RNA

RiboNucleic AcidAlso made of monomers called nucleotides• 5 carbon sugar: Ribose • Phosphate functional group: PO4

• 1 of 4 nitrogen bases– Cytosine– Guanine– Adenine– Uracil replaces Thymine

• Molecule is a single strand

Purpose of RNAThere are many types of RNA, so function varies depending upon type:• Directly involved in converting genetic code on

DNA into proteins • Determines which genes are expressed • Some regulate chemical processes, so, they

function like enzymes• Small sections of RNA act as place holders for

DNA polymerase during DNA replication

3 TYPES OF RNA USED IN PROTEIN SYNTHESIS

messenger RNA(mRNA)• Long, single strand• Made in the nucleus • Carries genetic code from DNA to ribosomesIn cytoplasm

transfer RNA (tRNA)

• Folded chain of RNA• Carries the amino acid monomers that make a protein• “Translates” the genetic code

ribosomal RNA (rRNA)

Ribosomes• Globular strand of RNA• 2 parts• Located on the ER andfree floating in cytoplasm• Site of protein synthesis

Why is RNA needed in the first place?DNA carries the code, however, DNA cannot leave the nucleus. RNA carries the “genetic message” to ribosomes

Why change a base? The nuclear envelope “recognizes” the bases on DNA and keeps it in the nucleus; by changing a single base on RNA, its chemical signature changes, so the nuclear envelope will allow it to pass through to cytoplasm

Why not just make proteins in the nucleus? Proteins are “huge” molecules. There’s just not enough room in the nucleus to make all the proteins an organism requires, so the protein factory (ribosomes, ER and Golgi Body) is out in the cytoplasm where there’s plenty of space.

Why are ribosomes on the ER and in the cytoplasm?Proteins are needed for many functions. The proteins made in ribosomes on the ER are transported either to the nucleus or out of the cell(hormones). Those ribosomes in the cytoplasm make proteins that stay in the cell(replace organelles)

MAKING mRNA1st step in protein synthesis

Messenger RNA is made in the nucleus in a process called: TRANSCRIPTION

Here the genetic code on DNA is “rewritten” in a strand of mRNA.

BIRTHDAY CAKE

ONCE THE RNA IS FORMED IT DETACHES FROM THE DNA TEMPLATE.

THE DNA EITHER CLOSES OR TRANSCRIPTION CONTINUES UNTIL ENOUGH mRNA IS MADE FOR PROTEIN SYNTHESIS

A

BC

Next Steps….2. The newly formed messenger RNA leaves the nucleus 3. Together with the ribosomes and tRNA a new protein is formed during

TRANSLATION

TRANSLATION

TRANSLATION

THE ORDERING OF AMINO ACIDS BY mRNA TO FORM A POLYPEPTIDE(PROTEIN)

TRANSLATION TAKES PLACE IN RIBOSOMES. (ROUGH E.R. OR CYTOPLASM)

EACH TYPE OF RNA CONTROLS THE ORDER OF AMINO ACIDS

Let’s see how the code is read…

MESSENGER RNA (mRNA) CODONSMESSENGER RNA (mRNA) CODONS

The genetic code is translated in ‘3 letter words”

mRNA – THREE NUCLEOTIDE BASES MAKE A CODON.

A CODON CODES FOR A SPECIFIC AMINO ACID.

TRANSFER RNA – (tRNA)The TranslatorTRANSFER RNA – (tRNA)The Translator

THREE NUCLEOTIDES AT THE HEAD(one end) OF THE tRNA IS THE ANTICODON

THE ANTICODON WILL MATCH UP WITH THE mRNA CODON

THE TAIL OF THE tRNA CARRIES THE AMINO ACID THAT CORRESPONDS TO THE CODON.

THE RIBOSOMETHE RIBOSOME CELL ORGANELLE

THAT IS MADE UP OF rRNA.

SITE OF PROTEIN SYNTHESIS

RIBOSOME HAS 2 ‘PARKING PLACES’ FOR tRNA

A site: tRNAs enterP site: Amino Acids bondE site: tRNA exits(not shown)

SUMMARY OF TRANSLATION SUMMARY OF TRANSLATION

1. The small ribosome subunit attaches to the mRNA at the start codon.

This START codon is always AUG!Then the large subunit of the ribosome attaches over the small subunit

#1

2.The A site of the ribosome is filled with the first tRNA carrying the first amino acid (always Methionine) The ribosome shifts

down one codon, moving the tRNA to the P site and opening the A site again

#2

3. The mRNA codon is ‘read’ and the second corresponding tRNA carrying the second amino acid fills the A site

4. At this time a peptide bond is formed between the two amino acids in the A and P sites

PEPTIDE BOND

#3

#4

5. The first tRNA in the P site releases its amino acid and is moved to the E site where it is released from the ribosome

6-7.The ribosome again shifts down 1 codon, moving the second tRNA to the P site leaving the A site empty for the third tRNA carrying the next amino acid

8. This process of building the polypeptide chain continues until the ribosome reaches the STOP codon at the end of the mRNA and detaches

#5

#6-#8

READING THE AMINO ACID WHEELThere are only 20 amino acids(that is 20 monomers) for all proteins. However, there are far more than 20 codons. For example: Find LEUCINE( be careful, its listed more than once…how many codons are there for this one amino acid? What might this imply about certain amino acids?

mRNA Codon

CAGStart with first letter in center andwork your way out foreach letter inthe codon untilyou identify theamino acid

GLUTAMINE

6

Amino acids that are used more often in proteins have more than one codon; this helps increase the rate of protein synthesis and decreases chance mutation will alter the protein.

Let’s Practice!Write the DNA sequence below, in one color; TAC CTT AAC GAG CTA AAA GTT AGC TGG TTG ACTNow, transcribe the mRNA codons from the DNA;AUG GAA UUG CUC GAU UUU CAA UCG ACC AAC UGAGood Job! Now, what would the tRNA anticodon’s be?UAC CUU AAC GAG CUA AAA GUU AGC UGG UUG ACUAwesome! Now, let’s make a polypeptide by translating the mRNA codon’s to amino acids:MET- GLU A-LEU-LEU-ASP A-PHY-GLU-SER-THR-ASP-STOP