Survey of the Natural Sciences Study Guide for the DAT

Biology (continued)

Genetics

Genetics studies how and why organisms display the traits they do as it relates to the traits of their parents. Additionally, genetics offers us the opportunity to attempt to alter genetic code to avoid mutation or harmful genes by mechanisms of genetic technology. This technology is at the forefront of biological research by pharmaceutical and medical companies.

Molecular Genetics

In the simplest terms, deoxyribonucleic acid (DNA) codes for genes at specific locations on a chromosome. Genes are found in many forms called alleles. The genetic code for traits is called genotype, and the visible traits are called the phenotype.

Punnett squares are used to explain and predict the passage of alleles from parents to offspring. A Punnett square is a technique used to perform a test cross between a given set of parents and their alleles. The cross predicts the percent of offspring that will show a given dominant or recessive trait. A homozygous individual has two similar alleles of a gene (either both dominant or both recessive), while a heterozygous individual has two different alleles of a gene (one of each).

Human Genetics

In humans, dominant traits are inherited as long as at least one dominant allele is present in the genetic makeup for a given trait. Recessive traits are those that have to be doubly present, meaning they present in both the mother’s and father’s genetic material that is passed on to the offspring. Some traits are sex-linked recessive traits, which are carried on the X chromosome. This means that men only have to have one copy of the recessive allele for it to be present whereas women need two copies of the alleles to show the trait. This is because men have only one X chromosome and women have two. Karyotypes are visual depictions of chromosomes that are useful in explaining human genetics.

Classical Genetics

Classical genetics is described by the central dogma, which is that DNA codes for genes and that it replicates itself. Additionally, DNA is transcribed into RNA, which is later translated to proteins. Mutations occur in the genetic code, and they contribute to genetic diversity.

Gregor Mendel proposed two laws to describe patterns of genetic inheritance using molecular genetics. The first law states that alleles segregate during meiosis, giving each gamete one allele per gene. The second law is the law of independent assortment, which explains inheritance patterns exemplified in the Punnett square model.

Crossing over occurs between chromosomes, which contributes to the genetic diversity of offspring and impacts the way traits are passed down. It happens between nonsister chromatids of homologous chromosomes. Classical genetics also encompasses mitosis and meiosis principles as well as sex determination and sex linkage.

Chromosomal Genetics

Mutations can occur at the chromosome level via a variety of mechanisms. Sometimes, mutations occur at random, and sometimes there is a causative factor like a carcinogen. Chromosomes are made of a code of nitrogenous bases that, in triplets, code for amino acids, which compose proteins. When there is an addition, deletion, or substitution of a nitrogenous base, mutations can occur.

A point mutation is when one nucleic acid is replaced by another. Sometimes this replacement is a silent mutation, meaning that the amino acid being coded for is the same and the resulting protein is the same. Sometimes the replacement results in a missense mutation where the amino acid is changed, which can impact the resulting protein. A nonsense mutation is when the change in code results in a stop codon, which is often a very harmful mutation. In frameshift mutations, no nucleic acid is replaced, but one is inserted into the genetic code, which alters all of the following coded amino acids.

Genetic Technology

Genetic technology is always on the forefront of biologic research and development. Recombinant DNA technology includes changing an organism’s DNA by adding foreign DNA to their genome. This is an attempt by scientists to change the course of genetic disease by replacing harmful genes in a genome with normally functioning genes.

Developmental Mechanisms

Sometimes, homologous chromosomes don’t separate properly during cell division of meiosis, which is called nondisjunction. This could lead to various mutations, including trisomy, meaning an individual has three chromosomes where there are only intended to be two. Other genetic errors include chromosomal breakage caused by external factors like X-rays and mutations, which are changes in the genetic code.

Genomics

The genome is a large area of current genetics research and includes mapping all genetic markers, positions, and chromosomes in the human genome. Genomics utilizes this map of the human genetic makeup to understand the structure, function, mechanisms, and patterns of evolutionary change that occurs in genetics. This is the basis of future realms of research, including genetic technologies, because the complete set of genes in an organism must be understood prior to changing areas via recombinant technology.

Gene Expression

Organisms have various mechanisms in place to regulate which genes are expressed and which are not. During transcription, RNA polymerase binds to operator and promoter regions of the DNA, which guide the polymerase to the correct location to begin reading the genetic code. These regions are not read by the RNA polymerase, but they instead guide the RNA polymerase to the correct region to begin reading the genetic code.

Other regulatory systems include inducers and repressors. In an inducible system, an inducer substance must be present to allow for transcription to occur. This inducer will bind the repressor and prevent it from binding to the operator region of the DNA and stopping transcription. In a repressible system, transcription will occur unless a repressor or corepressor is present, which indicates to the RNA polymerase to “turn off” transcription of that region of DNA.

Epigenetics

Epigenetics is the study of how behaviors and environment of an organism affect the function of the genes in the genetic makeup of the respective organism. These mechanisms include DNA methylation, histone modification, and non-coding RNA. DNA can be methylated through a variety of methods and causes. This methylation is reversible and not a permanent change. Often, when the DNA is methylated, the gene is turned off, whereas the gene is “on” when demethylated.

Referring back to DNA structure and function, histones are a protein found in chromosomes that are tightly wrapped by genetic material. When the histones are more loosely packed, genetic code can be read, whereas it cannot be read when tightly bound. Similar to DNA methylation, this process changes depending on the environment and is reversible. Non-coding RNA is important to turn off the transcription of certain genes as well as to modify histones to cause changes in genetic expression.

Integrated Relationships

Genetics determines all of the functions and structures in a resulting organism, so its value to overall biology is clear. Through the understanding of mutations, genetic technology was born. Additionally, understanding how changes to the genetic code and genetic expression occur has allowed us to better understand developmental biology.

Evolution and Ecology

Every organism has a given environment composed of living (biotic) and nonliving (abiotic) components. Ecology is the study of organisms and their interactions with their surrounding environments and the impact of those interactions on the development, or evolution, of the organism.

Natural Selection

All organisms living in a given environment are constantly competing for resources. Because of this, there are traits and characteristics that allow a certain organism to survive more successfully. This observation is the basis of natural selection, which, in the simplest terms, is survival of the fittest. This theory describes a phenomenon in which traits that lead to greater survival or greater success in mating will become more common in future offspring because organisms with those traits survive longer and produce more offspring than those without the given trait. This competition could lead to extinction of a less favored species. Additionally, two species in competition with each other may evolve to differ from one another so as not to compete with each other any longer, a process called divergence.

Population Genetics/Speciation

Organisms within a population interact with one another in a variety of mechanisms. Mutualism is a process in which the interaction of two organisms is beneficial to both parties, whereas in parasitism, one species benefits and the other is harmed. A middle grounds is called commensalism where one organism benefits and the other is unaffected by a given interaction.

When a group within a given species begins to develop its own unique, identifiable characteristics, speciation can occur. This is when one species splits into two different groups.

Animal Behavior

Animals interact with each other and with their surroundings. Through doing this, they learn and adapt to changes in their environment to maximize survival. These animals are not born instinctively knowing how to perform this behavior, but they learn it as they recognize that protection from predators and keeping warm, for instance, will aid in their survival.

In many instances in nature, males will compete in some fashion to garner attention from their female counterparts. In doing this, the female will mate with the more fit male, which in turn selects for the favored traits of the more fit male. This will lead to the selection of certain traits over time that can lead to the evolution of the species.

Ecology

Ecology is broken down into levels of organization. Multiple individual organisms form a species, and multiple species form a population. A group of populations is called a community, and all of the living things in a given habitat is called an ecosystem. Ecology aims to study the interactions that occur at each level and observe how those interactions shape the evolution of the organisms involved.

Population Ecology

Similar organisms that together produce viable offspring are called a species. A group of organisms of the same species living together in a given environment is called a population.

Community Ecology

A community consists of many populations together in a given environment. These populations can be animal or plant populations.

Ecosystem Ecology

Ecosystems encompass communities and their environments, including elements from a variety of the six kingdoms. Many different ecosystem types are present in our world, but all will follow these characteristics:

• stability of abiotic and biotic factors

• a constant energy source and biotic environment with the ability to make organic compounds with this energy

• recycling of materials.

All ecosystems within a geographic region compose biomes. Biomes are unique to one another based on the unique living and nonliving components that comprise them.

Integrated Relationships

Much like the integration of all parts of the ecosystem, ecology topics interconnect with one another. It is through the understanding of types of relationships (symbiotic, commensal, parasitic) that we can understand animal behavior. Animal behavior provides reasoning for evolution of species and trait selection over time. Understanding the flow of energy through all parts of an ecosystem helps scientists understand laws of conservation of energy as well.