How do genes work?
[note: if you choose not to read all this info, at least look at number 15 , 16, and 17 sumarries below. But a thougough understanding is essential]

1. Inside the nucleus DNA is constantly being “turned on” in certain places.  By this we mean regulatory proteins will bind to a specific site on the DNA and cause the DNA to start to activate a gene.  Remeber, genes, like DNA, are made up of nucleotides (the A, T, G, or C we see so much about)

2. This area on the DNA that produces (term we use in biology is “codes”) for an end product is called a gene.

3. The basic terminology for this end product is called a polypeptide, which will be talked about shortly.

4. Two examples of polypeptides are: proteins and enzymes.

5. In a process called “TRANSCRIPTION”, the DNA makes a copy of the gene (not the full DNA strand) called ribonucleic acid (RNA for short).  Think of this as “ the DNA has just transcribed a gene into RNA.” This process occurs within the cell’s nucleus.

6. This RNA copy of the DNA is composed of two parts. One part is cut away from the RNA leaving behind a shorter piece of RNA.  The parts that are cut away are the non-coding regions of a RNA strand called “introns”.  What is left behind in one singular RNA strand are called the “extrons”. This might sound confusing, but a good way to remember this is that the introns will be left inside the nucleus, while the remaining strand of RNA, now composed solely of extrons will be exported outside the nucleus. [introns = left inside the nucleus ; extrons = exported outside the nucleus]

7. To make things more confusing, the left over piece of RNA that is composed of extrons has a new special name, it’s now called messenger RNA, or mRNA.  Not to hard to remember, since delivery messengers (i.e., FED EX couriers) routinely export their packages outside the country.

8. The mRNA now passes outside the nuclear pores into the cytoplasm where it meets up with ribosomes.  These ribosomes may be freely floating, or attached to the Endoplasmic Reticulum.  In fact, one usually finds many ribosomes attached to just one piece of mRNA.

9. At this point, the ribosomes “holds” the mRNA in place in a process known as translation.  Here, another type of ribonucleic acid called transfer RNA, or tRNA, will bring in an amino acid to a specific site on ribosome attached to the mRNA.

10. The ribosome simply starts at one end of the mRNA and reads right on down it till it reaches the opposite end.  Every three nucleotides, called a codon, is translated into one amino acid. Indeed, this process is known as “TRANSLATION.”  By the time the ribosome reaches the end of the mRNA, the ribosome has catalyzed the assembly of a strung together piece of amino acids.  This strung together piece of amino acids is called a polypeptide.  Depending on how it functions, we then call this polypeptide a protein or an enzyme.

11. Note tRNA’s are not made into the polypeptides.  They carry amino acids to the ribosomes and drop them off.

12. More information you probably never use.  Those polypeptides made on the ER will pass through the ER and Golgi apparatus to be exported directly outside the cell.  These are called secretory proteins.  Those polypeptides synthesized on the freely floating ribosomes are destined to function inside the cell’s cytoplasm or in mitochondria.

13.  Even more knowledge.  Mitochondria also have their own complement of DNA, which likewise codes for its own polypeptides to be used only inside the mitochondria.  Thus, mitochondria use DNA both made from its own DNA and from DNA that is in the nucleus whose polypeptide products were brought into the mitochondria from the cytoplasm.

14. Note that apart from the DNA in a cell’s nucleus and in its mitochondria, no other subcellular organelle contains DNA. Weird, eh?
 
 
 
 
 

15. Take home message #1

        Flow of information is:

    DNA (gene) ---->  RNA, introns removed inside nucleus --->   mRNA moved to outside the nucleus ---> ribosomes ---> Protein
 

16. Take home message #2

        Transcription - occurs inside cell's nucleus and is the step from DNA to RNA
        Translation -  occurs outside the cell's nucleus and is the step from mRNA to polypeptide

17. Take home message #3

                ___intron___|___extron___|_____intron___|_extron_____|___intron__|__extron_____    <---- RNA strand

    introns removed:

                _____extron_____|_____extron___|___extron____   <-----  now called  mRNA strand & moves to outside nucleus