From DNA to mRNA (College Board AP® Biology)

Protein Synthesis - An Overview

• All heritable characteristics are coded for in DNA

• Information is taken from the nucleotide base sequence in the DNA to produce an mRNA sequence, this is then converted into the sequence of amino acids in a protein

• Expression of a gene implies the creation of a functional protein which plays a role in determining characteristics

The Gene to Protein Diagram

DNA is the ultimate source of heritable information, and all characteristics are brought about by proteins coded for by DNA

• This process of protein synthesis occurs in two stages:

  • Transcription – DNA is transcribed and an mRNA molecule is produced

    • mRNA is a single stranded RNA molecule that transfers the information in DNA from the nucleus into the cytoplasm

    • mRNA production requires the enzyme RNA polymerase

  • Translation – mRNA (messenger RNA) is translated and an amino acid sequence is produced with the help of specific tRNA molecules

RNA Molecules in Protein Synthesis

Messenger RNA acts as a messenger

• It copies the code on the DNA, in short sections, and delivers it to the ribosomes for translation

• mRNA is a short single-stranded molecule with exposed, unpaired bases

mRNA Molecule Diagram

mRNA carries short sections of the DNA code to the ribosome for translation

Transfer RNA transfers amino acids to the ribosome

• tRNA is single-stranded, but folded with hydrogen bonds between some complementary bases, securing its 'clover' shape

• tRNA has an amino acid binding site which carries amino acids specific to an anticodon on the same tRNA

• The anticodon is complementary to a specific codon on the mRNA

tRNA Molecule Diagram

tRNA transfers amino acids to the ribosome and matches up to the codons using the complementary anticodon base sequence

Ribosomal RNA and protein form the ribosome structure

• rRNA facilitates the binding of mRNA and tRNA to catalyze the formation of peptide bonds during translation

• Ribosomes have three tRNA binding sites and one mRNA binding site

• A ribosome has a large subunit and a small subunit

• The mRNA binding site sits in between the two subunits

Ribosome Structure Diagram

Ribosome subunits are made of rRNA and proteins which facilitate translation

Transcription

The Process of Transcription

• This stage of protein synthesis occurs in the nucleus of the cell

• Part of a DNA molecule unwinds (the hydrogen bonds between the complementary base pairs break)

• This exposes the gene to be transcribed (the gene from which a particular polypeptide will be produced)

• A complimentary copy of the code from the gene is made by building a single-stranded nucleic acid molecule known as mRNA (messenger RNA)

• Free RNA nucleotides pair up (via hydrogen bonds) with their complementary (now exposed) bases on one strand (the template strand) of the ‘unzipped’ DNA molecule

• The sugar/phosphate groups of these RNA nucleotides are then bonded together by the enzyme RNA polymerase to form the sugar/phosphate backbone of the mRNA molecule

• When the gene has been transcribed (when the mRNA molecule is complete), the hydrogen bonds between the mRNA and DNA strands break, and the double-stranded DNA molecule reforms

• The mRNA molecule then leaves the nucleus via a pore in the nuclear envelope

  • This is where the term messenger comes from - the mRNA is despatched, carrying a message, to another part of the cell

  • DNA can't make this journey; it's too big to fit through the pores in the nuclear envelope

The Production of mRNA in Transcription Diagram

DNA is transcribed and an mRNA molecule is produced

Sense and antisense strands

• In the transcription stage of protein synthesis, free RNA nucleotides pair up with the exposed bases on the DNA molecule but only with those bases on one strand of the DNA molecule

• The RNA will have a complementary base sequence to the DNA strand (with the substitution of Thymine with Uracil)

• The strand of the DNA molecule that carries the genetic code is called the sense strand

• The opposite DNA strand is called the antisense strand

  • Or alternatively, the minus strand or noncoding strand

• To get an RNA transcript of the sense strand, the antisense strand is the one that is transcribed to form the mRNA molecule

  • This mRNA molecule will later be translated into an amino acid chain

Reading the Antisense Strand of DNA in Transcription Diagram

The antisense strand of the DNA molecule is the one that is transcribed

Post-Transcriptional Modification

Addition of a poly-A tail

• A poly-A-tail is a long chain of around 100-250 adenine nucleotides

• This sequence is added to the mRNA code at a location marked by a specific base sequence (AAUAAA) found in the mRNA  

• The process is called polyadenylation

• The poly-A tail plays a role in the termination of transcription

  • Causing the RNA polymerase enzyme to detach from the antisense strand and release its transcript

  • The poly-A tail also helps mRNA to exit the nucleus, to prepare it for translation

Addition of a GTP cap

• GTP is guanine triphosphate

• This molecule is added at the 5' end of the mRNA transcript

  • This is the opposite end to the poly-A-tail

• The GTP cap helps to stabilize the mature mRNA's structure

• It is required because transcription happens in the 5' to 3' direction, leaving the 5' end exposed

• There is an exposed free triphosphate group from the first nucleotide built into the chain

• This triphosphate group is replaced with a 'cap', catalyzed by an enzyme

Splicing

• Within eukaryotic genes, there are both coding and noncoding sequences of DNA

  • The coding sequences are called exons and these are the sequences that will eventually be translated into the amino acids that will form the final polypeptide

  • The noncoding sequences are called introns and are not translated (they do not code for any amino acids)

• When transcription of a gene occurs, both the exons and introns are transcribed

• This means the messenger RNA (mRNA) molecule formed also contains exons and introns

  • This RNA molecule is often referred to as primary mRNA or pre-mRNA

• As the introns are not to be translated, they must be removed from the pre-mRNA molecule

• The exons are then all fused together to form a continuous mRNA molecule called mature mRNA that is ready to be translated

• This process is sometimes called ‘splicing’ and is part of the process of posttranscriptional modification (referring to the modification of the RNA molecule after transcription but before translation occurs)

• Splicing ensures that only the coding sections of mRNA are used to form proteins by translation

  • If any introns were included in the mature mRNA, the resulting protein would not be formed properly and may not function as it should

Alternative Splicing

• In certain cases, different combinations of introns and some exons are spliced out of pre-mRNA

  • This can lead to different versions of mature mRNA being produced

  • And in turn, different proteins being expressed

• This is known as alternative splicing

Post Transcriptional Splicing of mRNA Diagram

 The RNA molecule (known as pre-mRNA) produced from the transcription of a gene contains introns that must be removed (to form mature mRNA) before translation can occur