Replication, transcription, translation and expression of

nucleic acid

Central dogma of molecular biologyCentral dogma of molecular biology

translationtranscription

replication

DNA RNA PROTEIN

Solid arrow indicate types of information transfers that occur in cells. DNA directs its own replication to produce new DNA molecule; DNA is transcribes into RNA; RNA is translated into protein. The dashed lines represent information transfers that occur in certain organisms.

Describe the flow of genetic information from DNA through RNA and eventually to protein

Information FlowInformation FlowDNA DNA RNARNA ProteinProtein

Replication: DNA duplicates itselfTranscription: RNA is made on a DNA templateTranslation: Protein is synthesizedfrom AAs and the three RNAs.Reverse Transcription: RNA directs synthesis of DNARNA replication: RNA replicates itself

DNA replication DNA replication

DNA replication is an anabolic polymerization DNA replication is an anabolic polymerization process, that process, that allows a cell to pass copies of its allows a cell to pass copies of its genome to its descendantsgenome to its descendants..

Must occur before every cell divisionMust occur before every cell division After two strands of DNA separate, each serves as After two strands of DNA separate, each serves as

template for the synthesis of a complementary strand.template for the synthesis of a complementary strand. Biologists say that DNA replication is Biologists say that DNA replication is

semiconservative replication semiconservative replication because each daughter because each daughter DNA molecule is composed of one original strand DNA molecule is composed of one original strand and one new strand.and one new strand.

PRINCIPAL OF DNA REPLICATION

c) Synthesis of lagging strand

DNA REPLICATION PROCESS

Initial Processes in DNA ReplicationInitial Processes in DNA Replication

DNA replication begins at a specific sequence of DNA replication begins at a specific sequence of nucleotides called an nucleotides called an originorigin. .

First, a cell removes chromosomal proteins, exposing First, a cell removes chromosomal proteins, exposing the DNA helix. the DNA helix.

Next, an enzyme called Next, an enzyme called DNA helicaseDNA helicase locally locally ""unzips/unwindunzips/unwind" the DNA molecule by breaking the " the DNA molecule by breaking the hydrogen bonds between complementary nucleotide hydrogen bonds between complementary nucleotide bases, which exposes the bases in a bases, which exposes the bases in a replication forkreplication fork. . Other protein molecules stabilize the single strands so Other protein molecules stabilize the single strands so that they do not rejoin while replication proceedsthat they do not rejoin while replication proceeds

After helicase untwists and separates the strands, a molecule of an After helicase untwists and separates the strands, a molecule of an enzyme called enzyme called DNA polymerase IIIDNA polymerase III binds to each strand. binds to each strand.

DNA polymerases IIIDNA polymerases III replicate DNA in only one direction - 5' to replicate DNA in only one direction - 5' to 3' - like a jeweler stringing pearls to make a necklace, adding 3' - like a jeweler stringing pearls to make a necklace, adding them one at a time, always moving from one end of the string to them one at a time, always moving from one end of the string to the other. the other.

Because the two original (template) strands are antiparallel cells Because the two original (template) strands are antiparallel cells synthesize new strands in synthesize new strands in two different ways:two different ways:

1) One new strand, called the 1) One new strand, called the leading strandleading strand, is synthesized , is synthesized continuously as a single long chain of nucleotides. continuously as a single long chain of nucleotides. 2)The other new strand, called the 2)The other new strand, called the lagging strandlagging strand, is synthesized , is synthesized in short segments that are later joined. in short segments that are later joined.

Synthesis of the Leading StrandSynthesis of the Leading Strand A cell synthesizes a leading strand toward the replication fork A cell synthesizes a leading strand toward the replication fork in the following series of five steps in the following series of five steps

1) An enzyme called 1) An enzyme called primaseprimase synthesizes a synthesizes a short RNA moleculeshort RNA molecule that is complementary to the template DNA strand. This that is complementary to the template DNA strand. This RNA RNA primer primer provides the 3' hydroxyl group required by DNA provides the 3' hydroxyl group required by DNA polymerase.polymerase.

2) 2) Triphosphate deoxyribonucleotidesTriphosphate deoxyribonucleotides form hydrogen bonds with form hydrogen bonds with their complements in the parental strand. Adenine nucleotides their complements in the parental strand. Adenine nucleotides bind to thymine nucleotides, and guanine nucleotides bind to bind to thymine nucleotides, and guanine nucleotides bind to cytosine nucleotides. cytosine nucleotides.

3) Using the energy in the high-energy bonds of the triphosphate 3) Using the energy in the high-energy bonds of the triphosphate deoxyribonucleotides, DNA polymerase III covalently joins deoxyribonucleotides, DNA polymerase III covalently joins them one at a time by dehydration synthesis to the leading them one at a time by dehydration synthesis to the leading strand. strand.

4) 4) DNA polymerase III also performs a proofreading function. About DNA polymerase III also performs a proofreading function. About 1 out of every 100,000 nucleotides is mismatched with its 1 out of every 100,000 nucleotides is mismatched with its template; for instance, a guanine might become incorrectly paired template; for instance, a guanine might become incorrectly paired with a thymine. with a thymine.

DNA polymerase III recognizes most such errors and removes the DNA polymerase III recognizes most such errors and removes the incorrect nucleotides before proceeding with synthesis. This role, incorrect nucleotides before proceeding with synthesis. This role, known as the known as the proofreading exonucleaseproofreading exonuclease function, acts like the function, acts like the delete key on a keyboard, removing the most recent error. delete key on a keyboard, removing the most recent error.

Because of this proofreading exonuclease function, only about Because of this proofreading exonuclease function, only about one error remains for every ten billion (10one error remains for every ten billion (101010) base pairs replicated. ) base pairs replicated.

5) 5) Another DNA polymerase - Another DNA polymerase - DNA polymerase IDNA polymerase I - replaces the - replaces the RNA primer with DNA. Note that researchers named DNA RNA primer with DNA. Note that researchers named DNA polymerase enzymes in the order of their discovery, not the order polymerase enzymes in the order of their discovery, not the order of their actions. of their actions.

Synthesis of the Lagging StrandSynthesis of the Lagging Strand

The steps in the synthesis of a lagging strand are as The steps in the synthesis of a lagging strand are as follows : follows :

As with the leading strand, primase synthesizes RNA As with the leading strand, primase synthesizes RNA primers. primers.

Nucleotides pair up with their complements in the Nucleotides pair up with their complements in the template-adenine with thyamine, and cytosine with template-adenine with thyamine, and cytosine with guanine. guanine.

DNA polymerase III joins neighboring nucleotides and DNA polymerase III joins neighboring nucleotides and proofreads. In contrast to synthesis of the leading strand, proofreads. In contrast to synthesis of the leading strand, however, the lagging strand is synthesized in discontinuous however, the lagging strand is synthesized in discontinuous segments called segments called Okazaki fragmentsOkazaki fragments. Each Okazaki fragment . Each Okazaki fragment requires a new RNA primer and consists of 1000 to 2000 requires a new RNA primer and consists of 1000 to 2000 nucleotides. nucleotides.

DNA polymerase I replaces the RNA primers of Okazaki DNA polymerase I replaces the RNA primers of Okazaki fragments with DNA and further proofreads the daughter fragments with DNA and further proofreads the daughter strand. strand.

DNA ligase DNA ligase seals the gaps between adjacent Okazaki seals the gaps between adjacent Okazaki fragments to form a continuous DNA strand. fragments to form a continuous DNA strand.

TranscriptionTranscription

TRANSCRIPTION is the synthesis of RNA TRANSCRIPTION is the synthesis of RNA under the direction of DNAunder the direction of DNA

DNA strand provide a template for assembling DNA strand provide a template for assembling a sequence of RNA nucleotidesa sequence of RNA nucleotides

The resulting RNA molecule is the transcript The resulting RNA molecule is the transcript of the gene’s protein-building instructionof the gene’s protein-building instruction

Called mRNA (messenger RNA) – carry Called mRNA (messenger RNA) – carry genetic message from DNAgenetic message from DNA

TRANSCRIPTIONTRANSCRIPTION Cells transcribe four main types of RNA from DNA :Cells transcribe four main types of RNA from DNA :

RNA primerRNA primer molecules for DNA polymerase to use during DNA molecules for DNA polymerase to use during DNA replication replication

messenger RNAmessenger RNA (mRNA) molecules, which carry genetic (mRNA) molecules, which carry genetic information from chromosomes to ribosomes information from chromosomes to ribosomes

ribosomal RNAribosomal RNA (rRNA) molecules, which combine with (rRNA) molecules, which combine with ribosomal polypeptides to form ribosomes-the organelles that ribosomal polypeptides to form ribosomes-the organelles that synthesize polypeptides synthesize polypeptides

transfer RNAtransfer RNA (tRNA) molecules, which deliver amino acids to (tRNA) molecules, which deliver amino acids to the ribosomes the ribosomes

The stages of transcriptionThe stages of transcription

1) Initiation1) Initiation

2) Chain elongation2) Chain elongation

3) termination3) termination

Initiation of TranscriptionInitiation of Transcription

RNA polymerasesRNA polymerases - the enzymes that synthesize RNA - the enzymes that synthesize RNA RNA polymerase bind to specific nucleotide sequences RNA polymerase bind to specific nucleotide sequences

called called promoter - promoter - include the transcription startpoint (the include the transcription startpoint (the nucleotides where RNA synthesis begin)nucleotides where RNA synthesis begin)

Initiation of TranscriptionInitiation of Transcription

Once it bind to the promoter sequence, RNA Once it bind to the promoter sequence, RNA polymerase unwinds and unzips the DNA molecule in polymerase unwinds and unzips the DNA molecule in the promoter regionthe promoter region

After unzip, RNA polymerase initiate RNA synthesis After unzip, RNA polymerase initiate RNA synthesis at the promoter on the template strandat the promoter on the template strand

Elongation of the RNA TranscriptElongation of the RNA Transcript

As RNA polymerase moves along the DNA, it continues to As RNA polymerase moves along the DNA, it continues to untwist the double helix for pairing with RNA nucleotidesuntwist the double helix for pairing with RNA nucleotides

The enzyme add nucleotides to the 3’ end of the growing RNA The enzyme add nucleotides to the 3’ end of the growing RNA molecule as it continues along the double helixmolecule as it continues along the double helix

Elongation of the RNA TranscriptElongation of the RNA Transcript In the wake of transcription, the DNA strands In the wake of transcription, the DNA strands

re-form the double helix and the new RNA re-form the double helix and the new RNA molecule peels away from its DNA templatemolecule peels away from its DNA template

Termination of TranscriptionTermination of Transcription

Transcription proceeds until shortly after the RNA Transcription proceeds until shortly after the RNA polymerase transcribes a DNA sequence called a polymerase transcribes a DNA sequence called a terminatorterminator

Terminator = sequence of nucleotides along the Terminator = sequence of nucleotides along the DNA, that signal the end of transcription unitDNA, that signal the end of transcription unit

After the RNA is released, the polymerase After the RNA is released, the polymerase dissociate from the DNAdissociate from the DNA

TRANSLATION Translation is the process whereby

ribosomes use the genetic information of nucleotide sequences to synthesize polypeptides composed of specific amino acid sequences.

In translation process, cell interprets a genetic message and builds a protein

Message = is a series of codons along an mRNA molecule

Interpreter = transfer RNA (tRNA) tRNA = transfer amino acids from

cytoplasm’s amino acid pool to ribosome The ribosome adds each amino acid

brought to it by tRNA to the growing end of a polypeptide chain

As a tRNA molecule arrives at a ribosome, it bears a specific amino acid at one end.

At the other end is a nucleotide triplet called an anticodon, which binds according to base-pairing rules to a complementary codon on mRNA.

How do ribosomes interpret the nucleotide sequence of mRNA to determine the correct order in which to assemble amino acids?

The genetic code Is a coding dictionary that specifies a

meaning for a base sequence the genetic code define as triplets of mRNA

nucleotides called codons that code for specific amino acids.

64 possible arrangements - more than enough to specify 21 amino acids.

61 codons specify amino acids and 3 codons

-UAA, UAG, and UGA-to stop translating UGA codes for the 21st amino acid,

selenocysteine. Codon AUG also has a dual function, acting as

both a start signal and coding for an amino acid – methionine.

AUG = start codon

Mutations of Genes:Mutations of Genes: Types of mutation Types of mutation

Mutations range from large changes in an Mutations range from large changes in an organism's genome, such as the loss or gain of an organism's genome, such as the loss or gain of an entire chromosome, to the most common type of entire chromosome, to the most common type of mutation - mutation - point mutationspoint mutations - in which just one - in which just one nucleotide base pair is affected. nucleotide base pair is affected.

Mutations include Mutations include base pair insertionsbase pair insertions, , deletionsdeletions, and , and substitutionssubstitutions. .

Effects of MutationsEffects of Mutations Some base-pair substitutions Some base-pair substitutions

produce produce silent mutationssilent mutations because the substitution does because the substitution does not change the amino acid not change the amino acid sequence because of the sequence because of the redundancy of the genetic redundancy of the genetic code.code.

For example, when the DNA For example, when the DNA triplet AAA " is changed to triplet AAA " is changed to AAG, the mRNA codon will AAG, the mRNA codon will be changed from UUU to be changed from UUU to UUC; however, because both UUC; however, because both codons specify the amino acid codons specify the amino acid phenylalanine, there is no phenylalanine, there is no change in the phenotype - the change in the phenotype - the mutation is silent because it mutation is silent because it affects the genotype only. affects the genotype only.

Of greater concern are substitutions that Of greater concern are substitutions that change a codon for one amino acid into a change a codon for one amino acid into a codon for a different amino acid. codon for a different amino acid.

A change in a nucleotide sequence A change in a nucleotide sequence resulting in a codon that specifies a resulting in a codon that specifies a different amino acid is called a different amino acid is called a missense missense mutationmutation; what gets transcribed and ; what gets transcribed and translated makes sense, but not the right translated makes sense, but not the right sense. sense.

The effect of missense mutations depends The effect of missense mutations depends on where in the protein the different on where in the protein the different amino acid occurs. amino acid occurs.

When the different amino is in a critical When the different amino is in a critical region of a protein, the protein becomes region of a protein, the protein becomes nonfunctional; however, when the nonfunctional; however, when the different amino acid is in a less important different amino acid is in a less important region, the mutation has no adverse effect. region, the mutation has no adverse effect.

A third type of mutation A third type of mutation occurs when a base-pair occurs when a base-pair substitution changes an substitution changes an amino acid codon into a amino acid codon into a stop codon. stop codon.

This is called a This is called a nonsense nonsense mutationmutation. Nearly all . Nearly all nonsense mutations result nonsense mutations result in nonfunctional proteins. in nonfunctional proteins.

Frameshift mutationsFrameshift mutations (that is, insertions or (that is, insertions or deletions) typically deletions) typically result in drastic result in drastic missense and nonsense missense and nonsense mutations, except when mutations, except when the insertion or deletion the insertion or deletion is very close to the end is very close to the end of a gene of a gene