rna & protein synthesis 12-3. intro genes code dna instructions that control the production of...
TRANSCRIPT
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.