RNA, Ribosomes and Protein Synthesis

Learning Objectives

Contrast RNA and DNA. Explain the process of transcription. Identify the genetic code and explain how it is read. Summarize the process of translation. Describe the central dogma of molecular biology.

Comparing RNA and DNA

The sugar in RNA is ribose instead of deoxyribose. RNA is generally single stranded, not double stranded.RNA contains uracil in place of thymine.

The Role of RNA

The roles played by DNA and RNA molecules in directing protein production are like the two types of plans builders use: A master plan A blueprint

Types of RNA

The three main types of RNA are:

Messenger RNA

Ribosomal RNATransfer RNA

Messenger RNA An mRNA molecule is a copy of the portion of DNA that will be

used to make a protein. After being made in the nucleus, mRNA travels to the

cytoplasm, the site of protein synthesis.

Ribosomal RNA Protein synthesis occurs on ribosomes, which are made up of

two subunits. Both subunits consist of several molecules of ribosomal RNA

(rRNA).

Transfer RNA

During protein synthesis, transfer RNA molecules (tRNA) carry amino acids from the cytoplasm to the mRNA.

RNA Synthesis: TranscriptionIn transcription, segments of DNA serve as templates to produce complementary mRNA molecules.

RNA Synthesis: Promoters

RNA polymerase binds only to regions of DNA that have specific base sequences. These regions are called promoters.

C

A

G

U

TA

RNA Synthesis: RNA Editing

New RNA molecules sometimes require a bit of editing before they are ready to be read.

IntronsExons

Cap Tail

The Genetic Code

RNA has four bases: adenine, cytosine, guanine, and uracil. These bases form a “language”: A, C, G, and U.

The Genetic Code: Codons

The genetic code is read in three-letter groupings called codons.

A codon is a group of three nucleotide bases in messenger RNA that specifies a particular amino acid.

AUG AAC UCU

Genetic Code Table

There are 64 possible three-base codons in the genetic code.

Reading CodonsStart at the middle of the circle with the first letter of the codon and move outward.

CAC = Histidine

Start and Stop Codons

The methionine codon AUG serves as the “start” codon for protein synthesis. There are three “stop” codons.

AUG = methionine = “start” codon

UAA, UAG, and UGAare “stop” codons

Translation

Transcribed mRNA directs the translation process. Translation is the process that makes proteins using the

copy of the DNA code on the mRNA.

Translation: Transfer RNA

To start translation, tRNA molecules bind to mRNA codons, carrying amino acids with them.

anticodon

Translation: The Polypeptide AssemblyThe ribosome helps form a peptide bond. It breaks the bond holding the first tRNA molecule to its amino acid.

Translation: Completing the PolypeptideThe ribosome reaches a stop codon, releasing the newly synthesized polypeptide and the mRNA molecule, completing the process of translation.

Roles of RNA in Translation

All three major forms of RNA—mRNA, tRNA, and rRNA—are involved in the process of translation.

The Molecular Basis of HeredityThe central dogma of molecular biology is that information is transferred from DNA to RNA to protein.

Gene ExpressionWhen a gene (segment) of DNA code is used to build a protein, scientists say that gene has been expressed.