RNA: Structure and RNA: Structure and FunctionFunction

BiologyBiology

VickVick

What is RNA?What is RNA?

RRiboibonnucleic ucleic AAcidcid It carries out the instructions It carries out the instructions

encoded in DNAencoded in DNA Remember DNA=blueprints for Remember DNA=blueprints for

life… but “someone” has to be able life… but “someone” has to be able to read those blueprints and get to read those blueprints and get that info outside of the nucleusthat info outside of the nucleus

DNA in its “pure form” cannot DNA in its “pure form” cannot leave the nucleus…leave the nucleus…

RNA StructureRNA Structure

Made of Made of nucleotides that nucleotides that have 3 have 3 components (very components (very similar to DNA):similar to DNA):– Sugar (ribose)Sugar (ribose)– Phosphate group Phosphate group

(phosphate + (phosphate + oxygen)oxygen)

– Nitrogen base (A, Nitrogen base (A, U, C, G)U, C, G)

Differences from DNA:Differences from DNA:

– Sugar is riboseSugar is ribose

– Single strandedSingle stranded

– Contains uracil Contains uracil instead of instead of thymine as one of thymine as one of the nitrogen the nitrogen basesbases

3 Types of RNA:3 Types of RNA:mRNAmRNA Messenger RNA Messenger RNA

(mRNA)(mRNA)– Carry copies of Carry copies of

instructions from instructions from DNA for the DNA for the purpose of purpose of assembling assembling proteinsproteins

– Remember where Remember where proteins are proteins are assembled in the assembled in the cell?cell?

rRNArRNA

Ribosomal RNA (rRNA)Ribosomal RNA (rRNA)– a molecular component a molecular component

of a of a ribosomeribosome, the cell's , the cell's essential essential proteinprotein factory. factory.

– Strictly speaking, Strictly speaking, ribosomal ribosomal RNARNA (rRNA) (rRNA) does not make proteins. does not make proteins. It makes polypeptides It makes polypeptides (assemblies of amino (assemblies of amino acids) that go to make acids) that go to make up up proteinsproteins. .

Ribosome Review:Ribosome Review:

Ribosomes are ancient molecular machines that Ribosomes are ancient molecular machines that are responsible for production of protein in all are responsible for production of protein in all living cells. living cells.

They are large macromolecular complexes They are large macromolecular complexes composed of RNA (2/3) and protein (1/3). composed of RNA (2/3) and protein (1/3).

Ribosomes read the nucleotide sequence of a Ribosomes read the nucleotide sequence of a messenger RNA (mRNA) into a protein sequence, messenger RNA (mRNA) into a protein sequence, using the genetic code. (What are the building using the genetic code. (What are the building blocks of proteins?)blocks of proteins?)

They use transfer RNAs (tRNAs) to mediate this They use transfer RNAs (tRNAs) to mediate this process of translation from the nucleotide process of translation from the nucleotide language of RNA and DNA into the amino acid language of RNA and DNA into the amino acid language of proteins. language of proteins.

Ribosome StructureRibosome Structure

EF-G is an elongation factor found in the EF-G is an elongation factor found in the ribosomeribosome

tRNAtRNA

Transfer RNA (tRNA)Transfer RNA (tRNA)– carries amino acids to the ribosomes carries amino acids to the ribosomes – enables the ribosomes to put this enables the ribosomes to put this

amino acid on the protein that is amino acid on the protein that is being synthesized as an elongating being synthesized as an elongating chain of amino acid residues, using chain of amino acid residues, using the information on the mRNA to the information on the mRNA to "know" which amino acid should be "know" which amino acid should be put on next. put on next.

– For each kind of amino acid, there is For each kind of amino acid, there is a specific tRNA that will recognize a specific tRNA that will recognize the amino acid and transport it to the amino acid and transport it to the protein that is being synthesized the protein that is being synthesized and tag it on to the protein once the and tag it on to the protein once the information on the mRNA calls for it.information on the mRNA calls for it.

Steps of Protein Steps of Protein SynthesisSynthesis

11stst Step Step– TranscriptionTranscription

DNA is transcribed to produce a molecule of DNA is transcribed to produce a molecule of mRNAmRNA

22ndnd Step Step– TranslationTranslation

tRNA is translated into amino acids or proteinstRNA is translated into amino acids or proteins

DNA mRNAtranscription Proteintranslation

TranscriptionTranscription

RNA Polymerase is RNA Polymerase is used in the process used in the process of transcriptionof transcription

DNA section with DNA section with desired gene is desired gene is unzipped unzipped

A complementary A complementary mRNA strand is mademRNA strand is made

DNA is rezipped and DNA is rezipped and mRNA moves into the mRNA moves into the cytoplasmcytoplasm

• VocabularyVocabulary– Codon – 3 letter sequence or Codon – 3 letter sequence or

“word” on mRNA“word” on mRNA– Anti-codon – three bases on tRNA Anti-codon – three bases on tRNA

that are complimentary to the that are complimentary to the mRNA codonmRNA codon

– Codons specify a single Codons specify a single amino acid that is to be amino acid that is to be added to the proteinadded to the protein

– Ex: UCGCACGGU=Ex: UCGCACGGU=UCG-CAC-UCG-CAC-GGUGGU

– Specific codons:Specific codons:• AUG is the start codonAUG is the start codon

– Starts protein synthesisStarts protein synthesis

• Stop codonsStop codons• 3 of them3 of them– Stop protein synthesisStop protein synthesis– Do not code for any amino acidDo not code for any amino acid

TranscriptionTranscription to to TranslationTranslation

Start CodonsStart Codons These are used, along with certain other These are used, along with certain other

initiation factors to start initiation factors to start translation/protein synthesistranslation/protein synthesis

The most common start codon is AUG The most common start codon is AUG which is read as methionine or, in which is read as methionine or, in bacteria, as formylmethionine. bacteria, as formylmethionine.

Alternative start codons (depending on Alternative start codons (depending on the organism), include "GUG" or "UUG", the organism), include "GUG" or "UUG", which normally code for valine or leucine, which normally code for valine or leucine, respectively. However, when used as a respectively. However, when used as a start codon, these alternative start start codon, these alternative start codons are translated as methionine or codons are translated as methionine or formylmethionine.formylmethionine.

What do those names mean and where What do those names mean and where do they come from?do they come from?

codons are deciphered in a decoder chart to codons are deciphered in a decoder chart to determine the correct amino acids needed determine the correct amino acids needed

in protein synthesisin protein synthesis

Stop Codons:Stop Codons: 3 codons are needed to stop the 3 codons are needed to stop the

process of translation:process of translation:– UAG is UAG is amberamber – UGA is UGA is opalopal (sometimes also called (sometimes also called umberumber) ) – and UAA is and UAA is ochreochre

("Amber" was named by discoverers Richard ("Amber" was named by discoverers Richard Epstein and Charles Steinberg after their friend Epstein and Charles Steinberg after their friend Harris Bernstein, whose last name means Harris Bernstein, whose last name means "amber" in German.) "amber" in German.)

(The other two stop codons were named "ochre" (The other two stop codons were named "ochre" and "opal" in order to keep the "color names" and "opal" in order to keep the "color names" theme.) theme.)

What are their amino acid names?What are their amino acid names?– STOPSTOP– (Remember we already stated they do not (Remember we already stated they do not

code for any amino acids?)code for any amino acids?)

Review:Review:

mRNAmRNA– Messenger RNAMessenger RNA, a copy of DNA blueprint of the , a copy of DNA blueprint of the genegene

to be expressed. (to be expressed. (Information)Information) tRNAtRNA

– Aminoacyl Aminoacyl transfer RNAtransfer RNA, also called , also called anticodonanticodon or or adaptoradaptor molecule.  One or more tRNAs for each amino molecule.  One or more tRNAs for each amino acid. (acid. (Supply)Supply)

RibosomeRibosome– A very large complex of several rRNAs (ribosomal RNA) A very large complex of several rRNAs (ribosomal RNA)

and many protein molecules. Total molecular weight and many protein molecules. Total molecular weight over 2 million dalton. (over 2 million dalton. (Factory)Factory)

ProteinProtein– Polypeptide chain with sequence dictated by the mRNA Polypeptide chain with sequence dictated by the mRNA

sequence.  Also called the sequence.  Also called the gene (gene (Product)Product)