HEC USAT Biology Syllabus

During which stage of meiosis does crossing over occur?

Prophase I, when homologous chromosomes pair to form bivalents (tetrads) and exchange segments at chiasmata.

What are the three main classes of carbohydrates based on the number of sugar units?

Monosaccharides (single sugar), disaccharides (two sugars), and polysaccharides (many sugars).

Give one example each of a storage polysaccharide in plants and in animals.

Starch is the storage polysaccharide in plants; glycogen is the storage polysaccharide in animals.

What type of bond links two monosaccharides together, and what is released?

A glycosidic bond, formed by a condensation (dehydration) reaction that releases one molecule of water.

What are the building blocks (monomers) of proteins, and what bond joins them?

Amino acids are the monomers; they are joined by peptide bonds formed via condensation reactions.

Distinguish between saturated and unsaturated fatty acids.

Saturated fatty acids have no carbon-carbon double bonds (single bonds only); unsaturated fatty acids contain one or more carbon-carbon double bonds.

What are the components of a triglyceride (fat)?

One molecule of glycerol bonded to three fatty acid chains by ester bonds.

Name the four levels of protein structure.

Primary (amino acid sequence), secondary (alpha-helix/beta-pleated sheet), tertiary (3D folding), and quaternary (multiple polypeptide subunits).

What are the two types of nucleic acids and their basic monomer?

DNA and RNA; their monomer is the nucleotide.

What three components make up a nucleotide?

A pentose sugar, a nitrogenous base, and a phosphate group.

Name the nitrogenous bases found in DNA.

Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

Which nitrogenous base in RNA replaces thymine found in DNA?

Uracil (U) replaces thymine in RNA.

Classify the DNA/RNA bases into purines and pyrimidines.

Purines are Adenine and Guanine (double-ring); pyrimidines are Cytosine, Thymine, and Uracil (single-ring).

What is an enzyme and how does it affect a chemical reaction?

An enzyme is a biological catalyst (usually a protein) that speeds up reactions by lowering the activation energy without being consumed.

What does the 'lock and key' model of enzyme action describe?

It describes the active site of an enzyme having a specific shape that is exactly complementary to its substrate, like a key fitting a lock.

How does a competitive inhibitor differ from a non-competitive inhibitor?

A competitive inhibitor binds the active site and competes with substrate; a non-competitive inhibitor binds a different (allosteric) site, changing the enzyme's shape.

What generally happens to enzyme activity above its optimum temperature?

The enzyme denatures—its tertiary structure and active site shape are lost—so activity sharply decreases.

In a monohybrid cross between two heterozygous parents (Aa x Aa), what is the genotypic and phenotypic ratio of offspring?

Genotypic ratio 1:2:1 (AA:Aa:aa); phenotypic ratio 3:1 (dominant:recessive).

State Mendel's Law of Segregation.

During gamete formation, the two alleles of each gene separate so that each gamete carries only one allele of the pair.

State Mendel's Law of Independent Assortment.

Alleles of different genes assort independently of one another during gamete formation (applies to genes on different chromosomes).

What is the phenotypic ratio of offspring in a dihybrid cross between two heterozygotes (AaBb x AaBb)?

9:3:3:1.

Define the terms homozygous and heterozygous.

Homozygous means having two identical alleles for a gene (e.g., AA or aa); heterozygous means having two different alleles (e.g., Aa).

According to Chargaff's rule, what are the base-pairing ratios in DNA?

Adenine pairs with Thymine and Guanine pairs with Cytosine, so amount of A = T and G = C (A+G = T+C).

How many hydrogen bonds form between A-T and between G-C base pairs?

Adenine-Thymine form 2 hydrogen bonds; Guanine-Cytosine form 3 hydrogen bonds.

Why is DNA replication described as semi-conservative?

Each new DNA molecule consists of one original (parental) strand and one newly synthesized strand.

What is the role of DNA polymerase and DNA ligase in replication?

DNA polymerase adds new nucleotides to the growing strand (5' to 3'); DNA ligase joins Okazaki fragments on the lagging strand.

What are the two main stages of protein synthesis and where does each occur?

Transcription (DNA to mRNA) occurs in the nucleus; translation (mRNA to protein) occurs at the ribosomes in the cytoplasm.

What is a codon and how many nucleotides does it contain?

A codon is a sequence of three nucleotides on mRNA that codes for one specific amino acid (or a start/stop signal).

What is the function of tRNA in translation?

Transfer RNA carries specific amino acids to the ribosome and matches its anticodon to the complementary mRNA codon.

In the digestive system, which enzyme begins starch digestion and where?

Salivary amylase (ptyalin) begins starch digestion in the mouth.

What is the main site of nutrient absorption in the digestive system, and what feature increases its surface area?

The small intestine; villi and microvilli greatly increase its absorptive surface area.

What is the role of the alveoli in the respiratory system?

Alveoli are the site of gas exchange, where oxygen diffuses into the blood and carbon dioxide diffuses out, due to their thin walls and large surface area.

Trace the path of blood through the four chambers of the heart starting from the body.

Deoxygenated blood enters the right atrium, goes to the right ventricle, to the lungs, returns oxygenated to the left atrium, then the left ventricle pumps it to the body.

Compare arteries and veins in terms of wall thickness and blood pressure.

Arteries have thick, muscular, elastic walls and carry blood at high pressure; veins have thinner walls, valves, and carry blood at low pressure.

What is the function of hemoglobin in red blood cells?

Hemoglobin binds and transports oxygen (as oxyhemoglobin) from the lungs to tissues, and helps carry carbon dioxide back to the lungs.

What are the three main parts of a neuron?

Dendrites (receive impulses), the cell body (soma, contains the nucleus), and the axon (transmits impulses away).

What is the function of the myelin sheath on a neuron?

It insulates the axon and speeds up the conduction of nerve impulses through saltatory conduction at the nodes of Ranvier.

What is the key difference between the nervous system and the endocrine system in coordinating responses?

The nervous system uses electrical impulses for fast, short-lived, localized responses; the endocrine system uses hormones in the blood for slower, longer-lasting, widespread responses.

Which hormone lowers blood glucose, and which gland secretes it?

Insulin lowers blood glucose; it is secreted by the beta cells of the islets of Langerhans in the pancreas.