IB Biology SL 2025 Exam - Unit 3 Study Guide

Covering topics from Unit 3 that could be on the 2025 test!

What is DNA replication used for?

Reproduction - offspring need copies of the base sequences from the parents

Growth and tissue replacement - each cell needs a full set of the organism’s base sequences. When the cell divides into the two daughter cells, DNA needs to be replicated. This is used to replace cells and for growth

Complementary Base Pairing

Where certain bases pair with one other base

Adenine pairs with thymine (uracil in RNA)

Guanine pairs with cytosine

This ensures that the new cells receiving the DNA from the parent cell are identical, and makes it possible to check the base and replace the incorrect nucleotides

Helicase

Ring-shaped protein that separates the DNA strands to start DNA replication

Helicase breaks the hydrogen bonds between bases to allow the strands to be templates

DNA Polymerase

Assembles new strands of DNA, using the two original strands as templates

The replisome contains separate DNA polymerases for each strand

The polymerase moves along the template strands, adding one nucleotide at a time. Once hydrogen bonds have formed and the correct base is in position, the nucleotides are linked via covalent bonds

Polymerase Chain Reaction

Automated method of DNA replication that can take a small amount of DNA and constantly reproduce to create large amounts of DNA. Requires 3 steps:

Melting: Heating to 95C for 30-60 sec to break hydrogen bonds but not the covalent bonds

Annealing: Cooling to 54C for 30-60 sec to allow two different primers to bind to the DNA strands

Elongation: Heating to 72C for Taq DNA polymerase to bind to the DNA strand adjacent to the primer and assembles new DNA strands in 30-60 sec

Gel Electrophoresis

Process where DNA is separated into certain lengths

Done in a sheet of gel that’s about 3-4 mm thick. Wells on one end are filled with DNA and an electrolyte solution is poured over the gel as it is put into a tank with electrodes at both ends

Once a voltage is applied, the DNA’s negative charges move towards the positive anode

This can be used to determine paternity testing

Transcription

The synthesis of RNA using DNA as a template (only one strand)

Transcription will stop when a sequence is reached that indicates the end of the gene

Role of hydrogen bonding and complementary base pairing in transcription

Similar to DNA replication, complementary base pairing ensures that the assembled strand has the same code as the template strand, since only certain bases make hydrogen bonds with one other base

Stability of DNA Templates

Since RNA polymerase quickly moves through the template strands, the two DNA strands are only briefly vulnerable to mutations

Because they are relatively stable, it lowers the chances of mutations greatly

Transcription as a process required for the expression of genes

Since the proteins that are produced from reading and transcribing the RNA strand, it can determine gene expression as it relies on a certain polypeptide

Transcription is the first process that is required, and it’s the key stage at which this gene expression can be switched on or off

Translation as the synthesis of polypeptides from mRNA

To make a specific polypeptide, amino acids must be linked together in the correct sequence

It can be a chain of hundreds of amino acids

Process of polypeptide synthesis is called translation, and it happens in the cytoplasm

mRNA

mRNA, also called Messenger RNA, has the information for making a polypeptide.

Comes from transcribing DNA and is used in translation

Ribosomes in Translation

Complex structures consisting of a small and large subunit

Small subunit has binding site for mRNA

Large subunit has three binding sites for tRNA and a catalytic site to make peptide bonds between amino acids to create the polypeptide

tRNA

Transfer RNA translates base sequence of mRNA into amino acid sequence of a polypeptide

tRNA molecules have an anticodon at one end consisting of three bases and at the other end an attachment point for the amino acid corresponding to the anticodon

Each type of tRNA molecule has a distinctive shape that is recognized by a dedicated activating enzyme, to attach the correct amino acid onto the tRNA

Complementary base pairing between tRNA and mRNA

The three bases of the anticodon on the tRNA must be complementary to the bases of the mRNA in order for the tRNA to bind to the ribosome and deliver its amino acid

Codon

Sequence of three bases on the mRNA strand

All but three cause a specific amino acid to be added to the growing polypeptide

Anticodon

Sequence of three bases on the tRNA

Each tRNA has a certain amino acid that pairs with the mRNA on the strand

Degeneracy in Genetic Code

Different codons can code for the same amino acid

Universality in Genetic Code

Used by all living organisms and viruses, with only very minor changes

Using the genetic code expressed as a table of mRNA codons

Find the start codon, then work in groups of three

Stepwise movement of the ribosome along mRNA and linkage of amino acids by peptide bonding to the growing polypeptide chain

1. Activating enzyme with an active site that fits the tRNA binds to it and attaches the specific amino acid corresponding to the anticodon

2. The tRNA carrying a single, attached amino acid binds to the A (amino acyl) site on the ribosome, with the anticodon linked to the mRNA via complementary base pairing

3. The amino acid on the tRNA is linked to the growing polypeptide by peptide bonds

4. The tRNA moves from the A to the P (peptidyl) site as the ribosome moves along the mRNA by one codon

5. The polypeptide held by the tRNA is transferred to the tRNA that has arrived at the A site

6. The tRNA moves to the E (exit) site as the ribosome moves along by one codon. This causes the anticodon of the tRNA to separate from the codon on the mRNA and from the ribosome

How do mutations change protein structure?

A mutation is a change in the base sequence. Since a change in the base sequence means a different amino acid is coded, the protein structure will change

Gene mutations as structural changes to genes at the molecular level

It is possible, despite cells having methods to correct errors, for mutations to occur. This always happens randomly

3 types: substitution, insertion and deletion (multiple insertions and deletions can occur)

Consequences of Base Substitutions

Same-sense won’t have any effect, as the same amino acid will be coded for

Non-sense will likely result in the protein not functioning correctly, depending on the role of the protein

Mis-sense may not have much of an effect if the different amino acid has a similar structure and chemical properties to the original one. However, if it’s different, the protein will not be able to function (sickle-cell disease is the most common)

Consequences of Insertions & Deletions

Major insertions or deletions almost always result in the coded-for polypeptide ceasing to function

Minor insertions or deletions can also result in loss of function since they shift the reading frame for every codon from the mutation onwards (called frameshift mutations)

Causes of Gene Mutation

Base-pairing errors and mutagens (such as radiation and some chemical substances

Randomness in Mutation

Mutations cannot be directed by living organisms to achieve an intended outcome. Although there are certain bases that are more susceptible to mutations, the accuracy of correcting errors means that it’s nearly impossible to find a rate of randomness

Consequences of mutation in germ cells and somatic cells

Germ cells give rise to gametes, so genes in germ cells can be passed to offspring. Therefore, a new allele, produced by mutation in a germ cell can be inherited. Mostly it causes a genetic disease

Somatic cells are eliminated when the individual dies, so they mostly have limited consequences. However, if there’s mutation in the proto-oncogenes (responsible for cell cycle and cell division), it can cause cancer

Mutation as a source of genetic variation

Mutation changes the base sequence of a gene, so it changes one allele to another

Since mutation increases the number of different alleles of genes in a population, it increases genetic variation, allowing for species to evolve despite mutations being neutral or harmful

RNA Polymerase

Binds to a site on the DNA at the start of the gene that’s being transcribed

Unwinds the DNA double helix to separate it to two strands

Moves along template strands, positioning bases and linking them together to form a continuous strand of RNA

Detaches the assembled RNA from the template strand and allows DNA double helix to reform

Start Codon on mRNA

AUG (Methionine)

Stop Codons on mRNA

UAA, UAG, UGA

Same-Sense Mutations

When base substitutions change one codon for an amino acid into another codon for the same amino acid

Nonsense Mutations

Change a codon that codes for an amino acid into a stop codon, therefore terminating translation

Mis-Sense Mutations

Alter one amino acid in the sequence of amino acids in the polypeptide

Restriction Enzymes

Cut DNA at specific sequences