How Are Genes Expressed?

Chapter11

DNA codes for proteins, many of which are enzymes.

• Proteins (enzymes) can be used to make all the other molecules a cell needs: carbohydrates, lipids and nucleic acids.

• A segment of DNA that carries the instructions to make (codes for) a protein is called a gene.

• How can patterns of 4 bases hold information?

• The same way your computer works!

• The bases are read in groups of three bases called codons.

• Each codon “codes” for a specific amino acid (except for three which mean “stop”)

• We can make 64 different codons, but we only use 20 amino acids.

• One codon means “start here”

• Three codons are “stop” codons.

• More than one codon can be used for a particular amino acid.

• Because DNA is in the nucleus, and our “work benches” – ribosomes- are in the cytoplasm, we need a way to get just the information we need to the ribosome.

• We make a copy of the gene we need in messenger RNA. This process is called TRANSCRIPTION. (We have not changed the “language”, just the form of the information.

DNA vs. RNA

Uses A,T, C and G Uses A, U, C and G

Use deoxyribose Uses ribose

Double stranded Single stranded

• The strand of DNA that is copied is called the template or antisense strand.

• An enzyme called RNA polymerase looks at the DNA to find a special region called a promoter. This tells the enzyme where to start copying the DNA.

• It copies the DNA until it comes to a terminator.

• Prokaryotes can use the mRNA right after it is copied.

• Eukaryotes need to go through some additional steps, so at this point the mRNA is called pre-mRNA.

• Eukaryotes modify the pre-messenger RNA.

• The intervening sequences (Introns) are cut out and the expressed sequences (Exons) are spliced back together. This way, more than one protein can be made from a single gene!

• Now it is mature mRNA

• The mRNA leaves the nucleus through the pores in the nuclear envelope, and finds a ribosome in the cytosol or on rough E.R.

• Ribosomes are made of rRNA and proteins.

• Each ribosome is made of two subunits: – A large subunit – A small subunit

• These come together when they are needed.

Ribosome

The large subunit catalyzes the formation of the peptide bonds between the amino acids.

The small subunit acts as a reader of the RNA.The RNA must be read correctly, or the wrong amino acid will be used. The 3 base sequence the RNA looks at is called the “Reading frame.”

Changing from the “language” of nucleic acids to the “language” of proteins is called TRANSLATION.

Translation• We need a means of getting the correct

amino acid in the correct sequence. For this we use one more type of RNA : transfer RNA (tRNA).

• tRNA is a single strand of RNA that is folded into the shape of a clover. It has an anticodon that matches the codon on the mRNA, and a spot for holding the amino acid that matches the codon.

• To be sure that the correct tRNA always carries the correct amino acid, the two are put together by enzymes that match the anticodon and the amino acid.

• When the mRNA binds to the small subunit of the ribosome it is called initiation. The ribosome looks for the “start” codon, which is always AUG.

Elongation

The same three steps are repeated until the “stop” codon is read.

1. An amino acid is placed in position on the “A” site of the ribosome

2. The peptide bond is formed.

3. The peptide moves over to the “P” site so that the “A” site is available for the next amino acid. (The old tRNA is released.)

• When a ribosome has moved far enough down the mRNA, a second ribosome can pick up the mRNA and also start reading and translating it. It may be passed on to a third ribosome, and so on. When we have several ribosomes all translating the same mRNA at the same time, it is called a polysome.

Termination

• When the ribosome reads one of the stop codons, there is no matching tRNA. Instead, a protein called a release factor binds to the stop codon, the polypeptide is cut from the last tRNA, and the polypeptide (protein) is released into the ctyoplasm, where other proteins will help fold it.

Play with DNA

Now its your turn !

• Newly made proteins have a signal peptide – 4 to 12 amino acids – that direct the protein where to go.

• A typical eukaryotic cell transcribes only about 20 % of its DNA into RNA.

Regulation of protein synthesis

• To form each peptide bond requires 3 molecules of ATP. Since each protein can have from 50 to thousands of amino acids, much of the cell’s energy goes into protein synthesis.

• Protein synthesis is regulated at every step of the process. The most energy efficient, is to control protein synthesis by controlling transcription.

We can also have:

• Posttranscriptional control

• Translational control

• Posttranslational control

Genes can be of two types:

• Structural genes – codes for proteins used outside the nucleus

• Regulatory genes – help govern the amount of polypeptide made by the cell.– Transcription factors– Regulatory site