From Gene to Protein: The Connection Between Genes and Proteins

Overview:

The one gene-one polypeptide hypothesis states that each gene polypeptide, which can be-or can constitute a part of-a protein.

Transcription is the synthesis of RNA using DNA as a template. It takes place in the nucleus of eukaryotic cells.

Messenger RNA, or mRNA, is the type of RNA produced during transcription. It carries the genetic message of DNA to the protein making machinery of the cell in the cytoplasm.

Translation is the synthesis of polypeptides. Translation takes place in the cytoplasm of eukaryotic cells at ribosomes.

In eukaryotes, transcription results in pre-mRNA, which undergoes RNA editing and processing to yield the final mRNA that participates in translation.

In DNA, the instructions for building a polypeptide chain are written as a series of 3-nucleotide groups; this is called a triplet code, or codon.

In transcription, only one strand of the DNA is transcribed, and it is called the template strand. The mRNA that is produced is said to be complementary to the original DNA strand.

The complementary strand is made up of triplets called codons that are read, or translated, in the 3’ to 5’ direction along the mRNA. Each codon specifies one of the 20 amino acids, which are incorporated into a growing polypeptide strand.

The genetic code is redundant, meaning that more than one codon codes for each of the 20 amino acids. The codons are read based on a consistent reading frame- the groups of 3 must be read in the correct grouping in order for translation to be successful.

The Synthesis and Processing of RNA

The enzyme RNA polymerase separates the two DNA strands and connects the RNA nucleotides as they base pair along the DNA template strand.

The RNA polymerases can add only RNA nucleotides to the 3’ end of the strand, so RNA elongates in the 5’ to 3’ direction.The DNA sequence at which RNA polymerase attaches is called the promoter sequence.

The DNA sequence that signals the end of transcription is called the terminator.

The entire stretch of DNA that is transcribed into mRNA is called a transcription unit.

There are 3 main stages of transcription:

1. Initiation: In prokaryotes, a group of proteins plus RNA polymerase, bound to the promoter region of a DNA sequence, is collectively known as a transcription initiation complex. In eukaryotes, the process of initiation is more complex, but it also involves the binding of RNA polymerase to a promoter sequence.

2. Elongation: RNA polymerase moves along the DNA, continuing to untwist the double helix. RNA nucleotides are continually added to the 3’ end of the growing chain, and as the complex moves down the DNA strand, the double helix re-forms, with the new RNA molecule straggling away with the DNA template.

3. Termination: This occurs after RNA polymerase transcribes a terminator sequence in the DNA, and the transcribed RNA sequence is the actual termination signal.

mRNA Processing:

In eukaryotes, there are a couple of key post-transcription modifications to RNA-the addition of a 5’ cap, and the addition of a poly-A tail.

Another process, called RNA splicing, also takes place in eukaryotic cells. In RNA splicing, large portions of the newly synthesized RNA strand are removed or spliced out. The sections of the mRNA that are spliced out are called introns, and the sections that are left over-and subsequently spliced together by a splicesome- are called exons.

Questions:

1. Transcription is:

a) The actual synthesis of a polypeptide, which occurs under the direction of mRNA.

b) A number of genes give rise to two or more different polypeptides, depending on which segments are treated as exons during RNA synthesis.

c) The synthesis of RNA under the direction of DNA.

2. The sites of translation are:

a) 5' Cap

b) Poly-A tail

c) Ribosomes

d) Codons

3. The DNA sequence where RNA polymerase attaches and initiates transcription is known as the:

a) Transcription Unit

b) Terminator

c) Spliceosome

d) None of the above

4. The 5' end, the end transcribed first, is capped off with a modified form of which nitrogenous base?

a) Thymine

b) Cytosine

c) Uracil

d) Guanine

5. The 5' cap and Poly-A tail share which of the following functions:

a) They facilitate the export of the mature mRNA from the nucleus.

b) They help protect the mRNA from degradation by hydrolyzing enzymes.

c) They help ribosomes attach to the 5' end of the mRNA

d) All of the above

Answers: 1. C, 2. C, 3. E, 4. D, 5. D