Chapter 7Gene Expression and Control Part 2

Transcription: DNA to RNAThe same base-pairing rules that govern DNA

replication also govern transcription with one exception: In RNA, A pairs with U.

Transcription: DNA to RNADuring transcription, a strand of DNA (the antisense

strand) acts as a template upon which a strand of RNA is assembled, using RNA nucleotides.

Transcription is similar to DNA replication in that a DNA strand acts as a template for assembling another strand, in this case, of RNA.

However, a difference is that only part of the DNA strand (a gene) is used as a template, not the entire DNA strand.

Also, the enzyme, RNA polymerase, builds the growing RNA strand (whereas, in DNA replication, DNA polymerase builds the growing DNA strands).

The Process of TranscriptionTranscription occurs in the nucleus of cells.

1. RNA polymerase and several regulatory proteins attach to a specific binding site in the DNA, called a promoter, at a transcription site close to a gene.

2. The RNA polymerase moves along the DNA, over the gene. As it moves, the RNA polymerase unwinds the double helix so that it can “read” the base sequence of the antisense (noncoding) strand of DNA.

3. 3. The RNA polymerase joins free RNA nucleotides into a chain in the order dictated by the DNA sequence

4. When the RNA polymerase reaches the end of the gene, the DNA and the new mRNA strand are released.

The process of TranscriptionThe new mRNA strand is complementary in

base sequence to the DNA strand from which it was transcribed.

It is essentially an RNA copy of the gene (DNA).

Typically, many RNA polymerases transcribe a gene at the same time, so many new mRNA strands can be produced at the same time very quickly.

RNA Players in Translation: mRNA

mRNA is a disposable copy of a gene.

Its job is to carry DNA’s protein-building message to the other two types of RNA for translation.

This protein building message consists of a linear sequence of of genetic “words” spelled with the genetic alphabet of A, G, C, and U.

Each of these genetic “words” contains three letters and is called a codon.

Each codon codes for a particular amino acid.

Since there are 4 possible bases and three possible positions in a codon that each base could hold, there is a total of 64 (or 43) possible codons.

These 64 codons constitute the genetic code.

The Genetic Code

The Genetic CodeSince one codon follows another in the mRNA,

the order of codons in the mRNA determines the order of amino acids in the polypeptide that will be formed from it.

Thus the base sequence of a gene (DNA) is transcribed into the base sequence of an mRNA, which is in turn translated into an amino acid sequence in the polypeptide.

The Genetic CodeThere are only twenty amino acids found in

proteins.

So why are there 64 codons?

Many amino acids are specified by more than one codon. For example, GAA and GAG both code for glutamic acid.

Because of this, the genetic code is said to be a degenerate code.

The Genetic Code

In addition, some codons signal the beginning and end of a protein-coding sequence.

For example, in most species, the codon AUG in the mRNA signals for translation to start.

AUG also codes for the amino acid methionine. Therefore, methionine is always the first amino acid to added to a new polypeptide in most species.

In addition, the codons UAA, UAG, and UGA do not code for any amino acids at all. Therefore, their presence stops translation and so they are called stop codons.

These codons signal the end of a protein-coding sequence in an mRNA molecule.

RNA Players in Translation: rRNARibosomes consist of one large subunit and one

small subunit., both of which are composed of structural proteins and rRNA, ribosomal RNA.

In prokaryotes, the small subunit is a 30s and the large subunit is a 50s. Together these two form the 70s prokaryotic ribosome.

In eukaryotes, the small subunit is a 40s and the large subunit is a 60s. Together these two form the 80s eukaryotic ribosome.

These two ribosomal subunits come together to form an intact ribosome on an mRNA molecule during translation.

RNA Players in Translation: rRNA

RNA Players in Translation: tRNATransfer RNA’s (tRNA) deliver amino acids amino

acids to ribosomes in the order specified by the mRNA.

Each tRNA has two attachment sites.

One is the anticodon loop, where a triplet of nucleotides that base pair with the mRNA codons are located.

The second is where the amino acid binds, the amino acid specified by the codon.

tRNA’s with different anticodons carry different amino acids.

The tRNA carrying its appropariate amino acid is called a “loaded” tRNA.

Translation: RNA to ProteinTranslation occurs in the cytoplasm of the cell

upon ribosomes.

It has three stages: Initiation, Elongation Termination

Translation: RNA to ProteinInitiation: The small ribosomal subunit binds

to an mRNA molecule. The anticodon of a special tRNA called an initiator base pairs with the first AUG codon of the mRNA. Then a large ribosomal subunit joins the small subunit.

TACAUG

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Translation: RNA to Protein Elongation: The ribosome assembles a polypepide

chain as it moves along the mRNA. The initiator tRNA carries the amino acid methionine so the first amino acid of the new polypeptide chain is methionine. A second tRNA brings in a second amino acid as its anticodon base pairs with the second codon in the mRNA. The ribosome then joins the two amino acids together by way of a peptide bond. This process continues as new amino acids are added to the growing polypeptide chain.

Translation: RNA to ProteinTermination: When the ribosome reaches a

stop codon in the mRNA, the mRNA and polypeptide detach from the ribosome and the ribosomal subunits detach from one another. Translation is now complete. The new polypeptide chain will join other proteins in the cytoplasm or it will enter the rough ER.

Translation: RNA to Protein In cells that make a lot of protein, many

ribosomes may be translating the same mRNA.

These ribosomes are referred to as polysomes.

Translation: RNA to Protein

Not only do cells make many polypeptides from one mRNA, but they also make many copies of the same mRNA. Why?

Since RNA is not a very stable molecule, it lasts only minutes before being disassembled by cytoplasmic enzymes.

This fast turnover allows cells to adjust their protein synthesis rapidly in response to changing needs in a changing environment.

This also explains why cells make many copies of the same mRNA molecule.