AP Biology Unit 6: Gene Expression & Regulation | StarSpark

If you're studying for the AP Biology exam, Unit 6 is where genetics becomes real. This unit shows you how the genetic information stored in DNA actually gets turned into proteins that do the work in your cells. You need to understand DNA and RNA structure, how DNA is copied, how it's transcribed into RNA, how RNA is translated into proteins, and how genes are turned on and off. This is the heart of how your cells function.

Topic 6.1: DNA and RNA Structure

Genetic information is stored and passed down through DNA. Understanding the structure of DNA and RNA is the foundation for understanding everything that comes after.

DNA molecules exist in different forms depending on the organism. In prokaryotes, the DNA is typically circular and found in the nucleoid region. In eukaryotes, DNA is organized into multiple linear chromosomes. These chromosomes are incredibly long molecules, so they need to be packaged efficiently. This is where histones come in. Histones are proteins that DNA wraps around to form structures called nucleosomes. This wrapping allows the long strands of DNA to fit into the nucleus. Both prokaryotes and eukaryotes can also contain plasmids, which are small, circular DNA molecules separate from the main chromosome.

DNA itself is made of nucleotides. Each nucleotide contains a five-carbon sugar (deoxyribose in DNA), a phosphate group, and a nitrogenous base. The nitrogenous bases come in two types: purines and pyrimidines. Purines (guanine and adenine) have a double ring structure. Pyrimidines (cytosine, thymine, and uracil) have a single ring structure. In DNA, purines always pair with pyrimidines in a specific way: adenine pairs with thymine, and guanine pairs with cytosine.

RNA is similar to DNA but different in important ways. RNA uses ribose sugar instead of deoxyribose. RNA contains uracil instead of thymine. RNA is typically single-stranded instead of double-stranded. In RNA, adenine pairs with uracil instead of thymine.

Key concepts to know:

• Chromosomes and histones: Eukaryotic DNA is organized into linear chromosomes and wrapped around histone proteins. Prokaryotic DNA is typically circular and not associated with histones.
• Plasmids: Small circular DNA molecules found in both prokaryotes and eukaryotes, separate from the main chromosome.
• Purines and pyrimidines: Adenine and guanine (purines) are larger molecules with double rings. Cytosine, thymine, and uracil (pyrimidines) are smaller molecules with single rings. Purines pair with pyrimidines to form base pairs.
• DNA vs. RNA structure: DNA has deoxyribose, thymine, and is double-stranded. RNA has ribose, uracil, and is single-stranded.

Topic 6.2: DNA Replication

DNA replication is how your cells make copies of their DNA before they divide. It's a complex process that requires multiple enzymes working together. You don't need to memorize every enzyme name, but you do need to understand what each one does and how the process works.

DNA replication is semiconservative, which means each new DNA molecule consists of one original strand and one new strand. Here's how it works: the enzyme helicase unwinds the double helix by breaking the hydrogen bonds between base pairs. As the helix unwinds, it creates tension that would cause the DNA to tangle up. The enzyme topoisomerase relieves this tension by temporarily cutting one strand, allowing the DNA to rotate, and then resealing it.

Once the DNA is unwound, DNA polymerase can begin synthesizing new strands. DNA polymerase reads the template strand in the 3' to 5' direction and builds a new strand in the 5' to 3' direction. But here's the catch: DNA polymerase can't start synthesis from scratch. It needs a primer to get started. Short RNA primers are created to give DNA polymerase a starting point for synthesis.

Here's another complication: DNA has two strands running antiparallel (opposite directions). DNA polymerase only works in the 5' to 3' direction. This means it can build the leading strand continuously, following the replication fork. But for the lagging strand, it has to work in short fragments called Okazaki fragments, building in the opposite direction of the fork. These fragments are later joined together by the enzyme ligase.

Key concepts to know:

• Semiconservative replication: Each new DNA molecule has one original strand and one new strand. This was proven by Meselson and Stahl.
• Helicase: Unwinds the DNA double helix by breaking hydrogen bonds.
• Topoisomerase: Relieves tension created by unwinding the DNA.
• DNA polymerase: Synthesizes new strands by adding nucleotides in the 5' to 3' direction. It reads the template strand in the 3' to 5' direction.
• RNA primers: Short RNA sequences that DNA polymerase uses to initiate synthesis. These are later removed and replaced with DNA.
• Leading and lagging strands: The leading strand is synthesized continuously. The lagging strand is synthesized discontinuously in Okazaki fragments.
• Ligase: Joins the fragments on the lagging strand together to form a continuous strand.

Topic 6.3: Transcription and RNA Processing

Transcription is how the information in DNA gets converted into RNA. RNA polymerase reads the template strand of DNA and creates a complementary RNA strand. In eukaryotes, this RNA undergoes several modifications before it becomes a functional messenger RNA.

There are three main types of RNA. Messenger RNA (mRNA) carries genetic information from the nucleus to the ribosome. Transfer RNA (tRNA) brings specific amino acids to the ribosome during protein synthesis, and it has a special region called an anticodon that pairs with the codon on the mRNA. Ribosomal RNA (rRNA) is a component of the ribosome itself.

RNA polymerase synthesizes RNA in the 5' to 3' direction by reading the template DNA strand in the 3' to 5' direction. The RNA is built using the base-pairing rules: adenine pairs with uracil (instead of thymine), and guanine pairs with cytosine.

In eukaryotes, the mRNA undergoes processing. The poly-A tail, a stretch of approximately 200 adenine nucleotides, is added to the 3' end. This makes the mRNA more stable and helps it be recognized by the ribosome. The GTP cap is added to the 5' end, helping with ribosomal recognition.

But here's the really important part: eukaryotic genes contain introns and exons. Exons are the parts of the gene that are expressed (they code for amino acids). Introns are non-coding sequences that are removed. After transcription, the mRNA contains both exons and introns. The process of splicing removes the introns and joins the exons together. What's even cooler is that alternative splicing allows the same gene to produce different proteins by including or excluding different exons.

Key concepts to know:

• mRNA, tRNA, rRNA: Each type of RNA has a different function. mRNA carries information. tRNA delivers amino acids. rRNA is structural.
• Poly-A tail and GTP cap: These modifications protect the mRNA and help it be recognized by the ribosome.
• Introns and exons: Exons are expressed. Introns are removed during splicing.
• Alternative splicing: Different combinations of exons can be joined together, allowing one gene to produce multiple proteins.
• RNA polymerase direction: Synthesizes 5' to 3', reads template 3' to 5'.

Topic 6.4: Translation

Translation is the process of converting the instructions on mRNA into a protein. This happens on the ribosome, using tRNA molecules to bring the correct amino acids in the correct order.

Translation has three main stages: initiation, elongation, and termination. It starts when the ribosome finds the start codon (AUG) on the mRNA. This codon codes for methionine. The ribosome reads the mRNA in triplets called codons. Each codon specifies a particular amino acid or a stop signal. tRNA molecules carry specific amino acids. Each tRNA has an anticodon that base-pairs with the codon on the mRNA, ensuring that the correct amino acid is placed in the correct position.

The genetic code is nearly universal. Almost all living organisms use the same code to translate mRNA into protein. This is strong evidence for the common ancestry of all life. The exam won't ask you to memorize the entire genetic code, but it will ask you to understand how codons work and how to use a genetic code chart if one is provided.

Translation continues by adding amino acid after amino acid to the growing protein chain. The process terminates when the ribosome reaches a stop codon. The stop codons (UAA, UAG, UGA) don't code for amino acids. Instead, they signal that translation should end.

One important note: in prokaryotes, translation can begin while transcription is still happening. In eukaryotes, transcription happens in the nucleus and translation happens in the cytoplasm, so the mRNA has to be fully transcribed and processed before translation begins.

There's also a special case you should know: retroviruses have an alternate flow of genetic information. Instead of the usual DNA → RNA → protein, retroviruses use reverse transcriptase, an enzyme that copies the viral RNA genome into DNA. This DNA integrates into the host genome and is then transcribed and translated to produce new viral particles.

Key concepts to know:

• Start codon (AUG): Signals the beginning of translation and codes for methionine.
• Codons and anticodons: Codons are triplets on mRNA. Anticodons are on tRNA. They base-pair together.
• Genetic code: Nearly universal across all organisms, evidence of common ancestry.
• tRNA function: Brings the correct amino acid to the ribosome and base-pairs with the codon.
• Stop codons: UAA, UAG, UGA. Signal the end of translation.

Topic 6.5: Regulation of Gene Expression

Not every gene is expressed all the time. In fact, your cells would be a mess if they were. Gene regulation is how cells control which genes are turned on and off, and at what level they're expressed. This is crucial for cell differentiation, development, and response to environmental changes.

Regulatory sequences are stretches of DNA that interact with regulatory proteins to control whether a gene is transcribed. Some genes are constitutively expressed, meaning they're always on because they code for proteins that are always needed. Other genes are inducible, meaning they're turned on only when needed. For example, bacteria have genes that are only expressed when a particular nutrient is available.

Epigenetics is the study of how gene expression can be changed without changing the DNA sequence itself. DNA can be chemically modified, and histone proteins can be modified. These modifications can turn genes on or off. These changes can be reversible, which is why they're called "epigenetic" rather than genetic changes.

In prokaryotes, groups of related genes are often organized into operons. An operon is a cluster of genes that are regulated together. The classic example is the lac operon in bacteria, which is turned on when lactose is present and turned off when it's not.

In eukaryotes, regulatory sequences are often far from the genes they control. These sequences include promoters (where RNA polymerase attaches) and enhancers (which increase transcription). Transcription factors are proteins that bind to these regulatory sequences and control whether RNA polymerase can do its job.

Key concepts to know:

• Regulatory sequences: DNA sequences that control whether genes are transcribed.
• Constitutive expression: Genes that are always on.
• Inducible genes: Genes that are turned on only when needed.
• Epigenetics: Reversible modifications of DNA or histones that affect gene expression without changing the DNA sequence.
• Operons: In prokaryotes, clusters of genes regulated together. Operons can be inducible (turned on when needed) or repressible (turned off when the product is abundant).
• Promoters and enhancers: Regulatory sequences that control transcription.
• Transcription factors: Proteins that bind to regulatory sequences and control transcription.

Topic 6.6: Gene Expression and Cell Specialization

Your body has many different types of cells. All of them have the same DNA, but they look and function completely differently. This is because different genes are expressed in different cell types. A nerve cell expresses genes for proteins used in nerve function. A muscle cell expresses genes for muscle proteins. Gene regulation is what makes cell specialization possible.

RNA polymerase and transcription factors bind to promoter and enhancer sequences to initiate transcription. Negative regulatory molecules can inhibit gene expression by binding to DNA and blocking transcription. Small RNA molecules can also play roles in regulating gene expression, turning genes on or off.

The combination of genes that are expressed and the levels at which they're expressed determine the phenotype of the cell and the organism. During development, induction of transcription factors leads to sequential gene expression. Early genes turn on, produce transcription factors, which turn on other genes, which produce more transcription factors, and so on. This cascade of gene expression is how development happens.

Key concepts to know:

• Promoters and enhancers: Where RNA polymerase and transcription factors bind to initiate transcription.
• Negative regulation: Proteins that inhibit gene expression.
• Sequential gene expression: During development, one gene's product can activate other genes.
• Small regulatory RNA molecules: Can regulate gene expression through various mechanisms.

Topic 6.7: Mutations

Mutations are changes in the DNA sequence. They can be caused by errors in DNA replication, errors in DNA repair, or external factors like radiation or chemicals. Mutations are the source of genetic variation, which is the raw material for evolution. Most mutations are neutral or harmful, but some can be beneficial.

There are several types of mutations. Point mutations occur when one nucleotide is replaced with a different one. Frameshift mutations occur when nucleotides are inserted or deleted, shifting the reading frame and changing all the codons downstream. Nonsense mutations are point mutations that create a stop codon, causing translation to terminate early. Silent mutations are changes in the DNA that don't result in a change in the amino acid sequence (usually because of the redundancy in the genetic code).

Whether a mutation is beneficial, harmful, or neutral depends on the environment. A mutation that helps an organism survive in one environment might be harmful in another. Errors during mitosis or meiosis can result in changes in chromosome number (aneuploidy), which often results in developmental limitations and disorders. Alterations in chromosome structure can also lead to genetic disorders.

Beyond mutations, prokaryotes can increase their genetic variation through horizontal gene transfer. This includes transformation (uptake of DNA from the environment), transduction (viral transmission of genetic information), conjugation (cell-to-cell transfer of DNA), and transposition (movement of DNA segments within and between DNA molecules). Related viruses can also recombine genetic information if they infect the same host cell. These mechanisms are important because they add genetic variation that can be subject to natural selection.

Key concepts to know:

• Point mutation: One nucleotide substituted for another.
• Frameshift mutation: Insertion or deletion of nucleotides, shifting the reading frame.
• Nonsense mutation: Point mutation creating a premature stop codon.
• Silent mutation: Change in nucleotide sequence with no change in amino acid sequence.
• Sources of mutations: Errors in DNA replication, errors in DNA repair, radiation, and chemicals.
• Environmental context: Whether a mutation is beneficial or harmful depends on the environment.

Topic 6.8: Biotechnology

Biotechnology is the use of DNA to solve practical problems. You don't need to know all the technical details of how these techniques work, but you do need to understand the basic principles and what each technique accomplishes.

Gel electrophoresis is a technique that separates DNA fragments based on size and charge. DNA is placed in a gel, and an electric field is applied. DNA molecules migrate through the gel at different rates depending on their size, allowing you to separate them and see how many different sizes are present. This is useful for comparing DNA samples.

The polymerase chain reaction (PCR) amplifies DNA segments. The DNA is repeatedly denatured (separated into single strands), primers anneal (attach) to the original strands, and DNA polymerase extends the new strands. This cycle is repeated many times, exponentially increasing the amount of the target DNA. PCR is incredibly powerful because it allows you to make many copies of a specific DNA region from a tiny starting sample.

Bacterial transformation introduces foreign DNA into bacterial cells. Scientists use this technique to insert genes into bacteria, often to make the bacteria produce useful proteins. For example, bacteria can be transformed to produce human insulin.

DNA sequencing determines the order of nucleotides in a DNA molecule. Modern DNA sequencing is fast and cheap, allowing scientists to sequence entire genomes. The unique pattern of DNA in each individual is called a DNA fingerprint and can be used to identify individuals or compare DNA from different sources.

Key concepts to know:

• Gel electrophoresis: Separates DNA fragments by size and charge.
• PCR (Polymerase Chain Reaction): Amplifies specific DNA sequences.
• Bacterial transformation: Introduces foreign DNA into bacterial cells.
• DNA sequencing: Determines the order of nucleotides in DNA.
• DNA fingerprint: The unique pattern of DNA in an individual, used for identification and comparison.

Study Tips for Unit 6

Use StarSpark's study tools to practice the prompts suggested in the Study Tips section. The central dogma and transcription/translation problems are critical for the AP exam.

Summary: What Actually Matters for the Exam

You've covered all the topics in Unit 6. Before you move on, test yourself with these scenario-based questions. If you can answer them confidently, you're in great shape for this section of the exam.

Explore the Full AP Biology Study Guide

Unit 6 builds on the genetic information stored in DNA from Unit 5 and shows you how that information is actually used by cells. The processes you learn here (transcription, translation, regulation) are happening in every cell in your body right now.

Check out the full AP Biology study plan to see how this unit connects to the rest of the course.

Other Unit Reviews:

• AP Biology Unit 1: Chemistry of Life
• AP Biology Unit 2: Cells
• AP Biology Unit 3: Cellular Energetics
• AP Biology Unit 4: Cell Communication and Cell Cycle
• AP Biology Unit 5: Heredity
• AP Biology Unit 7: Natural Selection
• AP Biology Unit 8: Ecology
• AP Biology Exam Prep Study Guide

For official AP Biology resources, visit apcentral.collegeboard.org.

This review is aligned with the AP Biology Course and Exam Description. AP is a registered trademark of the College Board, which was not involved in the production of this guide.