AP Biology 6.1 DNA and RNA Structure Study Notes - New Syllabus Effective 2025

AP Biology 5.5 Environmental Effects on Phenotype Study Notes- New syllabus

AP Biology 5.5 Environmental Effects on Phenotype Study Notes – AP Biology – per latest AP Biology Syllabus.

LEARNING OBJECTIVE

Describe the structures involved in passing hereditary information from one generation to the next.

Key Concepts:

Genetic Information: DNA & RNA
DNA Replication
Transcription & RNA Processing
Translation
Genetic Information In Retroviruses

AP Biology-Concise Summary Notes- All Topics

6.1.A – Describe the Structures Involved in Passing Hereditary Information from One Generation to the Next

🧬 What Does Heredity Mean?

Heredity is the passing of genetic information (DNA) from parents to offspring. This info determines traits like eye color, blood type, or the ability to roll your tongue.

🧠 Key Structures Involved in Heredity

1. DNA (Deoxyribonucleic Acid)

Main molecule that carries genetic information.
Double-helix structure (looks like a twisted ladder).
Made of repeating units called nucleotides, each with:
- A phosphate group
- A sugar (deoxyribose)
- A nitrogenous base (A, T, G, C)
Bases pair specifically:
- A-T (Adenine–Thymine)
- G-C (Guanine–Cytosine)
DNA is like a biological instruction manual for making proteins.

2. Genes

Segments of DNA that code for specific proteins.
Each gene has the info for one trait.
Located at specific spots (loci) on a chromosome.

🧠 Example: A gene for eye color may tell the body to make a pigment-producing protein.

3. Chromosomes

Long, tightly coiled strands of DNA + proteins (histones).
Found in the nucleus of eukaryotic cells.
Humans have 46 chromosomes (23 pairs) in each body cell.
- One set from mother, one from father

🔑 During reproduction, only gametes (egg & sperm) carry 23 chromosomes, so the offspring gets the full 46 again.

4. Nucleus

The control center of the cell.
Houses the DNA.
Ensures accurate copying and distribution of genetic info during cell division.

5. RNA (Ribonucleic Acid)

Temporary copy of DNA used to make proteins.
DNA → RNA → Protein = Central Dogma of Biology
RNA is single-stranded, with Uracil (U) replacing Thymine (T).

Types of RNA:

mRNA (messenger RNA) – Carries gene instructions out of nucleus
tRNA (transfer RNA) – Brings amino acids during protein building
rRNA (ribosomal RNA) – Part of ribosome, helps make proteins

6. Gametes (Sex Cells)

Eggs (♀️) and sperm (♂️)
Created through meiosis, a special kind of cell division
Carry half the DNA (haploid) so the zygote gets full DNA after fertilization

7. Zygote

Formed when egg and sperm fuse.
Now diploid (full 46 chromosomes).
This cell divides and grows into a full organism using the inherited DNA

🧠 Recap – Who’s Involved in Heredity?

Structure	Role
DNA	Stores genetic info
Genes	Code for traits/proteins
Chromosomes	DNA-packed structures
Nucleus	Cell’s DNA hub
RNA	Helps use DNA to make proteins
Gametes	Pass DNA to offspring
Zygote	First cell of a new organism

6.1.A.1 – How Genetic Information Is Stored & Passed Through DNA (and RNA)

📌 Big Idea:

Genetic information that determines all traits of living organisms is stored in DNA and in some viruses, it’s even stored in RNA. This info is what gets passed from generation to generation during reproduction.

🔁 DNA: The Molecule of Heredity

DNA (deoxyribonucleic acid) is the main molecule that carries heritable information.
It contains the instructions for building proteins, which control traits and cellular functions.
In some viruses, RNA takes over this role instead of DNA.

🧫 Types of Organisms & Their Chromosomes:

1. Prokaryotic Organisms (like bacteria)

Have circular chromosomes (imagine a rubber band shape).
Usually, one single circular DNA molecule floating in the cytoplasm (since they don’t have a nucleus).
May also have plasmids — small, extra DNA circles that carry special traits (like antibiotic resistance).

2. Eukaryotic Organisms (like plants, animals, fungi)

Have multiple linear chromosomes (like straight rods).
These chromosomes are found in the nucleus.
Made of DNA + proteins (mainly histones).
- DNA wraps around histones like thread around a spool → this helps organize & compact the long DNA.
- These DNA-histone bundles are called chromatin when loosely packed and chromosomes when fully condensed (during cell division).

📚 Summary Chart:

Feature	Prokaryotes	Eukaryotes
Chromosome Shape	Circular	Linear
Number of Chromosomes	Usually 1	Multiple
Location	Cytoplasm	Nucleus
DNA Packing Proteins	None	Uses histones
Extra DNA?	Yes, plasmids	Sometimes (e.g., mitochondria/chloroplasts)

💡 Bonus Tip:

Histones are like storage organizers. They keep DNA from getting tangled and help control which genes are active or silent.

6.1.A.2 – Plasmids: Extra DNA Circles in Cells

📌 What Are Plasmids?

Plasmids are small, circular pieces of DNA that exist outside the main chromosomes in a cell.

Think of them as bonus DNA packets separate from the cell’s main genetic material.

🧫 Where Do We Find Plasmids?

🌍 Organism Type	🔍 Plasmids Present?	📍 Location
Prokaryotes (like bacteria)	Very common	In the cytoplasm
Eukaryotes (like yeast)	Can be present	In the nucleus or organelles (rare)

🔄 Key Features of Plasmids:

Circular DNA molecules (just like a loop of string).
Carry a small number of genes usually non-essential, but helpful!
Commonly found in bacteria (prokaryotes).
Independent replication plasmids can copy themselves separately from chromosomes.
Often used in biotechnology to transfer genes (e.g., in genetic engineering).

💡 Why Are Plasmids Important?

Gene sharing: Bacteria can pass plasmids to each other in a process called conjugation, helping them spread traits quickly (like antibiotic resistance).
Survival advantage: Plasmids often carry genes that help bacteria survive tough environments (like toxins or antibiotics).
In biotech labs, scientists use plasmids to insert foreign genes into organisms (e.g., making insulin-producing bacteria).

🧠 Example:

A bacterium might have:

1 circular chromosome = main DNA (basic survival info)
2–3 plasmids = extra features (like resistance to antibiotics)

📌 Summary:

Plasmids = extra circular DNA, separate from chromosomes.
Found in both prokaryotes (commonly) and eukaryotes (rarely).
Carry non-essential but helpful genes.
Key players in horizontal gene transfer and biotechnology.

6.1.B – What Makes DNA the Perfect Hereditary Material?

🔑 What Is Hereditary Material?

DNA is the instruction manual of life – it carries the genetic code passed from parents to offspring. But why is DNA so good at this job? Let’s break it down 👇

🧩 1. Stable Structure

DNA is a double helix – two strands wound around each other like a twisted ladder.
This shape protects the genetic code inside 🧬.
The sugar-phosphate backbone gives it strength and stability — so the message doesn’t degrade easily over time.

🧬 2. Base Pairing Rules = Accurate Copying

The bases follow specific pairing:
- A (adenine) pairs with T (thymine)
- C (cytosine) pairs with G (guanine)
This ensures that when DNA is copied, it makes almost perfect duplicates, which is critical for heredity!

🔁 3. DNA Can Be Replicated

One of the most amazing things about DNA: it can make copies of itself!
Each strand serves as a template to build the other — like a zipper being unzipped and re-zipped.
This allows genetic information to be passed on to new cells and new generations.

✨ 4. Carries a Huge Amount of Info

The order of bases (A, T, G, C) is like letters in a language.
These sequences code for genes, which build proteins and control everything in the body.
Even with just 4 types of bases, DNA stores billions of instructions!

🛠 5. Can Be Modified, but Not Easily Broken

DNA can mutate (change), which allows evolution and adaptation over time.
But it’s also protected by repair enzymes and its tightly packed form (like wrapping it in bubble wrap).

6.1.B.1 – Base Pairing in Nucleic Acids: A Key to Genetic Accuracy

🧠 What Are Nucleic Acids?

Nucleic acids (DNA & RNA) are long chains made up of nucleotides, and each nucleotide has three parts:

A sugar (deoxyribose in DNA, ribose in RNA)
A phosphate group
A nitrogenous base (the “letter” of the genetic code: A, T, G, C, or U)

🧬 The Nitrogenous Bases:

There are two main types of bases in nucleic acids:

1. Purines

Double-ring structures (bigger molecules)
Includes:
- Adenine (A)
- Guanine (G)

2. Pyrimidines

Single-ring structures (smaller molecules)
Includes:
- Cytosine (C)
- Thymine (T) – in DNA
- Uracil (U) – replaces T in RNA

🔗 Base Pairing Rules (Chargaff’s Rules)

Molecule	Base Pairing
DNA	A 🧬 T G 🧬 C
RNA	A 🧬 U G 🧬 C

A pairs with T (or U in RNA) → 2 hydrogen bonds
G pairs with C → 3 hydrogen bonds (stronger pair)

⚖️ Why purines always pair with pyrimidines?

To keep the DNA double helix uniform in width – one big base with one small base = perfect fit.

🌍 Conserved Through Evolution

This base-pairing system is conserved across all living organisms – meaning it’s stayed the same through billions of years of evolution 🔁
Why? Because it’s efficient, accurate, and reliable for:

Copying DNA during cell division
Passing down genetic info
Building RNA & proteins correctly

A. Natural Selection
➢ Charles Darwin

British naturalist who sailed the world
Developed theory of evolution based on natural selection after studying animals on Galapagos Islands
Observed that there were similar animals on isolated animals, but they each had slight variations (ex. Beak shape, neck length, etc.)
■ There must have originally been a variety of beak lengths, but only the longest ones could survive. Since those with the longest beaks could reproduce better, they were more likely to contribute offspring with the same traits to the next generation
The Origin of Species
■ Variation exists in the population and some of this variation is heritable
■ Populations tend to make lots of offspring
■ Resources are limited, thus a struggle ensues
■ Those with better traits (phenotypes) will do a better job getting those resources and reproduce more
■ The genes that code for “better” traits in the current environment start increasing in the population
● The earth is always changing what what may be good now may not be in the future; evolution will always be occurring

➢ “Survival of the fittest”
➢ Jean-Baptiste Lamarck

Widely accepted theory of evolution in Darwin’s
Acquired traits inherited and passed onto offspring
■ “Law of use and disuse”
■ According to Lamarck, giraffes have long necks because they constantly use them
WRONG
■ Changes in somatic cells do not change gametes and thus cannot be passed onto offspring

➢ Evidence for Evolution
○ Paleontology
■ Study of fossils
■ Revealed great variety of organisms and the major lines of evolution
■ Tend to form in sedimentary rock

Biogeography
■ Study of the distribution of flora and fauna in the environment
■ Related species found in widely separated regions of the world
■ Suggests a common ancestor between species
■ Ex: pangea
Embryology
■ Study of the development of an organism
■ Early stages of vertebrate development all look alike
● All vertebrates show fishlike “gill slits”
Comparative anatomy
■ Study of anatomy of various animals
■ Vestigial Strucutres
● Structures with little-no function
● Remannts of structures that served important functions in the organism’s ancestors
■ Homologous structures
● Similar structures that serve different functions
■ Analogous structures
● Same function different structure

Molecular biology
■ Most compelling proof of evolution
■ Examining nucleotide/amino acid sequences of different organisms
■ Common genes and proteins
■ Ex: Hox Genes (body paint controller genes)
■ Shared structures help us to understand not only how structures develop but also supports shared shared ancestry
Artificial Selection
■ Humans selecting which organisms reproduce and survive so the future generations have traits that humans have selected
■ Ex. dog breeds

B. Common Ancestry
➢ Phylogenetic trees

Aka cladogram
Study the relationships between organisms
Begin with common ancestor and then branch out
Anytime there is a fork in the road, it is called a common ancestor node
■ Common ancestors likely do not exist anymore, but they are the point at which evolution went in two directions
Outgroup

■ Related to ingroup but diverged before others
■ Serves as reference group
■ Always stems from foot of tree

Monophyletic=ancestral species+all descendants
Paraphyletic=ancestral species+some descendants
Polyphyletic=ancestral species+descendants+distantly related organism
Taxonomic categories
■ DOMAIN
■ KINGDOM
■ CHORDATA
■ PHYLUM
■ CLASS
■ ORDER
■ FAMILY
■ GENUS
■ SPECIES

C. Genetic Variability
➢ Genetic variability: differences in each person/individual
➢ Only identical twins have exactly identical sets of alleles
➢ Survival of a species is dependent on genetic variation, allowing a species to survive in a changing environment

Natural selection only occurs if some individuals have more evolutionary fitness and can be selected

➢ Cause of genetic variability

Random mutations
DNA polymerase errors
Changes to DNA by transposons, etc.
Meiosis
■ Crossing-over
■ Independent assortment
Bacteria
■ Conjugation
■ Transduction
● Viruses pass around chunks of the bacterial genome during infection

D. Causes of Evolution
➢ Natural selection works internally through random mutations and externally through environmental pressures
➢ As long as a mutation does not kill an organisms before it reproduces, it may be passed on to the next generation
➢ Advantages brought about by a mutation will only be apparent until environmental pressure occurs
➢ Adaptation: variation favored by natural selection
➢ Any trati that causes an individual to reproduce better gives that individual evolutionary fitness
➢ Sexual selection can also spur evolution
➢ Speciation occurs once 2 populations can no longer reproduce together

Biological Species Theory
■ Species is a group of populations whose members have the potential to interbreed in nature and produce fertile, viable offspring
■ They do not breed successfully with other populations

➢ Catastrophic events speed up natural selection
➢ Genetic drift

Something that causes a change in a population besides natural selection
Caused by random events that drastically reduce the number of individuals in a population
Bottleneck effect
■ Sudden change in environment reduces size of population
■ New gene pool is not reflective of original population
Founder effect
■ Few individuals become so isolated from larger population and establish a new popluation that may not be representative of original population
Gene Flow
■ Tends to make a population more similar
■ When a poppulation gains/loses alles by genetic additions/subtractions from population
Only a few individuals are left to mate and regrow a population, so their traits become overemphasized without necessarily having any reproductive advantage

➢ Types of SElection

Directional selection favors one extreme of the normal distribution
Stabilizing selection: extreme traits are selected against
Disruptive selection favors both extremes,common traits selected against

E. Species

➢ Divergent evolution

In order for a population to split into different species, they must be reproductively isolated
Allows the two groups to undergo natural selection and evolve differently
a population evolves into 2 separate species due to different variation/environmental pressures until the 2 groups can no longer mate together
Prezygotic barriers prevent fertilization
■ Habitat isolation
● 2 species don’t encounter each other
● Same geographic area, but different habitats
■ Temporal Isolation
● Species breed during different times of da/season/etc.
■ Behavioral Isolation
● Different courting rituals
● Species do not respond to mating signals
■ Mechanical isolation
● Unsuccessful mating attempt
● Species are anatomically incompatible
■ Gametic isolation
● Sperm unable to fertilize eggs
● Gametes are unable to fuse to forma zygote
Post-zygotic barriers are related to the inability of the hybrid to survive/reproduce
■ Reduced hybrid viability
● Genes impair hybrid development
■ Reduced hybrid fertility
● Sterile hybrids
■ Hybrid breakdown
● Weaker hybrids over generations
● Offspring of htbrids are weak/sterile

➢ Convergent evolution

Process by which two unrelated and dissimilar species come to have analogous traits
Often due to exposure to similar selective pressures

➢ 2 types of speciation

Allopatric
■ Population becomes separated from the rest of the species by a geographic barrier so the 2 population can interbreed
Sympatric
■ New species form without any geographic barrier
■ Common in plants
■ Through polyploidy, sexual selection, habitat differentiation, etc.

➢ Polyploidy

Presence of extra sets of chromosomes due to accidents during cell division
Common in plants

➢ Autopolyploidy

Individual with more than two chromosome sets
derived from same species

➢ Allopolyploid

Species with multiple sets of chromosomes derived from different species (interbreeding)

F. Population Genetics
➢ Hardy-Weinberg Equilibrium

Law states that even with all the shuffling of genes that goes on, the relative frequencies of genotypes in a population still prevail over time, creating a stable gene pool
5 Conditions
■ Large population
■ No mutations
■ No immigration/emigration
■ Random mating
■ Natural selection
All populations violate one of these five–populations are always evolving

➢ p + q = 1
➢ $\square^2+2 \square \square+\square^2=1$

AP Biology 6.1 DNA and RNA Structure Study Notes