Notes, summaries, assignments, exams, and problems for Biology

Sexual Reproduction Basics

The male and female germ cells are called gametes. They have 23 pairs of chromosomes. This means 46 chromosomes. The chromosomes that make up each pair are called homologous. The sex chromosomes are referred to by the letters X and Y. The other 22 pairs of chromosomes are autosomes.

The gonads are represented by the testes in males and ovaries in females.

Male Reproductive System

The Scrotum

It is a membranous sac that hangs from the root of the penis. From the outside, it appears formed by a layer of skin, divided into two parts (right and left) by a raphe. The interior is divided by a membranous septum and a muscle called the tunica dartos muscle.

The Testicles

They are ovoid structures that are the male gonads, weighing... Continue reading "Male Reproductive Anatomy and Function" »

Respiratory System

The respiratory system is formed by the ducts through which air circulates, and by small cavities called alveoli, which are capable of entering the bloodstream. At the same time, blood circulates CO2 through the tubes of the respiratory system.

Upper Respiratory Tract

• Nose and nostrils: Two cavities located in the center of the face. Function: To warm and moisten the air.
• Pharynx: Connects the mouth with the larynx and nasal passages. It also communicates through the auditory tube. It forms part of the respiratory and digestive tracts.
• Larynx: Formed by cartilage and bone, it connects the pharynx with the trachea. It contains the vocal cords, which vibrate with airflow to produce the voice.

Lower Respiratory Tract

• Trachea: A tube

Cell Adhesion Molecules (CAMs): Types and Functions

Structurally, there are five families of CAMs: Cadherins, Immunoglobulin superfamily, Integrins, Selectins, and Proteoglycans.

Cadherins

Cadherins are monomeric molecules, which are receptors that bind *homophilically* (to identical cadherins) or *heterotypically* (to different cadherins) expressed on cells for cell-cell contact. Their action is calcium and temperature-dependent. A cell can express more than one type of cadherin. At least twelve types are known, but the most studied are:

• E-cadherin: Present in epithelia of various tissues, such as liver and kidney.
• N-cadherin: Found in neural tissue and heart muscle.
• P-cadherin: Found in the placenta.

Cadherins allow cell adhesion and maintenance... Continue reading "Cell Adhesion Molecules: Types and Functions" »

Touch

The skin is the largest organ of our body, composed of two layers:

• Epidermis: The outer layer is formed by epithelial tissue. The outer cells are dead and continuously shed as flakes and dandruff.
• Dermis: The inner layer, formed by connective tissue, provides elasticity to the skin. The dermis contains numerous blood vessels and tactile receptors.

Smell

The inside of the nostrils is lined by a mucosa called the pituitary gland. Two regions can be distinguished:

• Red pituitary: Occupies the lower part, has several blood vessels that warm the incoming air.
• Yellow pituitary: Occupies the upper part, contains numerous olfactory receptors.

Taste

At the top of the tongue are numerous projections called taste papillae. These contain sensitive cells (chemoreceptors)... Continue reading "Human Sensory and Locomotor Systems: Functions & Health" »

Structure and Location

The heart is a hollow, muscular organ about the size of a clenched fist. It's located in the chest cavity, between the lungs, within the mediastinum. Its shape resembles a triangular pyramid with a base and upper left and lower right edges. The heart is enclosed by the pericardium, a double-layered sac. The outer layer is fibrous, while the inner layer is the serous pericardium. Pericardial fluid between these layers lubricates the heart's movement.

Chambers and Valves

Internally, the heart has four chambers: two atria and two ventricles. The atria, located superiorly, have thin walls. The ventricles, situated inferiorly, have thicker walls (approximately 4mm). The right ventricle's wall is about 0.5cm thick, while the... Continue reading "Heart Anatomy and Function" »

Fundamentals of Genetics and Chromosomes

In humans, there are 46 chromosomes, occurring in 23 pairs. The diploid number refers to the total number of chromosomes in each body cell of a plant or animal.

Key Genetic Terms

• Gene: A length of DNA that is the code for making a specific protein molecule. Genes are always located in the same position on a chromosome.
• Allele: Different versions of the same gene.
  • A dominant allele is expressed and affects the appearance (phenotype) of the individual.
  • A recessive allele is only expressed if two copies are present or if the dominant allele is absent.
• Genotype: The specific combination of alleles an organism possesses for a trait.
  • Homozygous: Having two identical alleles for a trait (e.g., TT or tt).
  • Heterozygous:

DNA Replication

The process of DNA duplication, which allows identical genetic information to be shared between cellular daughters, consists of the following:

• The two strands of the double helix are separated.
• Free nucleotides can join the DNA nucleotides through their complementary bases. Only the nucleotides A can be combined with T, and C with G.
• The new nucleotides bind and form new DNA chains.
• Each of the two DNA chains serves as a template to create new strands.

Protein Synthesis

Proteins are formed by the joining of smaller molecules called amino acids. There are over 20 types of amino acids in different proteins. The order in which amino acids bind determines the type of protein.

Each cell produces thousands of proteins in its ribosomes, using... Continue reading "DNA Replication, Protein Synthesis, and Gene Therapy" »

Evolutionary Theories

The fossil record suggests that organisms appeared on Earth 3,000 million years ago and have been changing gradually, leading to the organisms present today.

Fixism

Considers that species have always been the same. It lasted until the eighteenth century. Cuvier studied fossils and explained the changes in specimens using a catastrophic theory that coincided with the Bible.

Lamarckism

Proposed in the book Philosophie Zoologique, it states that organisms change and adapt to the environment:

• Organisms have an innate tendency towards perfection, aiming for 'more perfect' species (e.g., humans).
• Believes in spontaneous generation.
• Proposes the Law of Use and Disuse and the Inheritance of Acquired Characters.

Darwinism

Published On the

Special Types of Cell Division

• Budding: Casts asymmetric cytoplasmic material; the daughter cell emerges as a bud from one side of the mother, such as in yeast or sponges.
• Sporulation: Several successive mitoses occur without cytokinesis; multinucleate cells are formed, for example, in fungi and protozoa.

Meiosis

To condense, look better.

Meiosis I

Stages of Prophase I

It is the most complex stage, with five sub-phases:

1. Leptotene: Chromosomes condense to become visible by light microscopy. Each is composed of two chromatids.
2. Zygotene: Homologous chromosomes pair up until they are completely aligned along their length. This pairing is called synapsis and occurs through a protein structure called the synaptonemal complex.
3. Pachytene: Crossover (chromatid