Biology Study Guide for the HESI Exam

Genetics and Heredity

Heredity is the passing on of genetic traits from one generation to the next. It is the reason that children resemble parents and why humans give birth to other humans. Genetics is the study of the principles underlying heredity.

Gregor Mendel

Mendel was a scientist in the 19th century who discovered the basic principles underlying the inheritance of traits. He cross-pollinated pea plants that had various characteristics and noted which characteristics were present in the offspring. For example, he cross-pollinated a tall pea plant with a short pea plant, and the result was a pea plant that was tall. Then he allowed that plant to self-pollinate, which resulted in offspring that were 75% tall and 25% short. From these experiments, he was able to determine the following principles of heredity.

Genes

Genes are sections of DNA that code for a specific trait. Each individual has two copies of each gene, one on each chromosome. The set of genes a person carries is known as their genotype, while the physical trait that the genes create is the phenotype.

Alleles

Many genes have different versions, which are known as alleles. These alleles help determine which phenotypes are present. For example, there were tall and short alleles (for the trait of height) in the pea plants Mendel studied. Dominant alleles (such as the tall trait) cover up, or mask, recessive alleles (such as the short trait). If the two alleles of an organism are the same, they are homozygous for that trait, and if the alleles are different, they are heterozygous. For a recessive phenotype to appear, both alleles must be recessive.

Punnett Square

A Punnett square is a diagram that helps predict the potential traits of the offspring of two parents. Each parent can only pass down one of their two copies of a gene. The Punnett square places one parent’s alleles at the top of the diagram and the other parent’s alleles at the side, then combines each row and column to determine the offspring’s potential genotype. The image shown here is a Punnett square about the trait of flower color:

The parent flowers are heterozygous for the trait of flower color, which has two phenotypes, yellow and white. The two alleles that code for those phenotypes are represented by letters, with a capital letter indicating a dominant trait and a lowercase letter indicating a recessive trait. If there is a capital letter paired with a lowercase letter, the dominant phenotype will appear. If we look in the boxes, there are three different possible genotypes: RR, Rr, and rr. Based on this, we can predict that these two plants would have a 75% chance of producing yellow flowered offspring and a 25% chance of producing white flowered offspring, because the yellow allele is dominant.

Deoxyribonucleic Acid (DNA)

DNA is the genetic material that is passed down from parent to offspring during both asexual and sexual reproduction. DNA codes for the genes that determine traits. This means DNA has all of the instructions needed to build a specific organism. DNA is made of nucleotides, each with a phosphate group, deoxyribose sugar, and a nitrogenous base. The four nitrogenous bases are adenine, thymine, cytosine, and guanine (often shortened to A, T, C, and G).

The shape of DNA is a double helix, in which two strands of DNA run opposite of each other like a twisted ladder. The “backbone” of DNA (the sides of the ladder) are made of alternating sugar and phosphate. The center of the molecule (the rungs of the ladder) is made of nitrogen bases bonded together, with A always bonding with T and C always bonding with G. The strands are complementary because the order of the bases of one strand provides the template for the other strand.

The bases are held together by hydrogen bonds. When DNA makes a copy of itself during replication, the hydrogen bonds break, the two strands separate, and nucleotides are added to match the exposed bases on each side of the molecule. The result is two new molecules of DNA.

Ribonucleic Acid (RNA)

DNA could not carry out its function without a host of other molecules and organelles. The first step in turning a gene into a functional protein is transcription, in which part of the DNA code is copied to create a molecule of messenger RNA, or mRNA. RNA is similar to DNA in structure, with some exceptions. It has a ribose sugar instead of a deoxyribose sugar, it is single-stranded, and it has the same bases as DNA, except that is has uracil instead of thymine. It is also much shorter, because it only carries the code for one amino acid rather than the entire genetic makeup of the organism. Each set of three nucleotides on the mRNA strand is known as a codon.

Once the mRNA has exited the nucleus and traveled to a ribosome (the site of protein synthesis), the process of translation begins. In this process, mRNA is threaded through the ribosome, and each codon is paired with an anticodon (a set of three complementary bases) attached to a structure called transfer RNA (tRNA). The tRNA has two ends, one with an anticodon and the other with a particular amino acid. So, the tRNA actually carries the amino acids so they can polymerize and be stored as glucose polymers.

Amino acids are the building blocks of proteins. As each codon passes through the ribosome, the corresponding amino acids are bonded together to create a polypeptide (long chain of amino acids). Eventually, a stop codon is encountered, and translation ends. The chain of amino acids is folded and modified, finally creating a functional protein with a specific role.

Retrieved from: https://openstax.org/books/biology-2e/pages/15-1-the-genetic-code

The Scientific Method

The scientific method is a way of devising and performing experiments that yield meaningful results. It involves a procedural approach to gaining information about the physical world that begins with a formulated question and ends with the acceptance or rejection of the hypothesis and the reevaluation of the experiment if indicated.

Steps in the Scientific Method

It is human nature to ask questions and seek answers. Early thinkers likely looked at the sky and wondered why it was blue, or looked at the grass and wondered why it was green. These steps of the scientific method are how such questions are answered:

1. Observation―A curiosity about a phenomenon arises, and, in response, a question is formulated. After formulating a question, a scientist looks for any relevant research or data already discovered and provided for the phenomenon in question. This helps give some direction on how to set up or approach the question.

2. Hypothesis―The scientist then generates a hypothesis, or an educated guess, as to what could be causing the phenomenon. This step helps narrow down the possible options for experimentation.

3. Experiment―Using available measuring tools and technology, an experiment is designed to provide valuable data for the scientist to investigate. For an experiment to be valid, it must be repeatable by others.

4. Conclusion―The data will then be analyzed and assessed for its validity and fully explained. Do the observations support (or disprove) the hypothesis, or do they support a different hypothesis? Finally, the scientist will decide if the hypothesis is confirmed. If the hypothesis is not confirmed, the scientist may choose to adjust some of the experimental methods or devise a new hypothesis.

Once a study is complete, other scientists will attempt to recreate the same experiment to confirm that the results hold true in a different time or place using the same methods. They will start by studying the methodology of the original experiment (observation) and then follow the subsequent steps of the scientific method.

Function of the Scientific Method

Overall, the scientific method provides a methodical approach for investigating experiments, studying data, and drawing conclusions. It is worthwhile to know that developments in scientific research do not arise from haphazard guessing and checking, but rather through logical design and reasoning. Even a basic familiarity with the method will prove useful when making sense of scientific experiments.