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Biochemistry & Cell biologyPHR213_Lecture 14

RNA

RNA nucleotides have: 1. A ribose sugar2. A phosphate group3. Any of the four bases:

- Adenine (A)- Uracil (U, only in RNA)- Cytosine (C)- Guanine (D)

Adenine Uracil

Guanine Cytosine

DNA & RNA

• There are several important differences between RNA and DNA:

• the pentose sugar in RNA is ribose, in DNA it’s deoxyribose

• in RNA, uracil replaces the base thymine (U pairs with A)

• RNA is single stranded while DNA is double stranded

• RNA molecules are much smaller than DNA molecules

DNA

tRNA gene Encodes tRNA

Protein coding gene Encodes mRNA

rRNA gene Encodes rRNA

sRNA gene Encodes sRNA

DNA & RNA

RNA

RNA Primary StructureIn RNA, A, C, G, and U are linked by 3’-5’ester bonds between ribose andphosphate

RNA

RNA can fold into specific structures• By forming conventional base-pairs with complementary sequences

found elsewhere on the same molecule.• Some nonconventional base-pair interactions to fold into a three-

dimensional structure.

Transcription

• In biology, transcription is the process of copying out the DNAsequence of a gene in the similar alphabet of RNA.

• Transcription is the first step in gene expression.

Stages of transcription

1. Initiation

2. Elongation

3. Termination

• RNA polymerase builds an RNA strand in the 5' → 3' direction,adding each new nucleotide to the 3' end of the strand.

Stages of transcription

1. Initiation:• Binding of RNA polymerase to promoter - RNA polymerase

recognizes and binds to a sequence of DNA called thepromoter, found near the beginning of a gene.

• RNA polymerase separates the DNA strands, providing thesingle-stranded template (3' → 5‘) needed for transcription.

Stages of transcription

Initiation

Gene to be transcribed

Promoter region

RNA polymerase

Gene to be transcribed

Promoter region

RNA polymerase

Binding of RNA polymerase to promoter

Separation of DNA strands

3’ 5’

3’ 5’

Stages of transcription

2. Elongation:

• The RNA polymerase builds an RNA molecule out ofcomplementary nucleotides, making a chain that grows from 5' to3'.

• New RNA strand contains the base uracil (U) instead of thymine(T).

Initiation

Synthesis of new strand (RNA)

Terminator region

Gene to be transcribed

Promoter region

RNA polymerase

3’ 5’

5’ 3’

Stages of transcription

3. Termination• Sequences called terminators signal that the RNA transcript is

complete.• Once they are transcribed, they cause the transcript to be released

from the RNA polymerase.

Translation

In this stage, the mRNA is "decoded" to build a protein (or achunk/subunit of a protein) that contains a specific series of aminoacids

Translation

Translation is divided into three stages:1. Initiation

2. Elongation

3. Termination

Translation

Initiation• Initiation complex formation - the ribosome assembles around

the mRNA to be read and the first tRNA (carrying the amino acidmethionine, which matches the start codon, AUG).

• This setup, called, is needed in order for translation to get started.

APE

Translation

Elongation:• mRNA codon is read one codon at a time• The amino acid matching each codon is added to a growing

protein chain.

Each time a new codon is exposed:• A matching tRNA binds to the codon• Existing amino acid chain (polypeptide) is linked onto the new

amino acid• mRNA continue shifting one codon and exposing a new codon

for reading

Amino acid is specific to the codon / anti-codon

Translation

The ribosome has three active sites:• A site - the point of entry for the aminoacyl tRNA

• P site - where the peptidyl tRNA is formed in the ribosome

• E site - which is the exit site of tRNA after it gives its aminoacid to the growing peptide chain.

Translation

Termination

• The stage in which the finished polypeptide chain is released

• A stop codon (UAG, UAA, or UGA) enters the ribosome binds witha release factor and terminates translation

Translation

Thank you

Mohammad Ahad A Khan, PhDAssistant Professor, Department of Pharmaceutical SciencesNorth South University, Bangladesh Room: SAC 1013email: mohammad.khan03@northsouth.eduCell: +880 17 0386 4707