y.molecular Biology-central Dogma

8/13/2019 y.molecular Biology-central Dogma

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Basics of Molecular BiologyCentral Dogma:

- DNA replication- Transcription

- Translation

Metabolic regulation:

- Genetic level- Metabolic pathway control- Cell receptor


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Central DogmaCentral Dogma: universal

- Genetic information is stored on the DNAmolecule.

- DNA replication: The genetic information canbe replicated directly to form a second identicalmolecule.- Transcription: Segments information on the

DNA molecule can be transcribed to yield m-, t-, r-RNAs.

- Translation: Using RNAs, this information istranslated into proteins.


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Cell Division Cycle (Mitosis)

DNA Replication in


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http://www.ncc.gmu.edu/dna/mRNAanim.htm

Transcription


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m-RNARNA polymerase

Promoter Operator Gene 1 Gene 2 Gene 3

Transcription

DNA template


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Transcription: sending the messageThe primary product of transcription: m-RNA, t-RNA and r-RNA.

- RNA polymerases consist of :sigma factor: a protein locate the beginning for the message.core enzyme: it contains the active sites.

Read 3 to 5 direction of DNA template RNA is synthesized in the direction of 5 to 3.

- Both strands of DNA could be transcribed.

- The base-sequence of RNA is the precise complement of the

DNA template sequence.DNA template RNA product

A UT AG CC G


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Transcription

Transcription requires:

- RNA polymerases

- Growth of RNA polymers is energy requiring.

- Activated ribonucleotide triphosphate: ATP, GTP, UTP, CTP.


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Transcription Process

- Initiation:The sigma factor recognizes a specific sequence ofnucleotides on a DNA strand promoter region.the strands unwound.

- Elongation:Transcription starts with the core enzyme then the sigmafactor is released.

- Termination:

RNA polymerase encounter a stop signal or transcriptionterminator (e.g. rho protein in procaryotes).

- the RNA polymerase dissociate from the DNA template- the RNA transcript is released.


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Translation


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TranslationTranslation is the final step on the way from DNA

to protein.- It is the synthesis of proteins directed by a

mRNA template.- The information contained in the nucleotide

sequence of the mRNA is read as three letterwords (triplets), called codons.

- Each word stands for one amino acid.- During translation amino acids are linked

together to form a polypeptide chain which willlater be folded into a protein.


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Translation: Message to Product

Universal three-letter codons on mRNA: A, G, C, U

- 64 codes for 20 standard amino acids

- more than one codon can specify a particularamino acids.- Nonsense codons: UAA, UAG and UGA

- Do not encode normally for amino acids.

- Act as stop points in translation.

- encoded at the end of each gene.


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Translation

Important Components:Ribosome:

The ribosome is the cellular factory responsible for the proteinsynthesis.

It consists of two different subunits, one small and one largeand is built up from rRNA and proteins.sedimentation coefficients rough mass determination:30S (small subunits) and 50S (big subunits) for procaryotes40S (small subunits) and 60S (big subunits) for eucaryotes

Inside the ribosome the amino acids are linked together into achain through multiple biochemical reactions.


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Translation-Componentst-RNA:The charged t-RNA (aminoacyl-t-RNA) carries an amino

acid at one end and has a triplet of nucleotides, ananticodon, at the other end.

It is formed by the energy from two phosphate bonds andenzymes (aminoacyl-t-RNA synthetases)

The anticodon of a t-RNA molecule can basepair, i.e form

chemical bonds, with the m-RNA's three letter codon.The t-RNA acts as the translator between m-RNA and

protein by bringing the specific amino acid coded for bythe m-RNA codon.


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Translation-Components

m-RNA template

The translation process also involves a

large number of protein factors thatfacilitate binding of mRNA and tRNA to theribosome.

Protein synthesis consumes a large part ofthe energy produced in the cell.


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Translation Processes

Translation consists of Initiation, Elongation andTermination.

Initiation results in the formation of an initiation complex in

which the ribosome is bound to the specific initiation (start)site on the mRNA while the initiator tRNA charged with (N-formyl)methionine is annealed to the initiator codon andbound to the ribosome.

- Protein synthesis begins with a AUG codon (less frequentlyGUG) on the m-RNA

AUG encodes for a modified methionine, N-formylmethionine(fMet).

In the middle of protein, AUG encodes for methionine.

http://www.phschool.com/science/biology_place/biocoach/translation/init.html


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Elongation joins amino acids to the growingpolypeptide chain according to the sequencespecified by the message.

- The formation of the peptide bond between the twoamino acids occurs on adjacent sites on theribosome: the P or peptidyl site and the A oraminoacyl site.

- The growing protein occupies the P site, while thenext amino acid to be added occupies the A site.

- As the peptide bond is formed, the t-RNAassociated with the P site is released.

http://www.phschool.com/science/biology_place/biocoach/translation/elong1.html


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Termination

- At a stop codon, a release factor reads the triplet,

and polypeptide synthesis ends.- the polypeptide is released from the tRNA.

- the tRNA is released from the ribosome, the tworibosomal subunits separate from the mRNA.

http://www.phschool.com/science/biology_place/biocoach/translation/term.html


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Posttranslation Posttranslational modification means the

chemical modification of a protein after itstranslation. It is one of the later steps in proteinbiosynthesis.

It may involve the folding of a proper structure,the formation of disulfide bridges and attachmentof any of a number of biochemical functionalgroups, such as phosphate, acetate, variouslipids and carbohydrates.

e.g. phosphorylation catalyzed by enzymes suchas with kinase.(e.g. phosphorylated TP53 protein functions astumor suppressor).


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Summary of Central Dogma

DNA replication DNA-RNA transcription

RNA-Protein translation Posttranslation

components, processes, features

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