lead to any change in gene sequence may lead to change in the protein productchange in the protein product..

Gene ExpressionGene Expression TranscriptionTranscription is the process by which mRNA is is the process by which mRNA is

made from DNA template to copy the information in made from DNA template to copy the information in the gene and pass it to mRNA (in the cytoplasm).the gene and pass it to mRNA (in the cytoplasm).

-- TranscriptionTranscription = mRNA from DNA (in the nucleus) = mRNA from DNA (in the nucleus)

TranslationTranslation is the process by which protein is made is the process by which protein is made from an mRNA template.from an mRNA template.

-- TranslationTranslation = Protein from mRNA (in the cytoplasm) = Protein from mRNA (in the cytoplasm)

Types of control in eukaryotes:Types of control in eukaryotes: Transcriptional,Transcriptional,

prevent transcription, prevent mRNA from being synthesized.prevent transcription, prevent mRNA from being synthesized. Posttranscriptional,Posttranscriptional,

control mRNA after it has been produced.control mRNA after it has been produced. Translational,Translational,

prevent translation; involve protein factors needed for prevent translation; involve protein factors needed for translation.translation.

Posttranslational,Posttranslational,

after the protein has been produced.after the protein has been produced.

Overview of TranscriptionOverview of Transcription

Transcription (DNA to RNA) is carried out by Transcription (DNA to RNA) is carried out by

RNA polymeraseRNA polymerase..

RNA polymerase uses DNA as template.RNA polymerase uses DNA as template.

RNA precursors are ATP, GTP, CTP, and UTP.RNA precursors are ATP, GTP, CTP, and UTP.

Chain growth is 5′ to 3′ just like DNA replication.Chain growth is 5′ to 3′ just like DNA replication.

Overview of TranscriptionOverview of Transcription

Only one of the two strands of DNA Only one of the two strands of DNA are are

transcribed by transcribed by RNA polymerase RNA polymerase for any gene.for any gene.

Genes are present on both strands of DNAGenes are present on both strands of DNA, but , but

at different locationsat different locations

RNA polymerase RNA polymerase has five different subunitshas five different subunits

RNA polymeraseRNA polymerase recognizes DNA sites called recognizes DNA sites called

promoterspromoters

Overview of TranscriptionOverview of Transcription PromotersPromoters: site of initiation of transcription: site of initiation of transcription

PromotersPromoters are recognized by are recognized by sigma factorsigma factor of RNA of RNA

polymerase.polymerase.

Transcription stops at specific sites called Transcription stops at specific sites called

transcription terminatorstranscription terminators ..

Unlike DNA replication, transcription involves Unlike DNA replication, transcription involves smaller smaller

units of DNAunits of DNA Often as small as a single gene.Often as small as a single gene. Allows cell to transcribe different genes at different rates.Allows cell to transcribe different genes at different rates.

Transcription: Steps in RNA Synthesis

Termination of TranscriptionTermination of Transcription

Termination of RNA synthesis is governed by a Termination of RNA synthesis is governed by a

specific DNA sequencespecific DNA sequence..

Unit of transcriptionUnit of transcription: unit of chromosome bounded : unit of chromosome bounded by sites where transcription of DNA to RNA is by sites where transcription of DNA to RNA is initiated and terminatedinitiated and terminated

mRNA have short half-lives (a few minutes)mRNA have short half-lives (a few minutes)

The The purinepurine and and pyrmidinepyrmidine bases of the DNA molecule are bases of the DNA molecule are the letters or alphabet of the genetic code. the letters or alphabet of the genetic code.

All information contained in DNA represented by four letters: All information contained in DNA represented by four letters: A,T,C,G.A,T,C,G.

Three nucleotides of DNA (1st, 2nd and 3rd) form Three nucleotides of DNA (1st, 2nd and 3rd) form triplet triplet codonscodons..

there are there are 64 possible codons64 possible codons, , most amino acids have more than one possible codon. most amino acids have more than one possible codon. Out of the 64 possible 3-base codons, 61 specify amino Out of the 64 possible 3-base codons, 61 specify amino

acids; the other three are stop signals (UAG, UAA, or UGA). acids; the other three are stop signals (UAG, UAA, or UGA).

The sequence of codons in the mRNA defines the primary The sequence of codons in the mRNA defines the primary structure of the final protein. structure of the final protein.

The Genetic CodeThe Genetic Code

The Genetic CodeThe Genetic Code

TranslationTranslation: the synthesis of proteins from RNA.: the synthesis of proteins from RNA.

Genetic codeGenetic code: a triplet of nucleic acid bases (: a triplet of nucleic acid bases (codoncodon) )

encodes a single amino acid.encodes a single amino acid.

Specific codons for starting and stopping translation.Specific codons for starting and stopping translation.

Degenerate codeDegenerate code: multiple codons encode a single : multiple codons encode a single amino acid.amino acid.

Anti-codonAnti-codon on tRNA recognizes codon. on tRNA recognizes codon.

Transfer RNATransfer RNA Transfer RNATransfer RNA: : at least at least

one tRNA per amino acid.one tRNA per amino acid.

Specific for both Specific for both a codon a codon

and its particular amino and its particular amino

acid.acid.

tRNA is cloverleaf in shapetRNA is cloverleaf in shape

Anti-codonAnti-codon: three bases : three bases

of tRNA that recognize of tRNA that recognize

three complementary three complementary

bases on mRNA.bases on mRNA.

The Genetic CodeThe Genetic Code

CodonsCodons

There are There are twenty amino acidstwenty amino acids, so a , so a three-basethree-base code is the code is the minimumminimum required. required.

Series of codons in part of a mRNA molecule. Each codon Series of codons in part of a mRNA molecule. Each codon consists of three nucleotides, representing a single amino acid.consists of three nucleotides, representing a single amino acid.

The Genetic CodeThe Genetic Code

The Genetic Code as Expressed by Triplet Bases of mRNA

Synthesis of the Informational Macromolecules

TheThe“Central Dogma”“Central Dogma”

Macromolecules and Genetic Macromolecules and Genetic InformationInformation

Central dogma of molecular biologyCentral dogma of molecular biology

DNA to RNA to proteinDNA to RNA to protein

EukaryotesEukaryotes: each gene is transcribed individually: each gene is transcribed individually

ProkaryotesProkaryotes: multiple genes may be transcribed : multiple genes may be transcribed

togethertogether

Transcription in ProkaryotesTranscription in Prokaryotes

Central Dogma of Molecular BiologyCentral Dogma of Molecular Biology

RibosomesRibosomes RibosomesRibosomes: :

Factory for protein synthesisFactory for protein synthesis; thay are composed of ; thay are composed of ribosomal RNA and ribosomal proteins (known as a ribosomal RNA and ribosomal proteins (known as a Ribonucleoprotein or RNP). Ribonucleoprotein or RNP).

They translate messenger RNA (mRNA) They translate messenger RNA (mRNA) to build to build polypeptide chainspolypeptide chains using amino acids delivered by using amino acids delivered by transfer RNA (tRNA).transfer RNA (tRNA).

Thousands of ribosomes per cellThousands of ribosomes per cell

Combination of rRNA and proteinCombination of rRNA and protein

RibosomesRibosomes Eukaryotic ribosomesEukaryotic ribosomes are larger than prokaryotic are larger than prokaryotic

ribosomes. They consist of two subunits: ribosomes. They consist of two subunits:

60S subunit 60S subunit holds (three rRNAs 5S, 5.8S, 28S and about holds (three rRNAs 5S, 5.8S, 28S and about 40 proteins).40 proteins).

40S subunit 40S subunit contains (an 18S rRNA and about 30 contains (an 18S rRNA and about 30 proteins), which come together to form an 80S particle proteins), which come together to form an 80S particle compared with prokaryotic 70S ribosome.compared with prokaryotic 70S ribosome.

Most mRNA are translated by more than one Most mRNA are translated by more than one ribosome at a time; the result, ribosome at a time; the result, a structure in which a structure in which many ribosomes translate an mRNA in tandemmany ribosomes translate an mRNA in tandem, is , is called a called a polysomespolysomes..

Four levels of protein structureFour levels of protein structurePolysomes

Ribosomes

Translation: The Process of Translation: The Process of Protein SynthesisProtein Synthesis

Translation is broken down into three main steps: Translation is broken down into three main steps:

1) 1) InitiationInitiation: two ribosomal subunits assemble with mRNA: two ribosomal subunits assemble with mRNA• Begins at an AUG start codon.Begins at an AUG start codon.

2) 2) ElongationElongation: amino acids are brought to the ribosome and : amino acids are brought to the ribosome and are added to the growing polypeptide.are added to the growing polypeptide.• Occurs in the A and P sites of ribosome.Occurs in the A and P sites of ribosome.• TranslocationTranslocation: movement of the tRNA holding the : movement of the tRNA holding the

polypeptide from the A to the P site.polypeptide from the A to the P site.• PolysomesPolysomes: a complex formed by ribosomes : a complex formed by ribosomes

simultaneously translating mRNAsimultaneously translating mRNA

Translation: The Process of Translation: The Process of Protein SynthesisProtein Synthesis

3) 3) TerminationTermination: occurs when ribosome reaches a stop codon.: occurs when ribosome reaches a stop codon.

• Release factors (RF)Release factors (RF): : recognize stop codon and cleave recognize stop codon and cleave

polypeptide from tRNA.polypeptide from tRNA.

• Ribosome subunits then dissociate.Ribosome subunits then dissociate.

• Subunits free to form new initiation complex and repeat Subunits free to form new initiation complex and repeat

process.process.

Translation: InitiationTranslation: Initiation

Translation: ElongationTranslation: Elongation

Translation: TerminationTranslation: Termination

MutationMutation MutationMutation includeinclude both gross alteration of chromosome and both gross alteration of chromosome and

more subtle more subtle alteration to specific gene sequencealteration to specific gene sequence. .

Gross chromosomal aberrations include: Gross chromosomal aberrations include: large deletions large deletions additionaddition translocation (reciprocal and nonreciprocal).translocation (reciprocal and nonreciprocal).

MutationMutation in a gene's DNA sequence in a gene's DNA sequence can alter the amino acid can alter the amino acid sequence of the protein encoded by the genesequence of the protein encoded by the gene. . Point mutations Point mutations are the result of the substitution of a are the result of the substitution of a single single

base. base. Frame-shift mutations Frame-shift mutations occur when the reading frame of the occur when the reading frame of the

gene is shifted by addition or deletion of one or more bases.gene is shifted by addition or deletion of one or more bases.

MutationMutation

MutationsMutations can have harmful, beneficial, neutral, or uncertain can have harmful, beneficial, neutral, or uncertain effects on health.effects on health.

MutationsMutations may be inherited asmay be inherited as autosomal dominantautosomal dominant, , autosomal autosomal recessiverecessive, , X-linked traitsX-linked traits. .

MutationsMutations that cause serious disability early in life are usually that cause serious disability early in life are usually rare because of their adverse effect on life expectancy and rare because of their adverse effect on life expectancy and reproduction.reproduction.