Central Dogma Genetic Code NEW

Genetic Information

• Central Dogma• Genetic codes

Lecture outline

• How the genetic information of living creatures is:– Written in an alphabet with just 4

letters, the four base pairs in DNA– Expressed during the growth and

development of an organism

• RNA/DNA? Plays a key role in the process of gene expression

1. To understand the central dogma theory

2. To understand the transcription and the translation process

3. To know what is the genetic code

OBJECTIVES 

Quiz1. What is central dogma?

_______________________________

2. What are the processes involve?

_________; ___________

3. What are the criteria of genetic code?

_________, ______________, __________

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No. Matrik:

Program:

The Central Dogma Theory

• 1. Transcription of DNA to RNA to protein: This dogma forms the backbone of molecular biology and is represented by four major stages.

• 2. The DNA replicates its information in a process that involves many enzymes: replication.

• 3. The DNA codes for the production of messenger RNA (mRNA) during transcription.

• 4. In eucaryotic cells, the mRNA is processed (essentially by splicing) and migrates from the nucleus to the cytoplasm.

• 5. Messenger RNA (mRNA) carries coded information to ribosomes. The ribosomes "read" this information and use it for protein synthesis. This process is called translation.

Genetic information flows;

1. From DNA to DNA during its transmission from generation to generation (genotypic function)

2. From DNA to protein during its phenotypic expression and involves 2 steps;

transcription: transfer of genetic information from DNA to RNA

translation: transfer of information from RNA to protein

-Transferring the genetic information from DNA to RNA or RNA synthesized: transcription

 central dogma:

DNA RNA Protein

Transcription Translation

- Transfer of genetic info from DNA to RNA is sometimes reversible whereas RNA to protein is always irreversible

Central dogma

4 types of RNA

tRNA (transfer RNA) -tRNA is the information adapter molecule. It is the direct interface between amino-acid sequence of a protein and the information in DNA. •mRNA -Messenger RNA is a copy of the information carried by a gene on the DNA. The role of mRNA is to move the information contained in DNA to the translation machinery.

•rRNA -Ribosomal RNA (rRNA) is a component of the ribosomes, the protein synthetic factories in the cell. Eukaryotic ribosomes contain four different rRNA molecules: 18 s, 5.8 s, 28 s, and 5 s rRNA.

•snRNA -Small nuclear RNA (snRNA) is the name used to refer to a number of small RNA molecules found in the nucleus.

Transcription Unit : area consists of promoter (the identification area of RNA Polymerase enzyme) until the termination area

RNA Pol

3’

5’

5’

3’

Promoter RNA

Initiation site Termination area

Figure 1: Transcription Unit consists of DNA sequences from promoter area until termination area

i) Initiation: - the promoter functions as a recognition site for transcription factors – enables RNA Polymerase to bind to the promoter – the DNA denatured into two strands (open complex). One of the helix strands functional as a template for RNA synthesis.

ii) Elongation: -synthesis of RNA transcript by RNA Polymerase in a 5’- 3’ direction same in DNA synthesis 

iii) Termination: a termination signal is reached that causes RNA Polymerase and the RNA transcript to dissociate from DNA.

3 stages of Transcription

In E. coli (prokaryotes) - RNA synthesized by RNA polymerase.

In mammals (eukaryotes) - 3 types of polymerases :

RNA Pol I, RNA Pol II and RNA Pol III

In E coli, RNA polymerase is a big and complex molecule. It consists of 6 subunit and called as Holoenzyme : α2, β, β’,,

-- - recognizes regions of the promoter and released from the enzyme after the RNA synthesized

--β – bind at the precursor

--β’- bind at the template DNA  

Figure 3: RNA polymerase activated by the sigma factor to recognize promoter sequence

3’5’

5’

3’

RNA Polymerase(Holoenzyme)

Faktor

DNA

Termination sequence

(i) Initiation(ii) Elongation5’

released(iii) Termination

mRNA mRNA

mRNA

Figure 2:Transcription cycle for RNA polymerase in E.coli

Enzim released

Promoter

Figure 4: RNA synthesized (mRNA)

RNA Modification

-For prokaryote - analysis bacterial genes showed that the sequence of DNA within the coding strand corresponds tothe sequence of nucleotides in the mRNA-providing the instructions for the correct amino acid sequence in polypeptide.

For eukaryote – their structural genes are note always colinear with mRNA.-Coding sequences within many eukaryotic genes are separated by DNA sequences that are not translated intoprotein. -The sequences in the RNA that correspond to the introns are removed. -The RNA derived from the exons are connected (spliced).

The coding sequences are called exonsThe sequences that interrupt them called intervening sequences or introns

Figure 5 : Excision of Intron from mRNA

 Transferring the genetic information from RNA into a polypeptide sequence (protein) or protein synthesized  DNA Transcription Translation

TRANSLATION

-Cistron : A length of DNA that contains the information for coding a specific polypeptide chain. A cistron codes for messenger RNA. It have sequence for initiation and termination

-Translation occur at ribosome in cytoplasm

3 stages of Translation

1. Initiation – involves the binding of mRNA and the initiator tRNA to the ribosomal subunits

2. Elongation – polypeptide synthesized

3. Termination – occurs when a stop codon is reached in the mRNA

1. Initiation : mRNA and the first tRNA bind to the ribosomal subunits to form a complex. In bacteria, the initiator tRNA (fmet-tRNA), carries a methionine . The start codon is AUG.

Initiation stage of protein synthesized in prokaryote

The formation of complex and showing the A and P site

2. -Elongation: a short polypeptide is attached to the tRNA located at the P site of the ribosome. A new tRNA carrying a single amino acid binds to the A site. This binding occurs because the anticodon in tRNA is complementary to the codon in the mRNA.

3. -Termination: Occurs when a stop codon is reached in the mRNA. In most species, the three stop codons (nonsense codon) are UAG, UGA & UAA

Genetic CodeProperties of the genetic code:

1. Composed of nucleotide triplets

2. Non-overlapping

3. Comma-free

4. Degenerate

5. Ordered

6. Contains start and stop codons

7. Nearly universal

There are 64 different codon - 61 of these have code a specific amino acid. - 3 codons are stop codon (nonsense codon- UAG,UGA,UAA)

INFORMATION PROCESSING  &  CENTRAL DOGMA       - the letters of the genetic alphabet... are the nucleotides A, T, G, & C of DNA     - unit of information is CODON   =   genetic 'word'     - a triplet sequence of nucleotides   CAT  in a polynucleotide     - 3 nucleotides  =  1 codon (word)  =  1 amino acid     - the definition of (codon) word = amino acid   

GENE Expression      

Central Dogma of Molecular Biology depicts flow of genetic information       Transcription - copying of DNA sequence into RNA      Translation     - copying of RNA sequence into protein   DNA sequence ------->RNA sequence -----> a. acid sequence     TAC                                   AUG                               MET

triplet sequence in DNA -->  codon in mRNA     ---->   amino acid in proteinInformation : triplet sequence in DNA is the genetic word [codon]

KEY QUESTIONS

HOW IS GENETIC INFORMATION STORED IN DNA TRANSFERRED TO RNA AND PROTEIN?

HOW DO RNA MOLECULES TRANSPORT GENETIC INFORMATION FROM DNA IN THE NUCLEUS TO THE SITES OF PROTEIN SYNTHESIS IN THE CYTOPLASMA?

WHAT ARE THE IMPORTANT STEPS IN RNA SYNTHESIS IN PROKARYOTES?

QUIZ

• Part of a DNA strand to be transcribed has the following sequence:

3’-TACTAACTTACGCTCGCCTCA-5’

a) What is the sequence of RNA transcribed from this part of the strand?

b) What sequence of amino acids does the RNA produce?