The Central Dogma: From DNA to Proteins
March 12 and 24, 2003


Readings: Ch 13, 200- 207. CD-Rom has excellent animations of this complex process!
Warm Up 8 (on transcription and translation) was due at 9:30 Weds March 12; Warm Up 9 (on evolution) is due at 9:30 Weds March 26


"A threefold cord is not quickly broken." - -Ecclesiastes iv. 12.

I. How in the information in DNA turned into Protein? The Central Dogma.

DNA is a huge information database that carries the complete set of instructions for making all the proteins a cell will ever need! Although there are only four different bases in DNA (A, C, G and T), the order in which the bases occur determines the information to make a protein, just like the 26 letters of the alphabet combine to form words and sentences:

Compare: RAT - TAR - ART - same 3 letters; completely different meanings.
And with DNA: GAC - AGC - CGA - same 3 'letters'; completely different meanings to the cell (specifies the amino acids Aspartic Acid, Serine, and Arginine)

Q: Review: What are genes?
A: We know from the results of the Human Genome project, that most of the cell's DNA (~97%) does NOT code for proteins, but has structural or regulatory functions. The DNA in each chromosome that
DOES provide the instructions for a protein is called a gene.

Q: If DNA is in the nucleus and proteins are synthesized in the cytoplasm, on ribosomes and in the rER, how to they "get together"?
A: The answer: use a "
messenger" to carry the instructions from DNA out into the cytoplasm. A nucleic acid very similar to DNA, called mRNA or messenger RNA, is a copy of a gene, and serves this function the "bridge" between DNA and protein:

The Central Dogma:

DNA encodes the information to make RNA.........and RNA molecules function together to make protein

II. What is RNA and how is it different from DNA?

Two big differences between DNA and RNA:

 

III. Transcription = Re-writing DNA into RNA

DNA is "transcribed" or re-written into RNA in a very complicated process called transcrption. (Think of a transcriptionist that takes spoken words from one source, like a court judge, and makes a copy of those words on paper. If that helps...).

Simply stated, during transcription, one gene (DNA) is 're-written' into an RNA in the nucleus:

IV. What's the connection between mRNA and protein?

The order of the bases in the DNA specifies the order of bases in the mRNA, and

The order of bases in the
mRNA specifies the order of amino acids in a protein.

The genetic code is a triplet code (handout)

1. Nucleotides on mRNA are read "three at a time" by the ribosome.

2. The amino acids corresponding to all 64 codons have been determined - this was all worked out in the 1960s by Marshall Nirenberg, Robert Holley, and Har Khorana (Nobel Prize!)

The genetic code chart represents the sequence on the mRNA codon.

All living organisms and viruses use this triplet genetic code - its that "biological unity" idea again!!!

Worksheet

V. Translation = De-coding RNA into protein (note: color coded!)

During translation, the mRNA transported to the cytoplasm is "de-coded" or "translated" to produce the correct order of amino acids in a protein.. Translation requires numerous enzymes. To know the full story, we need to look at two other RNA "Key Players" - rRNA and tRNA

rRNA = ribosomal RNA; these RNA molecules associate with other proteins to form the ribosomes. Each ribosome can accept two tRNAs at a time (carrying amino acids) and one mRNA.

tRNA = transfer RNA; small RNA molecules that carry a specific amino acid at one end and an anticodon region that recognizes and binds mRNA at the other end. The tRNA that binds to that mRNA codon determines what amino acid is added to a protein chain.

The Three RNAs (mRNA, tRNA, and rRNA) all work together to turn the information in DNA into a beautiful, 3-dimestional protein!!!

The steps of translation:

1. Initiation: mRNA enters the cytoplasm and becomes associated with ribosomes (rRNA + proteins).

tRNAs, each carrying a specific amino acid, pair up with the mRNA codons inside the ribosomes. Base pairing (A-U, G-C) between mRNA codons and tRNA anticodons determines the order of amino acids in a protein.

2. Elongation: addition of amino acids one-by-one:
As the
ribosome moves along the mRNA, the tRNA transfers its amino acid to the growing protein chain, producing the protein - codon by codon!

3. Termination: when the ribosomes hits a stop codon - UAA, UGA, or UAG - the ribosome falls apart!

The same mRNA may be used hundreds of times during translation by many ribosomes before it is degraded (broken down) by the cell.

 

How does translation relate to YOUR life? (Why do you need to know this, anyway?) All the proteins that make up YOU, your cells, your body, the foods you eat, all the living cells in the world, etc - are made this way! Every time your body needs more of a protein -

a gene carrying the information for that protein is transcribed into mRNA, and the mRNA is made into protein!
Better living through transcription and translation! We will talk about how this related to GENETICS and INHERITANCE very soon!


Objectives :

1. Name the two differences between DNA and RNA
2. Use the triplet code to determine the order of amino acids in a protein (worksheet)
3. Describe the "bottom line" of transcription
4. Describe the "bottom line" of translation
5. Define the words: gene, codon, amino acid, mutation, mRNA, tRNA, rRNA