RNAYUSRON SUGIARTO, STP, MP, MSc

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Central Dogma of Molecular Biology

• The flow of information in the cell starts at DNA, which replicates to form more DNA. Information is then ‘transcribed” into RNA, and then it is “translated” into protein. The proteins do most of the work in the cell.

Structure of RNA

A. Chain of nucleotides like DNAB. Parts of an RNA nucleotide

1. 5 carbon sugar: ribose2. phosphate group3. nitrogen base

2’-deoxyribose sugars

Phosphodiester linkages

Directional chain (5’ to 3’)

4 Bases

purines: adenine & guanine

pyrimidines: cytosine & thymine

DNA is a polymer of2’-deoxyribonucleotides

5’ end

3’ end

C

G

T

A

HO-CH2

O

H2N-C

C

C

HN

N

N

CH

C

O

N

O

O

O P O CH2

O

O

C

N

N

CHC

CH

NH2

NH2

C

CN

N

N

CH

C

NHC

O

O

O P O CH2

O

O-PO32

O

O

O P O CH2

O

N

CC

O

HN

CHCO

CH3

1’

2’3’

4’

5’

3’

RNA is a polymer of ribonucleotides

ribose sugars

Phosphodiester linkages

Directional chain (5’ to 3’)

4 Bases

purines: adenine & guanine

pyrimidines: cytosine & uracil

C

G

U

A

5’ end

3’ end

1’

2’3’

4’

5’

3’

OH

HO-CH2

O

H2N-C

C

C

HN

N

N

CH

C

O

N

O

O

O P O CH2

O

O

C

N

N

CHC

CH

NH2

OH

O

O

O P O CH2

O

N

CHC

O

HN

CHCO

OH

NH2

C

CN

N

N

CH

C

NHC

O

O

O P O CH2

O

O-PO32

OH

C. The RNA bases1. Purines

a. Adenine (A)b. Guanine (G)

2. Pyrimidinesa. Cytosine (C)b. Uracil (U)

D. Shape of the molecule: several shapes, but all are single stranded

E. Differences between DNA and RNA1. Different sugars: DNA=deoxyribose, RNA=ribose2. Different base: DNA=thymine,

RNA=uracil3. Different shape: DNA=double

helix, RNA=single strand

F. Types of RNA1. Messenger RNA (mRNA): long, single stranded molecule that

carries DNA message to the ribosomes

2. Transfer RNA (tRNA): small clover-leaf shaped molecules that pick up amino acids and take them to the ribosomes

3. Ribosomal RNA (rRNA): makes up the structure of ribosomes along with proteins

rRNA • rRNA—ribosomal RNA– Two subunits

• Ribosome “reads” mRNA and produces a polypepide

3 Types of RNA

• 1.mRNA– Messenger RNA

• Single strand• Serves as a

template (pattern for translation)

3 Types of RNA

• 2. tRNA– Transfer RNA

• 20+ types of tRNA

• Cloverleaf shape• Each tRNA is

specific for an amino acid

3 Types of RNA

• 3. rRNA– Ribosomal RNA

• Globular• 2 parts compose

the ribosome• Where are they

made?

F. The roles of RNA

1. RNA is not the genetic material and does not need to be capable of serving as a template for its own replication.

2. RNA functions as the intermediate, the mRNA, between the gene and the protein-synthesizing machinery.

3. RNA functions as an adaptor, the tRNA, between the codons in the mRNA and amino acids.

F. The roles of RNA

4. RNA also play a structural role, as in the case of the RNA components of the ribosome.

5. RNA is as a regulatory molecule, which through sequence complementarity binds to, and interferes with the translation of, certain mRNAs.

6. Some RNAs are enzymes that catalyze essential reactions in the cell.

The phosphate groups of DNA and RNA are

negatively charged

A phosphodiester group has a pKa of about 1, and so will always be ionized and negatively charged under physiological conditions (pH ~7).

Nucleic acids require counterions such as Mg2+, polyamines, histones or other proteins to balance this charge.

5’

3’

HO-CH2

ON

O

O P O CH2

OO

N

O

O P O CH2

OO

N

O

O P O CH2

OO

O-PO32

N

+M

+M

+M

+M

TranscriptionA. Transcription: the process in

which DNA makes a complementary copy of mRNA

B. Steps of transcription1. DNA untwists and bases separate2. Only a small section of the DNA is involved and only one strand acts as the template

3. RNA polymerase adds RNA nucleotides in the correct order as indicated by the DNA molecule

4. Base pairing rules applya. If DNA has a T, RNA

will match by adding Ab. If DNA has an A, RNA

will match by adding Uc. If DNA has a C, RNA

will match by adding Gd. If DNA has a G, RNA

will match by adding C

5. Fill in correct mRNA sequenceDNA: A T G C C T A G ARNA: U A C G G A U C U

Most RNA molecules consist of a single strand that folds back on itself to form double-helical regions

In RNA, G pairs with C and A pairs with U.

The loops and hairpins have few or no base-pairs

single strands

bulge

internal loop

hairpin

A-form double helix

A

A C

A

GA

CG

G

GCC

UCCU

AGGACGU

GCA

A AU

AA

GGAUGG

CUACC

GGAAC

AUGCU

AGCACCUUGA

G GC

A

T

AA

Transcription

• Enzyme: RNA polymerase (3 kinds in eukaryotes)

• “unzips” DNA and adds RNA nucleotides in the 5’ 3” direction

Transcription• Promotor

– Site where the polymerase attaches

• Termination site– Site where

transcription ends• Transcription Unit

– The stretch of DNA transcribed

Transcription• In eukaryotes, the

mRNA is modified after transcription

• A 5’ cap is added (guanine nuicleotide)

• Poly A tail (adenine)

• 50-250 nucleotides long

Transcription Graphics

RNA ProcessingA. There are large sections

of RNA molecules that are not used in making protein. These must be cut out before the RNA leaves the nucleus

B. Sections that are cut out (not used to make the protein) are called introns

C. Sections that are used to make the protein are called exons (they are expressed)

D. Some parts of RNA molecules may be exons when one protein is made and introns when another protein is made

The Genetic CodeA. Tells the cell how to assemble a

proteinB. Proteins determine the structure

and function of organismsC. Proteins are made of amino

acidsD. The bases in mRNA (as made

from DNA) determine what amino acids will be assembled into a protein

E. 20 amino acids can be assembled into thousands of proteins

F. This works similar to the way letters are assembled to make words- 26 letters in English alphabet make thousands of words

G. Codon: a sequence of 3 bases in mRNA that codes for 1 amino acid1. Examples

GUG = valineGUA = valineGUC = valine

GAC = aspartic acidGAU = aspartic acidUCU = serineUCC = serineUCG = serine

Which base could vary and still stand for the same amino acid?

- 3rd base

2. The genetic code is redundant: more than 1 codon can stand for 1 amino acid3. The genetic code is NOT ambiguous: 1 codon cannot stand for more than 1 amino acid

4. Special codonsAUG = methionine = “start” = the first codon of every proteinUAA = stopUAG = stopUGA = stop – these end a

protein

Fill in the chart…

mRNA codon Amino acid

AAG Lysine

CGU Argenine

GGG Glycine

Translation

A. Translation: the process in which the mRNA message is decoded and a protein is made.

B. Steps in translation1. mRNA made in the nucleus leaves and travels to a ribosome2. mRNA attaches to a ribosome

3. The ribosome reads the first codon, which is always AUG4. A tRNA that has a sequence of three complementary bases to mRNA brings in the appropriate amino acid. The complementary bases on tRNA are called an anticodon.

5. The ribosome reads the second codon and a tRNA with a matching anticodon brings in a second amino acid6. The ribosome joins the two amino acids with a linkage that is called a peptide bond7. The ribosome moves down and reads the next codon

8. tRNA molecules keep bringing in the appropriate amino acids9. The process continues until a “stop” codon is reached10. The polypeptide leaves the

ribosome and folds to become a protein

Translation• RNA protein• Structure of a

ribosome– Protein and

rRNA– Most common

form of RNA– Ribosomes are

formed in the nucleolus

Translation

• Three stages of translation1. Initiation2. Elongation3. Termination

Initiation

• Small ribosomal subunit binds to both the mRNA and the tRNA

• Large ribosomal subunit attaches

Elongation • Codon recognition--mRNA and tRNA form hydrogen bonds at the “A” site of the ribosome

• Peptide bond forms between amino acid at the “A” site and the growing polypeptide at the “P” site

• Translocation– Ribosome moves the

tRNA with polypeptide from the “A” to the “P”

Exit site

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Termination• Translation

continues until “stop” codon on mRNA—UAA, UAG, or UGA

• Polyribosomes• Multiple ribosomes

translating the same rRNA (polysomes)

Genetic Code Tablecodons

• Universal for almost all organisms– P. 308 in text– Use it to

decode the base sequence on the next slide

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Transcription and Translation in Cells

Prokaryotic Cell

Eukaryotic Cell

THANK YOUYUSRON SUGIARTO, STP, MP, MSc

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