DNA and Protein Synthesis

This is the most detailed topic General Science asks about at survey level, and the questions are narrower than the subject. They test base pairing, the two processes and where each happens, and very little else.

DNA structure

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Deoxyribonucleic acid is a double helix: two strands twisted around each other, often described as a twisted ladder.

Each nucleotide has three parts: a phosphate, a sugar (deoxyribose in DNA), and a nitrogenous base.

• The sugar and phosphate form the backbone, the sides of the ladder.
• The bases pair across the middle, the rungs.
• The two strands are held together by hydrogen bonds between the paired bases. They are weak individually, which is exactly what lets the strands be unzipped for copying.

The four bases and how they pair

A-T and C-G. Memorize it in that form and the whole topic opens up.

A useful way to hold it: the two letters that make a curve, C and G, go together. The other two are left over.

Given one strand, you can write the other:

Strand: A T G C C A

Complement: T A C G G T

That question shape appears constantly, and it is pure lookup once you have the pairs.

RNA, and what changes

Ribonucleic acid differs from DNA in three ways, all of which get asked.

Uracil (U) replaces thymine (T) in RNA. So when pairing DNA to RNA, A on the DNA pairs with U on the RNA.

Three kinds of RNA matter:

• Messenger RNA (mRNA) carries the instructions from the nucleus to the ribosome.
• Transfer RNA (tRNA) brings amino acids to the ribosome.
• Ribosomal RNA (rRNA) makes up part of the ribosome itself.

Replication

DNA copies itself before a cell divides.

The helix unzips, the two strands separate, and each acts as a template for a new partner strand built by base pairing. Each new molecule has one old strand and one new one, which is called semiconservative replication.

Three enzymes, one job each, and each is asked:

The names help: helicase works on the helix, and ligase ligates, which means ties together.

Transcription and translation

The two-step route from gene to protein. The order is alphabetical, which is a free mnemonic.

Transcription: DNA to RNA

Where: the nucleus. What: a section of DNA is used as a template to build a strand of mRNA. Enzyme: RNA polymerase.

The mRNA is a copy of the gene written in RNA, and it leaves the nucleus through a pore.

DNA stays in the nucleus. That is why a messenger is needed at all - the instructions cannot leave, so a copy goes instead.

Translation: RNA to protein

Where: the ribosome, in the cytoplasm. What: the mRNA is read three bases at a time, and each triplet specifies an amino acid. tRNA brings the matching amino acid and the chain is assembled.

A codon is three mRNA bases. The matching triplet on the tRNA is an anticodon.

Three bases per amino acid means a sequence of 30 bases codes for 10 amino acids, which is the arithmetic a question of this shape wants.

Every protein starts at the same codon: AUG, the start codon. Three other codons are stop codons that end the chain. With four bases read three at a time there are 64 codons in all, so 61 code for amino acids and 3 say stop.

The amino acids are linked by peptide bonds, and a finished chain is a polypeptide - a protein, or part of one.

Mutations

A mutation is a change in the DNA sequence.

• Substitution swaps one base for another. What that does depends on the codon it lands in:
  • Silent: the new codon still codes for the same amino acid, so nothing changes.
  • Missense: the codon now codes for a different amino acid.
  • Nonsense: the codon becomes a stop codon and the protein is cut short.
• Insertion or deletion adds or removes a base, which shifts how every subsequent triplet is read. This is a frameshift and its effects are usually far more severe, because every codon after the change is altered.

That contrast is the one the test asks. A frameshift is worse than a substitution, because the reading frame moves.

Mutations are the ultimate source of new alleles, which connects this topic to evolution.

What you can skip

The molecular machinery behind replication is a staple of biology courses and almost absent from the questions. Across all 2,104 General Science questions in our bank:

• Leading and lagging strands, and Okazaki fragments: zero mentions. Knowing that ligase joins fragments is as deep as it goes.
• Introns, exons, promoters and splicing: introns, exons and promoters never appear, and splicing appears once.
• Who discovered what. No question is answered by Watson, Crick, Franklin or Chargaff; Crick turns up once, as a wrong choice. The only discoverer the bank wants is Mendel, for inheritance, and that is in the genetics lesson.

Where people lose points

Pairing A with G or C with T. A-T, C-G.

Forgetting that RNA uses uracil.

Putting translation in the nucleus. Transcription is in the nucleus, translation is at the ribosome.

Saying DNA leaves the nucleus. mRNA does.

Saying a codon is three amino acids. It is three bases, specifying one amino acid.

Confusing replication with transcription. Replication copies DNA to DNA; transcription copies DNA to RNA.

Work one in under a minute

A DNA template strand reads T A C G G A. What is the corresponding mRNA sequence, and how many amino acids does it specify?

Pair each base, using U in place of T:

• T pairs with A
• A pairs with U
• C pairs with G
• G pairs with C
• G pairs with C
• A pairs with U

mRNA: A U G C C U.

Six bases at three per codon is two codons, so two amino acids.

Where this leads

The DNA here is what the chromosomes in the previous lesson are made of, and mutations are the source of the variation that evolution acts on.