Upload
others
View
1
Download
0
Embed Size (px)
Citation preview
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: [email protected]: +880 17 0386 4707