Review:

The process in which the information in genes is converted into the synthesis of proteins.

Now, what about the language of DNA?  What are these ‘elements’ that Mendel discovered?

 Remember, the language of cell metabolism is protein.

The language of proteins is written in a 20 letter alphabet in the form of amino acids.

But DNA is written in how many letters?
So, there is a disjunct. How to translate the DNA alphabet into the alphabet of proteins.
The problem is analogous to a code. What is the genetic code?

It turns out that in the early 1960s, this was perhaps the overriding question in  molecular biology: what sequence
    of DNA bases corresponded to amino acids?

The genetic code is written in triplets of bases, called CODONS.
  64 possible words, but 20 amino acids.
 Turns out that many amino acids are designated by multiple triplets.
  The code is redundant.
  Still, some left over as special signals -- stop codons.
   Mark the end of genes.

So, genes basically consist of a linear order of codons, or triplets, along the DNA .

Today's lecture:

Mutations consist of deletions, insertions, substitutions of nucleotides.
 Could be just one nucleotide, or entire gene, or many genes if part of  a chromosome is missing or altered.
But how is this information in DNA converted into a sequence of amino  acids?

Remember, DNA is the master set of genetic instructions.
 It’s housed in the nucleus (but remember, mitochondria and chloroplasts also have genetic information).

Soooooooo, we need to keep certain ideas in mind:
1. We want to use the instructions to make polypeptides (proteins) with a specific sequence of amino acids.
2. The cell makes lots of proteins simultaneously. In other words, lots of genes are being EXPRESSED at the same time.
3. But, not all genes are expressed at the same time and place in an organism. Some yes, but many others no.
4. Proteins are made in the cytoplasm, not the nucleus.
5. It’s advantageous to protect the master set of instructions. Don’t want to chop it up, for example.
6. There’s lots of RNA in the cytoplasm.

The solution is to make a working copy of the instructions, in the form of RNA.  THINK XEROX MACHINE.

     (remember the differences between DNA and RNA?)

There are 3 kinds of cellular RNA:
 Messenger, or mRNA.
  This is the actual working copy of a structural gene encoding a protein.
 Transfer, or tRNA.
  Relatively small. Carrier of amino acids.
 Ribosomal RNA.
  Variable size, serves structural and enzymatic role in ribosomes, the cytoplasmic particles in which protein
    synthesis occurs.

How does the cell make the working copy of a gene? I.e., an mRNA?
 By a process called TRANSCRIPTION.

Enzyme called RNA POLYMERASE binds to a special region of the gene.
The double  helix  unwinds, as in DNA synthesis, and the enzyme inserts ribonucleotides along one strand
of the DNA double helix.

The process continues until the polymerase reaches the end of the gene.

So mRNA has codons, and they're complementary to DNA codons.
   (Table 13.5)

But, we still haven’t made any proteins!
 To do that, the information in the working copy of mRNA must be
 TRANSLATED into protein. The process is called TRANSLATION.  (Fig. 13.6)

For that we need RIBOSOMES.
 Cytoplasmic granules whose function is synthesizing proteins.
  It’s a machine for making proteins.

 The machines are in place -- ribosomes. Awaiting instructions on what to make.  What are the instructions? mRNA copies!
  mRNA strand interacts with ribosome. In fact it passes through the ribosome, and as codons pass through, the commensurate amino acids line up and polymerize into a growing protein chain.
        Transfer RNA, or t'RNA, acts as an 'adapter' molecule, mating amino acids to specific codons on mRNA.
            What are 'anticodons'?
        As successive t'RNAs with specific amino acids line up along m'RNA, the amino acids are linked together.
     In other words, mRNA is moving through the ribosome, one codon at a time. Likewise, polypeptide is growing in length, one amino acid at a time.

Eventually, the final codon comes through and protein synthesis ends, with a complete protein.
Note: mRNA is very long, and can accomodate several ribosomes at the same time. So, several polypeptides are
    made simultaneously.  The oldest one is the longest one.
When a protein is finished it falls away from the ribosome, as mRNA exits.

As the protein is made, it also folds up to achieve its secondary and tertiary structure.

In summary, gene expression involves two processes,
 TRANSCRIPTION AND TRANSLATION.    (Fig. 13.7))

In the process, information in DNA determines sequence of amino acids in protein.

As the protein is made, it also folds up to achieve its secondary and tertiary structure.

Remember,
 Most plants have about 30,000 genes
  But not all genes are active all the time.
   Instead, genes are active when and where they’re needed.
    E.g. some genes that are important in defense against
    pathogens are turned on when the plant is challenged by a pathogen.
   Genes specific to make a flower aren’t active in roots.
  Why is this so?

So, gene expression is controlled in all organisms during development and throughout life. Genes are turned on and off in an orderly way. That's why are brains are in our heads, not in our.......  never mind.

 Promoters enable gene to be read, or copied into RNA.
  RNA polymerase binds to the promoter.
  Again, remember that a genome, or total genetic  information, or total  number of nucleotides, in the  nucleus of an organism is often much larger than the  number of nucleotides contained in its  structural   genes.