1. DNA, RNA, and Protein Synthesis
Molecular Genetics
DNA, RNA, and Protein Synthesis

2. DNA DNA – deoxyribonucleic acid DNA carries our genetic information.
Recall:
Where is DNA found?
in the nucleus
What type of macromolecule is DNA?
nucleic acid
What are the subunits of nucleic acids?
nucleotides

3. Finding the Structure of DNA: Chargaff’s Rule
Edwin Chargaff analyzed the amount of adenine, guanine, thymine, and cytosine in the DNA of various species. He found
the amount of thymine is almost equal to the amount of adenine
the amount of cytosine is almost equal to the amount of guanine

4. Finding the Structure of DNA: Franklin, Watson & Crick
A scientist named Rosalind Franklin was the first to use X-ray diffraction to take a picture of DNA.
James Watson and Francis Crick were able to determine the structure of DNA with the help of Franklin’s and Chargaff’s data
Watson and Crick proposed the double helix model of DNA

5. Structure of DNA
DNA is double-stranded and helical. It is composed of three things:
1) A 5-carbon sugar called deoxyribose
2) a nitrogenous base
3) a phosphate group.
There are four different types of
nitrogenous bases
- Adenine (A) Thymine (T)
- Cytosine (C) Guanine (G)

6. Nitrogenous Bases:
Purines & Pyrimidines
Double
Rings
Single
Rings

7. Structure of DNA DNA is like a twisted ladder
The “sides” of the ladder are made up of phosphates and sugars. The phosphate of one nucleotide is bonded to the sugar of another nucleotide forming a
“sugar-phosphate backbone”
The “rungs” of the ladder are made up of nitrogenous bases (A, G, T, or C).
The bases are held together by hydrogen bonds

8. DNA Replication
DNA replication occurs in preparation for cell division (mitosis)
Results in two copies of the DNA
Occurs during the S-phase of the cell cycle

9. DNA Replication First, the double-stranded DNA will unzip or unwind
DNA helicase is the enzyme responsible for the unzipping

10. DNA Replication
Each unzipped strand will serve as a template for semiconservative replication = final DNA molecule has one parent strand from the original molecule and one new strand
Another enzyme, DNA polymerase, will bind complementary nucleotides to form the new, complementary DNA strand.
The end result of DNA replication is two identical strands of DNA.

11. Structure of DNA versus RNA
Double-Stranded
RNA
Single-Stranded

13. Structure of RNA RNA – Ribonucleic Acid Single-stranded
Contains a ribose sugar, a nitrogenous base, and a phosphate group.
G-C and A-U There is no Thymine in RNA, Thymine is replaced with Uracil!!!!!

14. Types of RNA
mRNA, messenger RNA, used in protein synthesis to carry the information from the nucleus to the ribosome.
* Used in transcription (m=Messenger)
tRNA, transfer RNA, used in protein synthesis to carry or transfer amino acids to the ribosome
* Used in translation (t=Transfer)
rRNA, ribosomal RNA, used to form the structures of ribosomes (r=Ribosomal)

15. Central Dogma of Biology
DNA → RNA → Protein
(Transcription) (Translation)
DNA codes for RNA which guides protein synthesis

16. Protein Synthesis: How to Make a Protein
There are two steps involved in making a protein.
1st – The DNA must be transcribed. This means the DNA must be used as a template to make mRNA
2nd – The mRNA must be translated. This means the mRNA must be used as a template to make a sequence of amino acids (protein).

17. Transcription
Transcription involves copying DNA into mRNA (messenger RNA)
DNA cannot leave the nucleus. It must be transcribed into mRNA so the “message” can leave the nucleus.

18. Transcription Occurs in the nucleus of the cell
RNA polymerase, an enzyme, will unzip the DNA
The new RNA strand will be complementary to the unzipped DNA template strand.
It will follow the base pairing rule for RNA (no T in RNA, use U instead)

19. Transcription The product of transcription is mRNA
The mRNA will release from the template strand of DNA and leave the nucleus, heading to the ribosomes in the cytoplasm

20. Translation
Translation is converting the message coded in mRNA into a protein.
What are the subunits of proteins? Amino Acids
There are 20 amino acids.
Ribosomes make proteins; therefore, translation will occur at the ribosome.

22. Codons Codon = 3-base code in mRNA which codes for an amino acid
There are 64 codons
Only 61 codons actually code for an amino acid
There is 1 start codon – it also codes for the amino acid methionine
There are 3 stop codons
What is the start codon? AUG
What are the stop codons? UGA, UAA, UAG

23. Codon Chart

24. Codon Chart

25. Translation Continued
Occurs at the ribosome
mRNA connects to the ribosome
tRNA has a complementary sequence of bases to the mRNA, called the anticodon.
tRNA brings the amino acid to the ribosome
A tRNA carrying the amino acid methionine will bind to the start codon on the mRNA to start the process of translation.

27. Translation
Once the tRNA anticodon has attached to the mRNA codon, the appropriate amino acid will be released.
Now, the next codon will be read and the tRNA will release appropriate amino acid.
That amino acid will link with the previous amino acids.
Recall: What type of bond holds amino acids together? Peptide bond
These chains of amino acids will build a polypeptide chain which will create a specific protein.
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