RNA

How does the information move from the nucleus, where the DNA is located, to the cytoplasm, where the ribosomes are?

RNA, the other nucleic acid, that's how. Specifically mRNA. RNA, the middle player in the central dogma. This image is an abstract representation of tRNA. Without tRNA, mRNA, and rRNA, proteins cannot be made.

RNA

DNA alone cannot ‘‘tell’’ your cells how to make proteins. It needs the help of RNA, ribonucleic acid, the other main player in the central dogma of molecular biology. Remember, DNA ‘‘lives’’ in the nucleus, but proteins are made on the ribosomes in the cytoplasm. How does the genetic information get from the nucleus to the cytoplasm? RNA is the answer.

RNA vs. DNA

RNA, like DNA, is a nucleic acid. However, RNA differs from DNA in several ways. In addition to being smaller than DNA, RNA also

• consists of one nucleotide chain instead of two,

• contains the nitrogen base uracil (U) instead of thymine,

• contains the sugar ribose instead of deoxyribose.

Types of RNA

There are three main types of RNA, all of which are involved in making proteins.

1. Messenger RNA (mRNA) copies the genetic instructions from DNA in the nucleus, and carries the instructions to the cytoplasm.

2. Ribosomal RNA (rRNA) helps form ribosomes, the organelle where proteins are assembled.

3. Transfer RNA (tRNA) brings amino acids to ribosomes, where they are joined together to form proteins.

Shown are the three types of RNA and their roles: (1) mRNA contains the genetic message, (2) tRNA transfers the amino acids to the ribosome, (3) rRNA is the main component of the ribosome.

Transcription

How does a cell use the information in its DNA?

To transcribe means ‘‘to paraphrase or summarize in writing’’. The information in DNA is transcribed - or summarized - into a smaller version - RNA - that can be used by the cell. This process is called transcription.

Transcription

The process in which cells make proteins is called protein synthesis. It actually consists of two processes: transcription and translation. Transcription takes place in the nucleus of the eukaryotic cells. It uses DNA as a template to make an RNA molecule. RNA then leaves the nucleus and goes to a ribosome in the cytoplasm, where translation occurs. Translation reads the genetic code in mRNA and makes a protein.

Transcription is the first part of the central dogma of molecular biology: DNA → RNA. It is the transfer of genetic instructions in DNA to messenger RNA (mRNA). During transcription, a strand of mRNA is made that is complementary to a strand of DNA.

The concept of complementarity is related to the base-pairing rules discussed during the unit on DNA. In DNA, complementarity can be described as, adenine binds with thymine and guanine binds with cytosine. However, RNA does not contain thymine but is "replaced" with uracil (U). Uracil behaves in a similar fashion to thymine, meaning that it will also bind with adenine. Therefore, in RNA, complementarity can be described as, adenine binds with uracil and guanine binds with cytosine.

Steps of Transcription

Transcription takes place in three steps: initiation, elongation, and termination. The steps are illustrated in the Figure below.

1. Initiation is the beginning of transcription. It occurs when the enzyme RNA polymerase binds to a region of a gene called the promoter. This signals the DNA to unwind so the enzyme can ‘‘read’’ the bases in one of the DNA strands. The enzyme is now ready to make a strand of mRNA with a complementary sequence of bases.

2. Elongation is the addition of nucleotides to the mRNA strand. RNA polymerase reads the unwound DNA strand and builds the mRNA molecule using complementary base pairs. There is a brief time during this process when the newly formed RNA is bound to the unwound DNA. During this process, adenine (A) in the DNA binds to uracil (U) in the RNA.

3. Termination is the ending of transcription and occurs when RNA polymerase crosses a stop (termination) sequence in the gene. The mRNA strand is complete, and it detaches from DNA.

Processing mRNA after Transcription

In eukaryotes, the new mRNA is not yet ready for translation. It must go through additional processing before it leaves the nucleus. This may include splicing, editing, and polyadenylation. These processes modify the mRNA in various ways. Such modifications allow a single gene to be used to make more than one protein.

• Splicing removes introns from mRNA (see the Figure below). Introns are regions that do not code for proteins. The remaining mRNA consists only of regions that do code for proteins, which are called exons. Ribonucleoproteins are nucleoproteins that contain RNA. Small nuclear ribonucleoproteins are involved in pre-mRNA splicing.

• Editing changes some of the nucleotides in mRNA. For example, the human protein APOB, which helps transport lipids in the blood, has two different forms because of editing. One form is smaller than the other because editing adds a premature stop signal in the mRNA.

• Polyadenylation adds a string of nucleotides to the mRNA strand's end, consisting of a string of As (adenine bases). It signals the end of mRNA. It is also involved in exporting mRNA from the nucleus. In addition, the tail protects mRNA from enzymes that might break it down. This nucleotide string is often called a poly-A tail because "poly" refers to "many."