Biology

Homologous structures share a similar embryonic origin; analogous organs have a similar function. For example, the bones in the front flipper of a whale are homologous to the bones in the human arm. These structures are not analogous. The wings of a butterfly and the wings of a bird are analogous but not homologous.

Vestigiality is the retention, during the process of evolution, of genetically determined structures or attributes that have lost some or all of the ancestral function in a given species. Assessment of the vestigiality must generally rely on comparison with homologous features in related species.

Nature selects for less melanin when ultraviolet radiation is weak. This is why people who live in far northern latitudes have lighter skin. Having dark skin in this environment, is a disadvantage because it can prevent the body from producing enough vitamin D.

Humans living at latitudes far from the equator developed light skin in order to help absorb more vitamin D. People with light (type II) skin can produce previtamin D3 in their skin at rates 5–10 times faster than dark-skinned (type V) people.^

Melanin is a broad term for a group of natural pigments found in most organisms. Eumelanin is produced through a multistage chemical process known as melanogenesis, where the oxidation of the amino acid tyrosine is followed by polymerization.

Natural selection on polygenic traits can affect the distributions of phenotypes in three ways: directional selection, stabilizing selection, or disruptive selection.

Natural selection is the process through which populations of living organisms adapt and change. Individuals in a population are naturally variable, meaning that they are all different in some ways. This variation means that some individuals have traits better suited to the environment than others.

Darwin proposed that species can change over time, that new species come from pre-existing species, and that all species share a common ancestor. In this model, each species has its own unique set of heritable (genetic) differences from the common ancestor, which have accumulated gradually over very long time periods.

In Galapagos he found a remarkable population of plants, birds and reptiles that had developed in isolation from the mainland, but often differed on almost identical islands next door to one another and whose characteristics he could only explain by a gradual transformation of the various species.

The Hardy-Weinberg genotype frequencies, p2 + 2pq + q2, represent the binomial expansion of (p + q)2, and also sum to one (as must the frequencies of all genotypes in any population, whether it is in Hardy-Weinberg equilibrium).

The Hardy-Weinberg equilibrium can be disturbed by a number of forces, including mutations, natural selection, nonrandom mating, genetic drift, and gene flow. For instance, mutations disrupt the equilibrium of allele frequencies by introducing new alleles into a population.

Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble

The conducting zone consists of all of the structures that provide passageways for air to travel into and out of the lungs: the nasal cavity, pharynx, trachea, bronchi, and most bronchioles.

Respiration refers to a person's breathing and the movement of air into and out of the lungs. The respiratory system provides oxygen to body tissues for cellular respiration, removes the waste product carbon dioxide, and helps maintain acid–base balance.

Blood comes into the right atrium from the body, moves into the right ventricle and is pushed into the pulmonary arteries in the lungs. After picking up oxygen, the blood travels back to the heart through the pulmonary veins into the left atrium, to the left ventricle and out to the body's tissues through the aorta.

The cardiovascular system delivers oxygen, nutrients, hormones, and other important substances to cells and organs in the body. It plays an important role in helping the body meet the demands of activity, exercise, and stress.

Amylase (made in the mouth and pancreas; breaks down complex carbohydrates) Lipase (made in the pancreas; breaks down fats) Protease (made in the pancreas; breaks down proteins) as well as lysosome which secretes 40 types of digestive enzymes

Pepsin is an endopeptidase that breaks down proteins into smaller peptides. It is produced in the gastric chief cells of the stomach lining and is one of the main digestive enzymes in the digestive systems of humans and many other animals, where it helps digest the proteins in food

The processes of digestion include six activities: ingestion, propulsion, mechanical or physical digestion, chemical digestion, absorption, and defecation. The first of these processes, ingestion, refers to the entry of food into the alimentary canal through the mouth

The GI tract is a series of hollow organs joined in a long, twisting tube from the mouth to the anus. The hollow organs that make up the GI tract are the mouth, esophagus, stomach, small intestine, large intestine, and anus. The liver, pancreas, and gallbladder are the solid organs of the digestive system.

The digestive system breaks down food into nutrients such as carbohydrates, fats and proteins. They can then be absorbed into the bloodstream so the body can use them for energy, growth and repair. Unused materials are discarded as faeces

Antimicrobial resistance occurs when microbes evolve mechanisms that protect them from the effects of antimicrobials. All classes of microbes can evolve resistance. Fungi evolve antifungal resistance. Viruses evolve antiviral resistance.

Gram-negative bacteria are surrounded by a thin peptidoglycan cell wall, which itself is surrounded by an outer membrane containing lipopolysaccharide. Gram-positive bacteria lack an outer membrane but are surrounded by layers of peptidoglycan many times thicker than is found in the Gram-negatives.

Viruses cannot replicate on their own, but rather depend on their host cell's protein synthesis pathways to reproduce. This typically occurs by the virus inserting its genetic material in host cells, co-opting the proteins to create viral replicates, until the cell bursts from the high volume of new viral particles.

Viruses cause familiar infectious diseases such as the common cold, flu and warts. They also cause severe illnesses such as HIV/AIDS, Ebola, and COVID-19. Viruses are like hijackers. They invade living, normal cells and use those cells to multiply and produce other viruses like themselves.

Trees: They often use asexual reproduction through fragmentation to grow new plants. They do this by cutting a leaf or a stem off a plant and placing it in water or soil. This process is often called propagating from cuttings.

Stages:As with all living things, trees have a life cycle – from conception (seed), to birth (sprout), to infancy (seedling), to juvenile (sapling), to adult (mature), to elderly (decline), and finally to death (snag/rotting log).

Plants:Flowering plants reproduce sexually through a process called pollination. Flowers contain male sex organs called stamens and female sex organs called pistils. The anther is the part of the stamen that contains pollen.Flowering plants reproduce sexually through a process called pollination. Flowers contain male sex organs called stamens and female sex organs called pistils. The anther is the part of the stamen that contains pollen.

a diagram that depicts the lines of evolutionary descent of different species, organisms, or genes from a common ancestor.

A cladogram is an evolutionary tree that diagrams the ancestral relationships among organisms. In the past, cladograms were drawn based on similarities in phenotypes or physical traits among organisms. Today, similarities in DNA sequences among organisms can also be used to draw cladograms.

A dichotomous key is an important scientific tool, used to identify different organisms, based the organism's observable traits. Dichotomous keys consist of a series of statements with two choices in each step that will lead users to the correct identification.

In biology, taxonomy is the scientific study of naming, defining and classifying groups of biological organisms based on shared characteristics.

Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats.

a living thing that can move and eat and react to the world through its senses, esp. of sight and hearing: Mammals, insects, reptiles, and birds are all animals. In ordinary use, animal means all living beings except humans: A lion is a wild animal, and a dog is a domestic animal

Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclude the fungi and some algae, as well as the prokaryotes

Bacteria are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were among the first life forms to appear on Earth, and are present in most of its habitats.

A protist is any eukaryotic organism that is not an animal, plant, or fungus. While it is likely that protists share a common ancestor, the exclusion of other eukaryotes means that protists do not form a natural group, or clade

A fungus is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms

Donors who are AB-negative are the universal platelet and plasma donors because these blood components can be transfused to any patient. Donors who are O-negative are the universal blood type; O-negative red blood cells are compatible with all other blood types.

There are 4 main blood groups (types of blood) – A, B, AB and O. Your blood group is determined by the genes you inherit from your parents. Each group can be either RhD positive or RhD negative, which means in total there are 8 blood groups.

There are two differences that distinguish DNA from RNA: RNA contains the sugar ribose, while DNA contains the slightly different sugar deoxyribose which is a type of ribose that lacks one oxygen atom, RNA has the nucleobase uracil while DNA contains thymine.

Nucleic acid structure refers to the structure of nucleic acids such as DNA and RNA. Chemically speaking, DNA. Nucleic acid structure is often divided into four different levels: primary, secondary, tertiary, and quaternary.

Genetic abnormalities are conditions caused by changes to the genes or chromosomes. Inherited disorders are caused by gene mutations. These include disorders such as cystic fibrosis, sickle cell disease, and Tay-Sachs disease.

Curly hair is dominant over straight hair. Baldness is a dominant trait. Having a widow's peak is dominant over having a straight hairline. Freckles, cleft chin and dimples are all examples of a dominant trait.

In many ways, meiosis is a lot like mitosis. The cell goes through similar stages and uses similar strategies to organize and separate chromosomes. In meiosis, however, the cell has a more complex task. It still needs to separate sister chromatids (the two halves of a duplicated chromosome), as in mitosis. But it must also separate homologous chromosomes, the similar but nonidentical chromosome pairs an organism receives from its two parents.

Meiosis is a special type of cell division of germ cells in sexually-reproducing organisms that produces the gametes, such as sperm or egg cells. It involves two rounds of division that ultimately result in four cells with only one copy of each chromosome

In each round of division, cells go through four stages: prophase, metaphase, anaphase, and telophase.

Mitosis is a type of cell division in which one cell divides to produce two new cells (daughter cells) that are genetically identical to itself. In the context of the cell cycle, mitosis is the part of the division process in which the DNA of the cell's nucleus is split into two equal sets of chromosomes.

Today, mitosis is understood to involve five phases, based on the physical state of the chromosomes and spindle. These phases are prophase, prometaphase, metaphase, anaphase, and telophase.

The way in which traits are passed from one generation to the next-and sometimes skip generations-was first explained by Gregor Mendel. By experimenting with pea plant breeding, Mendel developed three principles of inheritance that described the transmission of genetic traits, before anyone knew genes existed.

Mendel crossed pure lines of pea plants. Dominant traits, like purple flower colour, appeared in the first-generation hybrids (F1), whereas recessive traits, like white flower colour, were masked. However, recessive traits reappeared in second-generation (F2) pea plants in a ratio of 3:1 dominant to recessive.