Final Exam Study Guide HBio 2026: Cell Cycle, Central Dogma, Heredity, Evolution, and Body Systems

The Cell Cycle and Mitosis

The cell cycle is a highly regulated series of events that leads to cell division. It is divided into several distinct stages, including Interphase ( $G_1$ , S, and $G_2$ ) and the M-phase (Mitosis and Cytokinesis). During $G_1$ (Gap 1), the cell grows and carries out normal cell functions; the DNA is found in the form of uncoiled chromatin. The S phase (Synthesis) is characterized by DNA replication, where the chromosomes are copied, and the DNA remains as uncoiled DNA/chromatin. In the $G_2$ phase (Gap 2), the cell prepares for division by duplicating organelles. The M-phase follows, consisting of Mitosis (division of the nucleus and DNA) and Cytokinesis (division of the cytoplasm resulting in two daughter cells).

Mitosis itself is a process of nuclear division categorized into four main stages: Prophase, Metaphase, Anaphase, and Telophase (often remembered by the mnemonic PMAT). During Prophase, chromatin condenses into visible chromosomes, spindle fibers form, and the nuclear envelope breaks down. In Metaphase, chromosomes line up at the center or equator of the cell. In Anaphase, sister chromatids are pulled apart to opposite sides of the cell. Finally, in Telophase, nuclear membranes reform around the chromosomes at each pole, and the chromosomes begin to uncoil back into chromatin. The main purpose of mitosis is to create genetically identical cells for growth, repair, and the replacement of somatic (body) cells. In humans, a somatic cell contains a diploid number of $46$ chromosomes.

A loss of cell cycle control is a critical medical concern because it can lead to cancer. This occurs when cells divide uncontrollably due to the failure of regulatory checkpoints. Unlike normal cells, cancer cells bypass the signals that usually tell a cell to stop dividing or to undergo programmed cell death, leading to the formation of tumors.

DNA Structure and Replication

DNA (Deoxyribonucleic acid) is a polymer made of monomers called nucleotides. Each nucleotide consists of three components: a deoxyribose sugar (often referred to in the transcript as a "diet glucose" or 5-carbon sugar), a nitrogenous base (Adenine, Thymine, Guanine, or Cytosine), and a phosphate group. According to base-pairing rules, Adenine ( $A$ ) always pairs with Thymine ( $T$ ), and Cytosine ( $C$ ) always pairs with Guanine ( $G$ ). This chemical complementarity allows for the calculation of base percentages; for example, if a DNA sample contains $30\%$ Adenine, it must also contain $30\%$ Thymine. The remaining $40\%$ is split equally between Cytosine ( $20\%$ ) and Guanine ( $20\%$ ).

DNA replication is described as semi-conservative because each new DNA molecule consists of one original (parental) strand and one newly synthesized strand. This process involves a suite of specialized enzymes. Helicase unwinds and "unzips" the DNA double helix at the replication fork. DNA Primase initiates synthesis by creating a short RNA segment (primer) that allows DNA polymerase to attach. DNA Polymerase builds the new strand by adding complementary bases in the $5'$ to $3'$ direction (while reading the template strand in the $3'$ to $5'$ direction). Because the double helix is antiparallel, the Leading strand is copied continuously toward the replication fork, while the Lagging strand is copied discontinuously in segments called Okazaki fragments. Finally, DNA Ligase binds these Okazaki fragments together to ensure a continuous sugar-phosphate backbone.

The steps of replication occur in a specific order: first, DNA helicase unwinds the DNA; second, primase lays down an RNA primer; third, DNA polymerase attaches to that primer; fourth, DNA polymerase adds nucleotides in the $5'$ to $3'$ direction; and fifth, Okazaki fragments are joined by ligase. Failures in these enzymes lead to specific issues: if helicase fails, the strands cannot separate; if DNA polymerase fails, the new strand cannot be built; if primase fails, the process cannot start; and if ligase fails, the lagging strand remains fragmented.

The Central Dogma: Transcription and Translation

The central dogma of biology explains the flow of genetic information: DNA is transcribed into RNA, which is then translated into protein ( $\text{DNA} \rightarrow \text{RNA} \rightarrow \text{Protein}$ ). This process is vital because it allows a copy of the genetic code to be sent out of the nucleus to the ribosomes to build the proteins necessary for life. Transcription occurs in the nucleus, where RNA polymerase uses a DNA template to synthesize a strand of messenger RNA (mRNA). Translation occurs in the cytoplasm at the ribosome, where the sequence of mRNA is used to assemble amino acids into a polypeptide chain.

Specific molecules play crucial roles in this process. DNA acts as the master template. mRNA carries the genetic message from the nucleus to the ribosome. Transfer RNA (tRNA) carries specific amino acids to the ribosome and uses its anticodon (a three-letter sequence) to match with the mRNA's codons. Ribosomal RNA (rRNA) forms part of the ribosome structure. Transcription begins at a site in the DNA called the promoter, and translation begins at a start codon (typically $AUG$ ) in the mRNA. Before leaving the nucleus, mRNA undergoes "editing" or RNA processing, where non-coding regions called introns are removed and coding regions called exons are spliced together.

The genetic code is read in groups of three nucleotides called codons. Each codon specifies one amino acid. For example, to specify three amino acids, a sequence of $9$ nucleotides is required. Using a codon chart, researchers can determine the protein sequence; for instance, the DNA sequence $TAC$ transcribes to the mRNA codon $AUG$ , matches the tRNA anticodon $UAC$ , and codes for the amino acid Methionine.

Genetic Mutations

Mutations are changes in the DNA sequence that most commonly occur during DNA replication. Point mutations involve the change of a single nucleotide and include substitutions (one base replaced by another), insertions (an extra base is added), and deletions (a base is removed). Insertions and deletions are particularly harmful because they cause a "frameshift," which shifts the reading frame of all subsequent codons, usually resulting in a completely different or non-functional protein.

Mutations are classified by their effect on the protein. A silent mutation occurs when a change in the DNA does not change the resulting amino acid (e.g., change from $TAC$ to $TAT$ both coding for the same product). A nonsense mutation occurs when a change creates a premature stop codon, terminating the protein early (e.g., an mRNA sequence changing to $UAG$ ). A missense mutation results in the substitution of one amino acid for another, which may or may not affect protein function depending on the chemical properties of the new amino acid.

Meiosis and Heredity

Meiosis is a specialized form of cell division used for sexual reproduction to produce gametes (sperm and egg cells). Unlike mitosis, which involves one division and produces two identical diploid somatic cells ( $46$ chromosomes in humans), meiosis involves two divisions and produces four genetically unique haploid cells ( $23$ chromosomes in humans). Meiosis introduces genetic variation through mechanisms such as "crossing over," which occurs during Prophase I when homologous chromosomes exchange segments of DNA.

Homologous chromosomes are pairs of chromosomes (one from each parent) that carry the same genes in the same order but may have different alleles. During Meiosis I, these homologous pairs line up and are separated. Sister chromatids, which are identical copies of a chromosome connected at the centromere, are separated during Meiosis II. After fertilization (the fusion of two haploid gametes), the resulting zygote is diploid, having two sets of chromosomes. An advantage of sexual reproduction is that it provides genetic diversity, which is essential for the survival of a species in changing environments.

Mendelian and Non-Mendelian Genetics

Genetics is the study of how traits are passed from parents to offspring. An allele is a version of a gene. An organism's genotype is its genetic makeup (e.g., $TT$ , $Tt$ , or $tt$ ), while its phenotype is its physical appearance (e.g., tall or short). Homozygous organisms have two identical alleles ( $TT$ or $tt$ ), and heterozygous organisms have two different alleles ( $Tt$ ). In Mendelian genetics, a dominant allele will mask a recessive allele. For example, if tall ( $T$ ) is dominant over short ( $t$ ), two heterozygous parents ( $Tt \times Tt$ ) have a $25\%$ probability of producing a short ( $tt$ ) offspring.

Non-Mendelian inheritance includes patterns that do not follow simple dominance. Incomplete dominance occurs when the heterogeneous phenotype is a blend of the two parents (e.g., red and white flowers producing pink offspring). Codominance occurs when both alleles are expressed equally (e.g., blood type $AB$ ). Polygenic inheritance involves many genes contributing to a single trait, showing continuous variation (e.g., human height). Sex-linked inheritance involve genes located on the sex chromosomes ( $X$ and $Y$ ); males are more likely to inherit sex-linked recessive disorders (like hemophilia or colorblindness) because they have only one $X$ chromosome and cannot be carriers.

Human blood types are a classic example of multiple alleles and codominance. The possible genotypes involve alleles $I^A$ , $I^B$ , and $i$ . Type $A$ can be $I^AI^A$ or $I^Ai$ ; Type $B$ can be $I^BI^B$ or $I^Bi$ ; Type $AB$ is $I^AI^B$ ; and Type $O$ is $ii$ . Pedigrees are used to track these traits through generations, helping identify if a pattern of inheritance is autosomal dominant, autosomal recessive, or sex-linked.

Evolution and Natural Selection

Evolution is the change in the inherited traits of a population over time. Natural Selection is the process by which individuals best suited to their environment survive and reproduce, a concept closely linked to "fitness," which measures an organism's ability to pass on its genes. Mechanisms of evolution include:

Natural Selection: Differential survival based on traits (e.g., darker beetles surviving better in a desert).
Genetic Drift: Random changes in allele frequencies, especially in small populations. This includes the Bottleneck Effect (a disaster leaves only a few survivors) and the Founder Effect (a few individuals start a new population).
Mutation: Permanent changes in DNA that introduce new alleles.
Gene Flow: The movement of alleles between populations through immigration or emigration.
Non-Random Mating: Choosing mates based on specific traits, including sexual selection (e.g., peahens choosing peacocks with bright tails) or artificial selection (e.g., farmers breeding high-milk cows).

Evidence for evolution includes homologous structures (structures shared by a common ancestor but with different functions, like a whale's fin and a human's arm) and vestigial structures (unused remnants of ancestral traits, like the human appendix). Cladograms are used to show evolutionary relationships. In a cladogram, organisms that share more recent common ancestors are more closely related; for example, humans and gorillas are more closely related than humans and lungfish. Traits such as "jaws," "lungs," "hair," and "grasping hands" are marked on the cladogram to show where these characteristics first appeared in the evolutionary lineage.

Human Body Systems

The human body maintains homeostasis through the coordinated efforts of various systems. Key interactions include:

Nervous System: Involved in rapid responses and reflexes (e.g., immediately pulling back a hand after hitting a wall). The central nervous system consists of the brain and spinal cord, which coordinate with the peripheral nervous system to send signals throughout the body.
Digestive System: Breaks down food into nutrients (e.g., processing a lunch of Mr. Empanada). It involves the mechanical and chemical breakdown of food across organs like the stomach and intestines.
Immune System: Protects the body from pathogens. A fever or cold is an immune response. Vaccines help by prompting the body to produce antibodies against specific diseases without the person having to get sick first.
Respiratory System: Its primary function is gas exchange (oxygen in, carbon dioxide out). The diaphragm contracts to take in air, often increasing in rate after physical activity like singing or exercising.
Muscular and Skeletal Systems: Work together to provide structure, support, and movement (e.g., standing upright for prayer or lifting a heavy desk).
Circulatory System: Transports blood, nutrients, and oxygen. A blockage in blood flow to the brain results in a stroke, while a blockage to the heart results in a heart attack.