RNA & Protein Synthesis

12-3

Intro

• Genes code DNA instructions that control the production of proteins within the cell.

• The first step in decoding these genetic messages is to copy DNA into RNA

• These RNA molecules contain coded information for making proteins.

The Structure of RNA

• RNA consists of a long chain of nucleotides.

• Each nucleotide is made up of 1. 5-carbon sugar2. a phosphate group3. a nitrogenous base.

3 Differences of RNA & DNA

• There are three main differences between RNA and DNA:

1. The sugar in RNA is ribose instead of deoxyribose

2. RNA is generally single-stranded

3. RNA contains uracil in place of thymine.

RNA

• You can think of an RNA molecule as a disposable copy of a segment of DNA.

• RNA molecule is a working copy of a single gene.

Types of RNA

• RNA molecules have many functions, but in the majority of cells most RNA molecules are involved in just one job—protein synthesis.

• The assembly of amino acids into proteins is controlled by RNA.  

• There are three main types of RNA:1. messenger RNA

2. ribosomal RNA

3. transfer RNA.

Messenger RNA

• The RNA molecules that carry copies of protein making instructions are known as messenger RNA (mRNA)

• They serve as “messengers” from DNA to the rest of the cell

Ribosomal RNA

• Proteins are assembled on ribosomes

• Ribosomes are made up of several dozen proteins and RNA-ribosomal RNA (rRNA)

Transfer RNA

• During the construction of a protein RNA transfers each amino acid to the ribosome based on the messages in mRNA.

• These RNA molecules are known as transfer RNA (tRNA).

Checkpoint

• Ribosomes are made of ribosomal RNA and A.) Transfer RNA B.) Protein C.) Messenger RNA

Transcription

• RNA molecules are produced by copying part of a sequence of DNA into a complimentary sequence in RNA• called transcription.

• Transcription requires an enzyme known as RNA polymerase.  

Transcription Continued

• RNA polymerase binds to DNA separates the DNA strands.

• RNA polymerase uses one strand of DNA as a template and nucleotides are assembled into a strand of RNA.

Promotors

• The enzyme will bind only to regions of DNA known as promotersThey have specific base sequences.

• Promoters are signals that “tell” enzymes where to bind to make RNA.

RNA Editing

• Many RNA molecules require editing before they are ready to go into action.

• Remember that an RNA molecule is produced by copying DNA.

Introns and Exons

• Eukaryotic genes contains sequences of nucleotides called introns

• Introns are not involved in protein synthesis

Introns and Exons

• The DNA sequences that code for proteins are called exons

• Exons are“expressed” in protein synthesis

• When RNA molecules are formed, both the introns and the exons are copied from the DNA.

Introns and Exons

• The introns are cut out of RNA molecules

• The remaining exons are then spliced back

Checkpoint

• If the introns are not removed from the pre-mRNA, the resulting protein would beA.) Made correctlyB.) Made incorrectlyC.) Not made at all

Why Throw Away RNA?

• Some RNA molecules may be cut and spliced in different ways so a single gene can produce several different forms of RNA.

• It is thought that introns and exons may play a role in evolution Small changes in DNA sequences could

have dramatic effects in gene expression.

Checkpoint

• Intervening sequences of DNA are known asA.) intronsB.) ExonsC.) Codons

The Genetic Code

• Proteins are made by joining long chains called polypeptides combination of any or all the 20

different amino acid

Properties of proteins depend on order of AA

• The “language” of mRNA instructions is called the genetic code.

The Genetic Code

• As you know, RNA contains four different bases: AdenineUracilCytosineGuanine.

• The genetic code is read three letters at a time

• Each “word” of the coded message is three bases long.

Codon

• Each three-letter “word” in mRNA is known as a codon

• A codon consists of three consecutive nucleotides that specify a single amino acid

• The amino acid will be added to the

polypeptide.

partnerwork

mRNA codeUCGCACGGUmRNA codonUCG-CAC-GGU

write the amino acids that correspond to the codons on page 303

Answer

• Serine• Histidine• Glycine

Codon Numbers

• Because there are four different bases, there are 64 possible, three-base codons

(4 *4 *4 = 64)

• Six different codons specify the amino acid leucine

• Six other codons specify arginine.

AUG

• AUG can either specify:1. methionine2. “start”

• There are three “stop” codons that do not code for any amino acid.

UAG, UAA, UGA

AUG

• Stop codons act like the period at the end of a sentence

• They signify the end of a polypeptide, which consists of many amino acids

Translation

• The decoding of an mRNA message into a polypeptide chain (protein) is known as translation.

• Translation takes place on ribosomes.  • During translation, the cell uses

information from messenger RNA to produce proteins.

Overall Summary

• Messenger RNA is transcribed from DNA in the nucleus and released into the cytoplasm

• Translation begins when an mRNA molecule attaches to a ribosome.

• In the ribosome, the amino acid is transferred to the growing polypeptide chain.

Anti-codon

• Each tRNA molecule carries only one kind of amino acid.

• In addition to an amino acid, each tRNA molecule has three unpaired bases.

• These bases, called the anticodon, are complementary to one mRNA codon.

Steps

• 1. mRNA attaches to a ribosome, where codons move thru ribosome, and are read 1 at a time

• 2. tRNA enters ribosome, carrying an amino acid and anticodon

Translation always starts with the start codon AUG (methionine)

• 3. Ribosomes form peptide bonds between each amino acid Also breaks the bond between amino acid and

tRNA

• tRNA is released back into cytoplasm

• Continues until stop codon is reached

• Protein is released and translation is complete

The Roles of RNA and DNA

• The cell uses the vital DNA “master plan” to prepare RNA “blueprints.”

• The DNA molecule remains within the safety of the nucleus, while RNA molecules go to the protein-building sites in the cytoplasm—the ribosomes.

Genes and proteins

• Many proteins are enzymes which catalyze and regulate chemical reactions

• A gene that codes for an enzyme can produce skin color or blood type

• Genes for certain proteins can regulate growth, controlling size and shape

• Proteins are the keys to almost everything that living cells do

• 1. List the three main types of RNA. • 2. What happens during transcription? • 3. What happens during translation?• 4.Describe the three main differences

between RNA and DNA.