FROM DNA TO PROTEINS

FROM DNA TO PROTEINS- TRANSCRIPTION AND TRANSLATION

We learned how DNA carries the instructions of heredity.  Heredity combines with our life experiences, our environment, and determines how we look and function, how we hear and see, how we think and feel.  DNA is made up of genes that are the set of instructions.....but the instructions for what?  DNA is the blue print for proteins.  Proteins build your body structures, carry out functions of your body at the cellular level, fight disease, carry oxygen,  and speed up reactions that help you absorb food and use energy.  Proteins are part of every cell in your body.  Every protein is synthesized (made) according to instructions contained in DNA.  The same steps for making protein from DNA instructions produce ALL of the worlds proteins.  Life can not exist without proteins.  

THE STORY OF MUSSELS

A mussel is an animal with a soft body, but a hard shell.  It has a foot that grabs onto the rocks to keep the ocean from washing it away.   If the Mussel were to be dislodged, the ocean might batter it against the rocks until it broke, making it just a bit of lunch for Sea Gulls.  When the mussels foot comes across a crevice in the rock, the foot sweeps the crevice clean of sand and debris, then the foot presses down and arches up, forming a suction.  This forms a vacuum sealed chamber under the foot, where the mussel excretes a sticky fluid made of keratin and other proteins.  It becomes a sticky foam, and by pumping its little foot, the mussel turns the foam into threads, which it them varnishes with an adhesive protein called Byssus.  The suction and the adhesive strands anchor the mussel to the rock. Byssus is the worlds best underwater adhesive.  Nothing we can manufacture does as good a job of sticking things together underwater.  A lot of industries are interested in manufacturing Byssus.  Material is made out of it, called Sea Silk, and the sticky adhesive fibers may one day be used in surgery or dentistry, or manufacturing.  Scientists are  inserting the "Byssus" genes into yeast, to see if they can make yeast mass produce the fibers.

Today pigs and tobacco plants have been bioengineered, genes inserted into them, to produce medicines and hormones that humans need. Can you think of any important proteins WE make that have value to others?

TRANSCRIPTION AND TRANSLATION OVERVIEW:

There are two steps to make a protein from DNA instructions: Transcription and Translation.  We learned that the sequence of bases in DNA is different for every type of organism....in other words, a fox has different genes, different base sequences, than a person. During the process of making proteins from DNA, sections of the DNA (genes),  are unwound and used to make a strand of RNA.  In a Eukaryotic cell (plant and animal cell)  transcription happens in the nucleus.  The process of making a protein is finished in the cytoplasm( the juice in the cell)  and organelles of the cell.  When we were learning about organic molecules, the molecules of life, we learned there are two types of nucleic acids, DNA and RNA.  DNA is a double stranded molecule,  and RNA, a single stranded molecule. There are three types of RNA that are used when protein is made.  The first kind is  Messenger RNA, which we abbreviate mRNA.  The next is ribosomal RNA, abbreviated rRNA.  and the last one is transfer RNA, abbreviated tRNA. 

TRANSCRIPTION- RNA and Protein Synthesis
Your DNA, or deoxyribonucleic acid, contains the genes that determine who you are. How can this organic molecule control your characteristics? DNA contains
instructions for all the proteins your body makes. Proteins, in turn, determine the structure and function of all your cells. What determines a protein’s structure? It begins with the sequence of amino acids that make up the protein. Instructions for making proteins with the correct sequence of amino acids are encoded in
DNA. DNA is found in chromosomes. The chromosomes in your cells always remain in the nucleus, but proteins are made at ribosomes in the cytoplasm.

How do the instructions in DNA get to the site of protein synthesis outside the nucleus? Another type of nucleic acid is responsible. This nucleic acid is RNA, or
ribonucleic acid. RNA is a small molecule that can squeeze through pores in the nuclear membrane. It carries the information from DNA in the nucleus to a
ribosome in the cytoplasm and then helps assemble the protein. The structure of RNA is similar to the structure of DNA, but they are not identical.
RNA contains three of the same nucleotide bases as DNA, but does not include thymine. Instead, RNA has the base uracil (U). The other major difference between DNA and RNA is that RNA is single stranded, instead of double stranded like DNA. This means that the bases of RNA are available to bond with other nucleotides.

RNA matching nucleotide bases are always A-U and G-C. Adenosine, Uracil, Guanine, and Cytosine. 

This part of RNA’s structure is very important to its function in making proteins.

There are three types of RNA directly involved in protein synthesis:
1. Messenger RNA (mRNA) carries the instructions from the nucleus to the
cytoplasm. mRNA is produced in the nucleus, as are all RNAs.
2. Ribosomal RNA (rRNA)

3. transfer RNA (tRNA)

The last two kinds, rRNA, and tRNA are involved in the process of ordering the amino acids to make the protein. Ribosomal RNA becomes part of the ribosome, which is the site of protein synthesis, and tRNA brings an amino acid to the ribosome so it can be added to a growing chain during protein synthesis. There are
numerous tRNAs, as each tRNA is specific for an amino acid. The amino acid actually attaches to the tRNA during this process. 

STEPS OF TRANSCRIPTION

Only one area of DNA serves as the template for a protein.  Only that section will be unwound and only a single nucleotide strand is created called mRNA.

1. Transcription starts at a base sequence  called a promoter.  Its a start signal at the beginning of a gene on the strand of DNA. An enzyme DNA Helicase begins to break the hydrogen bonds between the two strands of the DNA molecule, and unwinds are portion starting at the promoter.

2. Proteins position an enzyme, RNA Polymerase, on the DNA molecule, binding it with the promoter. 

3. RNA Polymerase moves along the DNA molecule joining nucleotide bases together, (from a big supply of free nucleotide bases in the nucleus) until it reaches a stop sequence. The strand that is being built is called mRNA, and the RNA Polymerase enzyme attaches bases ( A, U, G, C) making matches to the bases on the exposed portion of the DNA.  Transcription always happens in the same direction, its called 5' to 3'.  This is an odd expression, and difficult to understand until you have more chemistry education, but what you want to remember for now is transcription always happens in the same direction. 

4. Once the stop sequence is reached, the mRNA molecule is released.  The DNA molecule may wind back up, or proteins may continue making another mRNA strand. 

Finishing the mRNA before it leaves the nucleus:

1.  Enzymes then attach a "cap" at the start of the new mRNA molecule and on its tail.

2. Introns are sections in mRNA that do not code for a protein.  Exons are sections of mRNA that do code for proteins.  Introns and exons are both transcribed, but enzymes snip out the introns before the mRNA leaves the nucleus.

3. The "mature" mRNA  now leaves the nucleus through pores in the nuclear membrane and enters the cytoplasm of the cell.

STEPS OF TRANSLATION

Now the messenger RNA will be "translated"  on a little protein factory called a Ribosome in three steps: INTIATION, ELONGATION, and TERMINATION.  The cell of a mammal may have as many as 10 million ribosomes.  You have a lot of  protein factories working full time in each of your cells!! Ribosomes are made in the nucleus in two parts, from ribosomal RNA.  There is a large subunit and a small subunit and once made in the nucleus they are shipped out to the cytoplasm. .  They join together and form a little platform where  translation can happen.  The cytoplasm of the cell also has a big supply of free amino acids, which are the small molecules bonded together to make proteins and a supply of tRNA.  When translation happens, the mRNA bases are read three at a time.  Each combination of three bases is called a CODON.   There are 64 Codons.  These 64 codons "code" for more than twenty amino acids. There are also codons that give a start signal to begin translation and some that code for a stop signal. 

INITIATION:

In this step mRNA is loaded onto a ribosome. 

1. tRNA and mRNA  bind with the small ribosomal sub-unit

3. the large ribosomal unit then binds to the small unit

ELONGATION:

1. tRNA has an anticodon that matches the start codon.  It also has a hook that picks up a specific amino acid.

2. a second tRNA attaches to the ribosome carrying another amino acid.  Enzymes release the first tRNA while bonding the two amino acids together.

3. This continues on bonding amino acids together in a long chain.

TERMINATION:

1. A stop codon is reached on the mRNA and there is no corresponding anticodon on a tRNA.  

2. Enzymes release the polypeptide chain

3. The chain may be stored in the cytoplasm or be taken to the endoplasmic reticulum to be shaped.  

MET is a start Codon. UAA, UAG, UGA are stop codons.  

Putting it together:

We learned how important proteins are to our structure and function, and an interesting protein made by mussels.  Scientists are bioengineering organisms, inserting genes,  into plants and yeast to make an adhesive that mussels make.  

What protein so WE make that others may need?

We create antibodies, which are proteins, as well as hemoglobin, another protein,  in our blood.  When we donate blood or plasma our proteins may be used for medicine for others, or to help people who don't have enough blood cells or hemoglobin.