BDS Admission Test Biology Syllabus

What are the three components of a single nucleotide?

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

Name the nitrogenous bases found in DNA.

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

How does RNA differ from DNA in bases and sugar?

RNA contains uracil (U) instead of thymine, and its sugar is ribose instead of deoxyribose.

State the complementary base pairing rules in DNA.

Adenine pairs with Thymine (A-T, two hydrogen bonds); Guanine pairs with Cytosine (G-C, three hydrogen bonds).

Classify the nitrogenous bases into purines and pyrimidines.

Purines (double ring): Adenine and Guanine. Pyrimidines (single ring): Cytosine, Thymine, and Uracil.

What type of bond links nucleotides together in a DNA strand?

A phosphodiester bond, between the phosphate of one nucleotide and the sugar of the next, forming the sugar-phosphate backbone.

What is the overall shape of a DNA molecule, and who proposed it?

A double helix, with two antiparallel strands; proposed by Watson and Crick (1953).

List three structural differences between DNA and RNA.

DNA is double-stranded, has deoxyribose, and uses thymine; RNA is usually single-stranded, has ribose, and uses uracil.

Why is water described as a polar molecule?

Oxygen is more electronegative than hydrogen, creating a partial negative charge on oxygen and partial positive charges on hydrogens, making the molecule dipolar.

What property of water makes it an excellent solvent, and why is this biologically important?

Its polarity allows it to dissolve ionic and polar substances; this makes it the medium for metabolic reactions and for transport of substances in organisms.

Define cohesion and adhesion in water and give a biological role of cohesion.

Cohesion is attraction between water molecules; adhesion is attraction to other surfaces. Cohesion (via hydrogen bonds) enables the transpiration stream / continuous water column in xylem.

Why is water's high specific heat capacity biologically important?

Water absorbs much heat for a small temperature change, so it resists temperature fluctuations and helps organisms maintain a stable internal temperature.

List four key biological roles of water.

Solvent and transport medium, reactant (e.g., in hydrolysis and photosynthesis), temperature regulation (high specific/latent heat), and providing support (turgor/hydrostatic skeleton).

List three structural differences between prokaryotic and eukaryotic cells.

Prokaryotes lack a true (membrane-bound) nucleus, lack membrane-bound organelles, and have 70S ribosomes; eukaryotes have a true nucleus, membrane-bound organelles, and 80S ribosomes.

Where is the genetic material located in a prokaryotic cell?

In the cytoplasm in a region called the nucleoid; the DNA is a single circular chromosome (not enclosed by a membrane).

What is the cell wall of bacteria made of?

Peptidoglycan (murein) — a polymer of sugars and amino acids.

Describe the fluid mosaic model of the cell membrane.

A phospholipid bilayer (fluid) with proteins (mosaic) embedded in or attached to it, plus cholesterol and glycoproteins/glycolipids; components can move laterally.

What is diffusion?

The net movement of particles from a region of higher concentration to a region of lower concentration, down a concentration gradient, until equilibrium; passive (no ATP).

Define osmosis.

The net movement of water molecules from a region of higher water potential to lower water potential across a partially (selectively) permeable membrane; passive.

What is facilitated diffusion?

Passive transport of substances across a membrane through specific carrier or channel proteins, down the concentration gradient, without using ATP.

How does active transport differ from diffusion?

Active transport moves substances against their concentration gradient using carrier proteins and energy (ATP), whereas diffusion is passive and moves down the gradient.

Distinguish between endocytosis and exocytosis.

Endocytosis is bulk uptake of material into the cell by engulfing it in a vesicle; exocytosis is bulk release of material out of the cell via vesicles fusing with the membrane. Both require ATP.

What is the function of mitochondria, and what feature increases their efficiency?

Site of aerobic respiration (ATP production); the inner membrane is folded into cristae to increase surface area, and they contain their own DNA.

State the function of ribosomes and their two locations.

Ribosomes are the site of protein synthesis (translation); found free in the cytoplasm or attached to the rough endoplasmic reticulum.

Compare the functions of rough and smooth endoplasmic reticulum.

Rough ER (with ribosomes) synthesizes and transports proteins; smooth ER (no ribosomes) synthesizes lipids and steroids and helps detoxification.

What is the function of the Golgi apparatus?

Modifies, sorts, packages, and secretes proteins and lipids, and forms lysosomes; it produces secretory vesicles.

What are lysosomes and their function?

Membrane-bound vesicles containing hydrolytic (digestive) enzymes that break down worn-out organelles, waste, and engulfed material (autophagy and intracellular digestion).

What is the role of chloroplasts, and where is chlorophyll located?

Site of photosynthesis in plant cells; chlorophyll is found in the thylakoid membranes (stacked into grana), surrounded by the stroma.

What are the functions of the nucleus?

It controls cell activities, stores genetic material (DNA), and is the site of DNA replication and transcription; the nucleolus within it makes ribosomes.

What is a chromosome, and what is it made of?

A chromosome is a condensed structure of DNA wound around histone proteins (forming chromatin); it carries the genes and becomes visible during cell division.

What is the difference between chromatin and a condensed chromosome?

Chromatin is the loosely coiled, uncondensed DNA-protein complex seen during interphase; a chromosome is the highly condensed, visible form during cell division.

What is an enzyme, chemically?

A biological catalyst, mostly globular protein, that speeds up the rate of a biochemical reaction by lowering activation energy, without being used up or altering the equilibrium.

What is the active site of an enzyme?

A specific region (a cleft or groove) on the enzyme, with a shape complementary to the substrate, where the substrate binds and the reaction is catalyzed.

How do enzymes speed up reactions?

By lowering the activation energy required for the reaction to proceed, allowing it to occur faster at lower temperatures.

Explain the 'lock and key' model of enzyme action.

The substrate (key) has a shape exactly complementary to the rigid active site (lock); only a specific substrate fits, explaining enzyme specificity.

Explain the 'induced fit' model of enzyme action.

The active site is flexible and changes shape slightly to mould around the substrate when it binds, straining bonds and catalyzing the reaction; a refinement of the lock-and-key model.

What is an enzyme-substrate complex?

The temporary intermediate structure formed when the substrate binds to the enzyme's active site, before products are formed and released.

How does temperature affect enzyme activity?

Rate increases with temperature up to an optimum (more kinetic energy/collisions); beyond the optimum, the enzyme denatures and activity falls sharply.

How does pH affect enzyme activity?

Each enzyme has an optimum pH at which activity is maximal; deviations alter charges and bonds in the active site, reducing activity, and extreme pH causes denaturation.

Distinguish between competitive and non-competitive enzyme inhibitors.

Competitive inhibitors resemble the substrate and bind the active site (effect reduced by more substrate); non-competitive inhibitors bind elsewhere (allosteric site), changing the active site's shape (not overcome by more substrate).