1. Chapter 15: RNA
Ribonucleic Acid

2. I) What is RNA? RNA – Ribonucelic Acid
Single stranded nucleic acid
Formed from nucleotides
Contains 5-carbon sugar RIBOSE
4 Nitrogenous bases – A, U, G, C
Uracil replaces Thymine
RNA Functions – more than just one!
RNA is a temporary copy of a DNA base sequence
Carries genetic information from DNA to ribosomes
RNA has regulatory functions
RNAi can affect gene expression
RNA catalyzes peptide bond formation
Synthesis of proteins at ribosome

3. II) The Central Dogma of DNA
RNA is a complementary copy of a DNA sequence (a gene)
DNA gene sequence is transcribed into a strand of RNA
The RNA nucleotide sequence is translated into a polypeptide (forms protein)
DNA
RNA catalyzes protein synthesis
transcription
translation
RNA
PROTEIN
in nucleus
at ribosomes

4. III) Types of RNA Five types of RNA exist in cells
preRNA snRNA mRNA rRNA tRNA
Eukaryotes Only

5. III) Types of RNA Five types of RNA exist in cells
preRNA snRNA mRNA rRNA tRNA
Eukaryotes Only

6. IV) The genetic Code
Code of nucleotide bases universal to all living organisms
Based on a 3-base system called CODONS
A codon is a 3-base sequence that determines a specific amino acid
AUG = methionine GUU = valine
There are 64 codons for 20 amino acid sequences
Some amino acids have 2 or more codons
Valine = GUU, GUC, GUA, GUG
Degenerate code – multiple codons “code” for the same amino acid
Reduces point mutations
Reduces FRAME-SHIFT mutations
Change in reading frame
Ex) /CAG/CAG/CAG/CAG
C/AGC/AGC/AGC/AG
CA/GCA/GCA/GCA/G
Four codons code function as “signaling” codons
START Codon: AUG (also codes for methionine)
STOP Codons: UAA, UAG, UGA (no amino acid)

7. V) RNA Transcription
Transcription factors (proteins) attached to the DNA where a particular gene is located
RNA polymerase binds to the START codon near where these transcription factors are located
START codon is called the PROMOTER
RNA polymerase moves along the DNA strand in the 3’  5’ direction
RNA polymerase stops at the STOP codon and releases from the DNA
RNA polymerase produces a strand of mRNA (preRNA in eukaryotes)

8. VI) RNA Processing Occurs in eukaryotes ONLY
Newly transcribed preRNA is modified as follows:
1. A poly-A tail (repeating Adenines) is added to the 3’ end
2. A Methyl “cap” is added to the 5’ end (5’ cap)
The cap and tail prevent enzymes from hydrolyzing the RNA
3. Splicesosomes made of snRNA cut out non-coding sections
Remove introns and combine exons
Spliceosomes are made of RNA units called SnRNPs (Small nuclear Ribonuclear Particles)
Processed preRNA is now referred to as mRNA
mRNA transcript exits the nucleus and travels to a ribosome
Animation1
Animation 2

9. VII) The Ribosome – Protein Synthesis
Consists of 2 subunits
Large subunit
Contains 2 amino acid binding sites
Small subunit
Binds to mRNA

10. VIII) Transfer (tRNA)
Transfer RNA carries amino acids to the ribosome for proper placement in a polypeptide
Contains an ANTICODON which corresponds to the codons in mRNA
Ribosome aligns tRNA in proper sequence
tRNA contains three “hair-pin” loops with base- pairing

11. IX) Protein Synthesis
Occurs at the ribosome in both prokaryotes and eukaryotes
Involves three steps
Initiation
Elongation
Translocation occurs
Termination
Animation

12. XII) Mutations
Changes in DNA that are passed on to the next generation of cells
A) Point Mutations
Changes in one base or one codon.
May or may not have significant effects

13. Substitution – replace one base or codon with another

14. B) Chromosomal Mutations Mutations affecting an entire gene or chromosome Inversion – sequence of bases is reversed Deletion – one base or codon is removed

15. Translocation - a section of one chromosome attaches to a different chromosome Can result in trisomy and some rare cancers