Transcription & RNA Processing (College Board AP® Biology): Study Guide

mRNA, tRNA & rRNA

• In order for proteins to be produced the DNA base sequence needs to be converted, first into an mRNA base sequence, and then into a sequence of amino acids

• This process involves several types of RNA molecules, the function of which is determined by their base sequence and the resulting structure

mRNA: messenger RNA

• Single-stranded molecules of RNA

• The mRNA sequence is determined by the DNA sequence from which it is converted

• The role of mRNA is to carry information from DNA to ribosomes in the form of codons

tRNA: transfer RNA

• Single-stranded molecules of RNA that fold into a three-dimensional structure

  • The folded regions are held together by hydrogen bonds

• One end of each tRNA molecule binds to a specific amino acid

• The other end has an anti-codon sequence that base pairs with the corresponding mRNA codon

• tRNA is involved in translation, during which the mRNA sequence is converted to an amino acid chain

rRNA: ribosomal RNA

• Single-stranded molecules of RNA that also fold into a three-dimensional structure

• Combines with proteins to form ribosomes

Transcription

• Transcription is the process of converting information encoded in a DNA template strand into a molecule of mRNA

  • The DNA strand that acts as the template strand can also be referred to as the:

    • noncoding strand

    • minus strand

    • antisense strand

• The process of transcription is as follows:

  1. Part of a DNA molecule unwinds

     • This exposes the gene to be transcribed

  2. Free RNA nucleotides align with their complementary exposed bases on the template strand of the unzipped DNA

     • Selection of which DNA strand serves as the template strand depends on the gene being transcribed

  3. RNA polymerase enzyme joins the adjacent RNA nucleotides to form the sugar-phosphate backbone of the mRNA

     • RNA polymerase synthesizes mRNA in the 5’ to 3’ direction by reading the template strand in the 3’ to 5’ direction.

  4. The mRNA molecule detaches from the template strand and leaves the nucleus via a pore in the nuclear envelope

mRNA modifications

• In eukaryotic cells, the mRNA produced during transcription undergoes a series of enzyme-regulated modifications

• Examples of mRNA modifications include:

  • addition of a poly-A tail

    • A poly-A-tail is a long chain of adenine nucleotides added to the 3' end of the mRNA molecule

    • The poly-A tail increases the length of time that the mRNA can spend in the cytosol before it degrades

  • addition of a GTP cap

    • Guanine triphosphate (GTP) is added at the 5' end of the new mRNA

    • The GTP cap allows the mRNA to leave the nucleus and helps to stabilize its structure

  • removal of introns

    • Introns are noncoding regions of DNA present in eukaryotic cells

    • These introns are removed from pre-mRNA, leaving behind only the coding regions, known as exons

    • The resulting mRNA molecule is known as mature mRNA

    • The exons can be spliced together in different combinations, meaning that it is possible to produce more than one mature mRNA molecule from a single gene; this is known as alternative splicing