RNA (Ribonucleic Acid)
RNA is the other nucleic acid. It differs from DNA in three major ways:
- The sugar is ribose instead of deoxyribose
- There is only one strand instead of two
- RNA has uracil (U) instead of thymine. So, the base pairs in RNA are cytosine with guanine and adenine with uracil.
In a prokaryotic cell (one with no internal membrane-bound organelles like a bacterium), both DNA and RNA are found in the cytoplasm. In a eukaryotic cell (one with internal membrane-bound organelles, like humans), RNA can be found in the nucleus and cytoplasm, while DNA is only found in the nucleus.
Building a Protein: Transcription
Building proteins is very much like building a house:
- The master blueprint is DNA, which contains all of the information to build the new protein (house).
- The working copy of the master blueprint is called messenger RNA (mRNA), which is copied from DNA.
- The construction site is either the cytoplasm in a prokaryote or the endoplasmic reticulum (ER) in a eukaryote.
- The building materials are amino acids.
- The construction workers are ribosomes and transfer RNA molecules.
Let's look at each phase of the new construction more closely.
In a eukaryote, DNA never leaves the nucleus, so its information must be copied. This copying process is called transcription and the copy is mRNA. Transcription takes place in the cytoplasm (prokaryote) or in the nucleus (eukaryote). The transcription is performed by an enzyme called RNA polymerase. To make mRNA, RNA polymerase:
- Binds to the DNA strand at a specific sequence of the gene called a promoter
- Unwinds and unlinks the two strands of DNA
- Uses one of the DNA strands as a guide or template
- Matches new nucleotides with their complements on the DNA strand (G with C, A with U -- remember that RNA has uracil (U) instead of thymine (T))
- Binds these new RNA nucleotides together to form a complementary copy of the DNA strand (mRNA)
- Stops when it encounters a termination sequence of bases (stop codon)
mRNA is happy to live in a single-stranded state (as opposed to DNA's desire to form complementary double-stranded helixes). In prokaryotes, all of the nucleotides in the mRNA are part of codons for the new protein. However, in eukaryotes only, there are extra sequences in the DNA and mRNA that don't code for proteins called introns. This mRNA is then further processed:
- Introns get cut out
- The coding sequences get spliced together
- A special nucleotide "cap" gets added to one end
- A long tail consisting of 100 to 200 adenine nucleotides is added to the other end
No one knows why this processing occurs in eukaryotes. Finally, at any one moment, many genes are being transcribed simultaneously according to the cell's needs for specific proteins.
The working copy of the blueprint (mRNA) must now go the construction site where the workers will build the new protein. If the cell is a prokaryote such as an E. coli bacterium, then the site is the cytoplasm. If the cell is a eukaryote, such as a human cell, then the mRNA leaves the nucleus through large holes in the nuclear membrane (nuclear pores) and goes to the endoplasmic reticulum (ER).
Next, we'll learn about translation -- the assembly process.