TEAS Biology study guide

What this topic actually asks of you

Biology is one of the four sub-content areas of the ATI TEAS Science section. ATI publishes its size: it is 6% of the exam and 9 of the 44 scored science questions. The whole Science section is 50 items (44 scored, 6 unscored) and is timed separately — 60 minutes.

Two things follow, and they shape how to study it.

• Most of it is recall; a little of it is a ratio. The bulk of this topic is knowing the pieces — what each organelle does, which base pairs with which, what a codon is, what the monomer of each macromolecule is. A smaller part is a short genetic computation: reading a monohybrid cross off a Punnett square and turning it into a fraction, a percent or a count.
• The arithmetic is small; the vocabulary is where the points go. Every number here is small. The misses come from the words — the four macromolecule classes are easy to mix up, transcription is easy to confuse with translation, and "genotype" is easy to confuse with "phenotype".

Our practice items are single-answer and text-only. The real ATI TEAS also uses other item types: ATI's own content outline lists multiple-choice, multiple-select, fill-in-the-blank numeric, hot spot and ordered-response items. Elk Ridge Academy's practice items are single-answer only, and they carry no pictures or diagrams — a cross or a set of data is written as text — so treat the method, not any one format, as the thing you are drilling.

ATI's biology objectives. ATI's Content Outline names what this topic measures, and every practice item below maps to one of them: S.2.1 cell structure, function and organization · S.2.2 the relationship between genetic material and the structure of proteins · S.2.3 the concepts underlying Mendel's laws of inheritance · S.2.4 the structure and function of the basic macromolecules · S.2.5 the role of microorganisms in disease.

The eight procedures

Every practice item in this topic is one of these eight. Each is worked out in full below — the rule, the steps, and the trap it sets. (The ATI objective each one serves is in brackets.)

1. cell structure — the parts of a cell and their jobs [S.2.1]. Match each organelle or structure to the one thing it does. This is the largest recall family in the topic.
2. cell organization — the levels of organization [S.2.1]. Put the levels in order, from the single cell up to the organ system, and say what each level adds.
3. dna rna and proteins — the central dogma [S.2.2]. Base pairing, the three processes (replication, transcription, translation), where each happens, how the DNA sequence sets the protein, and what a mutation does to that chain.
4. mendels laws — the vocabulary of inheritance, and Mendel's three laws [S.2.3]. Dominant, recessive, genotype, phenotype, homozygous, heterozygous, allele — plus the law of segregation, the principle of dominance, and the law of independent assortment.
5. punnett squares — build the cross and count the boxes [S.2.3]. Set the parent gametes on the top and the side, fill the four inner boxes, and read off the genotypes.
6. cross probability — turn the boxes into a fraction, a percent or a count [S.2.3]. How many of four boxes, what that is as a percent, and how many of a real population that is.
7. macromolecules — the four classes and their monomers [S.2.4]. Monomer, a function and an example for carbohydrates, lipids, proteins and nucleic acids.
8. microorganisms and disease — the pathogens and how they spread and are treated [S.2.5]. Bacteria, viruses, fungi and protozoa: structure, spread, and what does (and does not) treat each.

Reference tables: the facts to know cold

Recall items are drawn from these rows, one fact per row. Read the whole row — the questions ask about the row, not just the name in it.

Table A — The parts of a cell. A cell is the smallest living unit. A eukaryotic cell (plants, animals, fungi) has a nucleus and membrane-bound organelles; a prokaryotic cell (bacteria) has none.

Table B — DNA, RNA and the central dogma. DNA holds the instructions; RNA carries them; proteins do the work. The usual order is DNA → RNA → protein.

Table C — The four macromolecule classes. "Macro" means big: carbohydrates, proteins and nucleic acids are polymers, long chains of small repeating units (monomers). Lipids are the exception — they are not chains, but a fat is built from smaller parts.

Table D — The levels of organization, smallest to largest. Each level is built from the one before it.

Table E — Microorganisms and disease. A pathogen is any germ that causes disease. Structure decides which treatment works: an antibiotic attacks parts that only bacterial cells have, so it works on bacteria and not on viruses.

Table F — How an infection spreads. A disease spreads when a pathogen leaves one host and reaches another. The route decides what prevention works — you break the chain at the route, not at the treatment. (Routes and examples: CDC, How Infections Spread.)

Prevention follows the route. Wash your hands (breaks the surface and contact routes), get vaccinated (trains your immune system before you meet the pathogen), and drink clean water and handle food safely (breaks the food-and-water route). An antibiotic is a treatment for someone already infected — it can make them less contagious — but it does not protect people who have not caught the infection the way hand-washing, clean water and vaccines do.

The vocabulary of inheritance

Eight words carry this whole topic. Learn them as pairs — the items test whether you can tell the pair apart.

• Allele — one version of a gene. An organism carries two, one from each parent.
• Dominant — the allele whose trait shows whenever it is present (written with a capital letter, T).
• Recessive — the allele whose trait shows only when both copies are recessive (written lowercase, t).
• Genotype — the two alleles an organism carries, written as a pair of letters: TT, Tt or tt.
• Phenotype — the trait the organism actually shows: tall or short.
• Homozygous — the two alleles are the same: TT or tt.
• Heterozygous — the two alleles are different: Tt.
• Gamete — a sex cell, which carries one allele of each gene, so a Tt parent makes T and t gametes.

Mendel's three laws

Mendel worked out three ideas from his pea plants. An entrance exam asks you to name the law that a statement describes, so learn each one in one sentence.

• The law of segregation. The two alleles of a gene separate when gametes form. Each gamete carries only one of the two. A Tt plant makes gametes T and t — never Tt. This is why a cross puts one letter from each parent into every box.
• The principle of dominance. When two different alleles are present for a gene with complete dominance, the dominant allele's trait is the one that shows, and the recessive allele is masked. (Not every trait is Mendelian, and dominance is rarely perfectly complete — "for a single gene with complete dominance" is the level an entrance exam uses.)
• The law of independent assortment. The genes for different traits (on different chromosomes) are sorted into gametes independently of one another — which allele a gamete gets for one gene does not decide the allele it gets for another. (One gene, one cross is all this topic computes; state the law, do not draw a two-gene cross.)

The three are easy to swap. Segregation is about the two alleles of one gene separating. Independent assortment is about two different genes not influencing each other. Dominance is about which of two different alleles shows.

From DNA to the protein, and what a mutation does

The DNA sequence is the instruction; the protein is the product. The chain ordinarily runs one way:

1. The DNA sequence sets the mRNA sequence. Replication copies the DNA → DNA. Transcription copies one gene's DNA → mRNA, using the base rule (C pairs with G, A pairs with U on the RNA side).
2. The mRNA codons set the amino acid order. Translation reads the mRNA at the ribosome, three bases at a time, and each codon names one amino acid.
3. The amino acid order sets the protein's shape and job. The order is what makes the protein fold into the shape that lets it do its work — an enzyme's active site, a channel's pore.

So a mutation — a change in the DNA sequence — can change the mRNA codon, which can change an amino acid, which can change the protein. Because the protein does the cell's work, a change in the DNA can change the trait. This is the ordinary direction of the central dogma: DNA → RNA → protein → the trait.

A four-step method for a genetic cross

The two computed procedures (punnett squares, cross probability) both run on the same four steps. Do them in order and the count falls out.

1. Write each parent's genotype. Two letters per parent — for example, a heterozygous tall plant is Tt. Capital = dominant allele, lowercase = recessive.
2. Split each parent into its gametes. Each gamete gets one letter. A Tt parent makes two kinds of gamete, T and t; a TT parent makes only T.
3. Build the square and fill the boxes. Put one parent's gametes across the top and the other's down the side, then combine across each row and down each column. A 2×2 grid gives four boxes.
4. Count, then scale. Count the boxes that match what was asked (a genotype or a phenotype), write it out of 4, then turn that into a percent (× 25) or a count in a population (× the total, × ¼, or × ½).

The one rule that keeps this honest: the box count is always out of 4 for a single-gene cross. "1 out of 4" is 25%, "3 out of 4" is 75%, "2 out of 4" is 50%. Get the fraction right first; the percent and the count are just that fraction scaled.

The eight worked examples

1. Cell structure — name the organelle by its job

A protein has just been built at the ribosome and now needs to leave the cell. Which organelle finishes it and packs it for shipping?

1. Read the verb: the question asks for the part that finishes and packs, not the one that builds the protein and not the one that carries it through the cell.
2. The ribosome builds a protein; the endoplasmic reticulum carries it through the cell.
3. Neither of those finishes or ships it — the part that does is the Golgi apparatus.

The trap: the ribosome builds proteins and the ER moves them, but only the Golgi packages and ships. Read which verb the question used.

2. Cell organization — put the levels in order

Which of these is the correct order from smallest to largest: organ, tissue, cell?

1. Each level of organization is built from the one before it, so the question is which level is built from which.
2. A cell is the smallest living unit; a group of similar cells is a tissue; two or more tissues working together are an organ. Nothing here is built out of an organ.
3. So the order from smallest to largest is cell → tissue → organ.

The trap: swapping tissue and organ. Cells make tissue; tissues make organs — never the reverse.

3. DNA, RNA and proteins — transcribe a strand

A DNA template reads G C A. What is the matching mRNA codon?

1. Transcription copies the DNA template base by base, using the C–G pair and the A–U pair (RNA has no T).
2. Take the bases in order: G pairs with C, C pairs with G, and A pairs with U.
3. Reading left to right, G C A transcribes to C G U.

The trap: writing T where RNA has U. RNA uses uracil in place of thymine; substitute U for T every time.

4. Mendel's laws — genotype or phenotype

Two pea plants are both tall. One is TT, one is Tt. Which word describes "TT", and which describes "tall"?

1. Ask which of the two names is something you can see and which is only letters.
2. The letter pair (TT, Tt) is the two alleles an organism carries — the genotype.
3. The trait you can observe (tall) is the phenotype.

The trap: calling both "genotype" because both are written as letters. Ask: can I see it? If yes, it is the phenotype.

5. Punnett squares — build the cross and count the boxes

A heterozygous tall plant (Tt) is crossed with a short plant (tt). What genotypes appear in the four boxes, and in what ratio?

1. By segregation each parent contributes one allele per gamete: the tall parent can give T or t, and the short parent can give only t.
2. Fill the 2 × 2 grid: Tt, tt, Tt, tt.
3. Count them: 2 Tt and 2 tt = 1 : 1.

The trap: reporting the Tt × Tt ratios out of habit. Two heterozygotes give 1 TT : 2 Tt : 1 tt — the genotype ratio 1 : 2 : 1 and the phenotype split 3 tall : 1 short — and when one parent is tt both of those change. Read who the parents are before you count.

6. Cross probability — scale the boxes to a population

A Tt × tt cross produces 200 offspring. About how many are expected to be short?

1. Only the tt boxes show the recessive (short) trait — 2 of the 4 boxes.
2. Write that as a fraction: 2/4 = 1/2.
3. Scale it to the population: 1/2 × 200 = 100 short offspring.

The trap: scaling the wrong fraction. Two heterozygotes give 1/4 short (50 of 200) and 3/4 tall (150 of 200); with one tt parent the split is 1/2. Read the cross before you scale it.

7. Macromolecules — name the monomer

A starch molecule is a large molecule built from many small repeating units. What is one of those units called?

1. The item asks for the unit the large molecule is built from — its monomer, not the class it belongs to.
2. Each class has its own monomer: carbohydrates are chains of monosaccharides, proteins are chains of amino acids (20 kinds) and nucleic acids are chains of nucleotides.
3. Starch is a carbohydrate, so its monomer is the monosaccharide.

The trap: naming another class's monomer. "Amino acid" is the protein monomer and "nucleotide" is the nucleic-acid monomer — neither builds a starch.

8. Microorganisms and disease — match the treatment to the pathogen

A patient has a bacterial infection. Which treatment is appropriate?

1. An antibiotic attacks structures that only bacterial cells have, so the question is which pathogen is a bacterium.
2. Of the four pathogen types, only bacteria have those structures; fungi and protozoa are living cells of a different (eukaryotic) kind, and a virus is not a cell at all.
3. So a bacterial infection calls for an antibiotic; a virus is not a cell and has none of the bacterial structures an antibiotic acts on, so an antibiotic has no effect.

The trap: reaching for an antibiotic for a virus (a cold, the flu). A virus is not a living cell and has none of the bacterial structures the drug acts on, so antibiotics do nothing; viral illness is prevented by vaccines and only sometimes eased by antivirals. Treatment follows structure.

The traps, collected

Most wrong answers in this topic's practice items are one of these.

• Organelles swapped: gave an organelle a job that belongs to another (ribosome vs Golgi vs ER; mitochondrion vs chloroplast).
• Plant-only part on an animal cell: put a cell wall or a chloroplast in an animal cell.
• Prokaryote given a nucleus: gave a bacterium a nucleus or a membrane-bound organelle.
• Levels out of order: put an organ before a tissue, or a cell after one of them.
• Thymine in RNA: wrote T where RNA uses U.
• Transcription vs translation: put protein synthesis in the nucleus, or mRNA copying at the ribosome.
• Genotype vs phenotype: called the visible trait the genotype (or the reverse).
• Dominant vs recessive read backwards: called the recessive allele the one that shows.
• Homozygous vs heterozygous swapped: called Tt homozygous or TT heterozygous.
• Genotype ratio for phenotype ratio: reported 1:2:1 when 3:1 was asked (or the reverse).
• The wrong count in a population: reported the dominant-outcome number when the recessive was asked, or used a 1/2 split where it is 1/4.
• One macromolecule class's monomer on another: nucleic-acid nucleotide on a protein, or a carbohydrate's monosaccharide on a lipid.
• Antibiotic for a virus: a very common microbiology error — treatment follows structure, and a virus lacks the bacterial structures the drug acts on.
• A helpful microbe called a pathogen: not every bacterium causes disease.
• The three laws swapped: called independent assortment "segregation", or described dominance as the law that separates alleles.
• The wrong transmission route: matched a vector-borne disease (malaria) to food or water, or a coughed-out disease to an insect bite.
• A mutation acting on the protein directly: skipped the mRNA — a DNA change moves through mRNA to the amino acid order before it reaches the protein.

Answer-choice strategies: three moves for when the word will not come

1. Read the verb in the stem. "Packages" → Golgi. "Builds" → ribosome. "Stores the DNA" → nucleus. "Releases energy" → mitochondrion. The question almost always gives the organelle's job before it asks for the name.

2. Say the base rule out loud. DNA pairs A–T and C–G; RNA swaps T for U. When a transcribed strand is asked for, cross out every T and write a U in its place before you choose.

3. Get the fraction before the percent. In any cross question, count the boxes first and write the fraction out of 4. Only then multiply — × 25 for a percent, × the population for a count. A choice that is not a multiple of 25 (as a percent) or of the population's quarters (as a count) is gone before you finish.

How to practice this

• Work the free Biology practice set with a pencil, and write the cross or the base rule out each time. The setup is where these items set their traps.
• Drill the procedures you get wrong, not the ones you already know. Sorting your errors by procedure beats counting them.
• Read the explanation on every practice item, including the ones you got right. A right answer reached with the wrong reason comes back on test day.
• Re-attempt what you missed a day later, not immediately: a day later measures whether the fact stuck.

Where to go next

• Next topic — Chemistry
• The rest of the section — Human anatomy and physiology, Chemistry, and Scientific reasoning.

This topic sits inside the Science section and carries ATI's published 6% of the exam and 9 of the section's 44 scored items — the blueprint's own split.