1. RNA and Protein Synthesis
Section 12-3

2. Objectives for this section
Compare and contrast DNA and RNA
Name the 3 main types of RNA
Describe transcription and the editing of RNA
Identify the genetic code
Summarize translation
Explain the relationship between genes and proteins

3. Introduction
The structure of DNA explains how it can be copied, but it does not tell how a gene works.
Genes—coded DNA instructions that control the production of proteins within the cell

4. Introduction
We now know that the central dogma of biology is DNA to RNA to protein
The first step in decoding the genetic message is to copy part of the DNA nucleotide sequence into RNA (ribonucleic acid)
These RNA molecules contain the coded instructions for making proteins

5. The Central Dogma of Bilogy

6. The Structure of RNA RNA, like DNA, is a long chain of nucleotides
RNA consists of the same components as DNA
A 5-carbon sugar
A phosphate group
A nitrogenous base

7. The Structure of RNA There are 3 main differences between DNA and RNA
The sugar in RNA is ribose instead of the deoxyribose in DNA
RNA is generally single-stranded
RNA contains uracil instead of thymine

8. The Structure of RNA RNA is like a disposable copy of a segment of DNA
In many cases, RNA is a copy of a single gene—the ability to copy a single DNA sequence into RNA makes it possible for a single gene to produce hundreds or even thousands of RNA molecules

9. 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
There are 3 main types of RNA

10. Types of RNA
Messenger RNA (mRNA)—carry copies of instructions for assembling amino acids into proteins; serve as “messengers” from DNA to the rest of the cell

11. Types of RNA
Ribosomal RNA (rRNA)—a component of ribosomes

12. Types of RNA
Transfer RNA (tRNA)—works during the construction of a protein; transfers each amino acid to the ribosome as it is specified by the coded messages in the mRNA

13. Types of RNA

14. Transcription
Transcription—when RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA
Transcription requires the enzyme RNA polymerase—binds to DNA and separates the DNA strands. Then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA

15. Transcription
How does the RNA polymerase know where to start and stop making the RNA copy?
The enzyme will only bind to regions of DNA known as promoters, which have specific base sequences
Similar signals tell the RNA polymerase where to stop

16. Transcription

17. RNA Editing
The first molecule of mRNA (known as the pre-mRNA) produced by copying the DNA sequence is like a rough draft and it requires editing
DNA contains sequences of nucleotides called introns, which are not involved in coding for proteins

18. RNA Editing
The DNA sequences that code for proteins are called exons, because they are expressed in the synthesis of proteins
When an RNA molecule is formed, it contains both introns and exons
The introns are cut out of the RNA molecule while it is still in the nucleus
The remaining exons are spliced back together and form the final pre-mRNA molecule

19. RNA Editing

20. The Genetic Code
Remember that proteins are made by joining amino acids into long chains called polypeptides
Each polypeptide contains a combination of any or all of the 20 different amino acids
The properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides

21. The Genetic Code
The “language” of mRNA instructions is called the genetic code
RNA contains 4 different nitrogenous bases (U, C, G, A)
How can a code with only 4 letters translate into 20 different amino acids?
The genetic code is read 3 letters at a time, so each “word” of the coded message is 3 bases long

22. The Genetic Code Each 3-letter “word” in mRNA is known as a codon
A codon consists of 3 consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide
Example: UCGCACGGU would be read as UCG-CAC-GGU. These codons represent 3 amino acids: serine-histidine-glycine

23. The Genetic Code
Because there are 4 different bases, there are 64 possible 3-base codons.
Note that some amino acids can be specified by more than one codon
There are also “start” and “stop” codons
Start codons (AUG) tell where protein synthesis is to begin
Stop condons (3 different ones) tell where the end of the polypeptide is

24. The Genetic Code

25. Translation
The mRNA molecule has been transcribed and serves as instructions, but we need something to read the instructions and put them to use
In the cell, the ribosome takes care of this
Translation—the decoding of an mRNA message into a polypeptide chain (protein)

26. Translation Steps in translation
Begins when an mRNA molecule in the cytoplasm attaches to a ribosome
Each codon of the mRNA moves through the ribosome and the proper amino acid is brought to the ribosome by the tRNA

27. Translation
Each tRNA carries only one kind of amino acid and picks it up based on the anitcodon it is carrying
Example: if the anticodon is UUU, the tRNA would pick up the amino acid with the codon AAA (they are opposites)

28. What codon would these anticodons pick up??
ACG
UGG
CAG
GGG
GCC
CAA

29. Translation
The ribosome forms a peptide bond between the first and second amino acids
At the same time, it breaks the bond with the tRNA molecule and releases it
The ribosome moves on the third amino codon, where a tRNA molecule brings it the amino acid specified by the third codon

30. Translation
The polypeptide chain continues to grow until the ribosome reaches a stop codon on the mRNA molecule
When the stop codon is reached, it releases the newly formed polypepetide and mRNA molecule, completing the process of translation

31. Genes and Proteins
What does protein synthesis have to do with the color of a flower, eye color, or height?
Remember that many proteins are enzymes, which catalyze and regulate chemical reactions.
A gene that codes for an enzyme to produces pigment controls flower color.
Proteins are specific tools that build or operate components of living cells.