Campbell Biology · Chapter 16
The Molecular Basis of Inheritance
pp. 305–324 · 3 sections
This chapter asks how biologists proved that genes are made of DNA, how DNA's double-helix shape works, and how a cell copies its DNA accurately before dividing. It finishes with how a cell packs a couple of meters of DNA into a tiny nucleus. DNA structure and replication are core AP material (Topics 6.1 and 6.2), and chromatin packing sets up gene regulation in Topic 6.5.
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16.1 How we know genes are DNA, and the double helix
pp. 305–310
Topic 6.1 tests base pairing, purines pairing with pyrimidines and antiparallel strands, and Topic 1.6 tests a nucleotide's parts. The course doesn't name the classic experiments or scientists, but a question can describe a similar experiment and ask what it shows.
In the course: Topic 6.1 DNA and RNA Structure, Topic 1.6 Nucleic Acids, Topic 6.7 Mutations (notes, videos and more questions)
Key points
- In the early 1900s most biologists bet that genes were proteins. Proteins come in endless shapes and jobs, while DNA has only four kinds of nucleotides and looked too plain to hold so much information.
- Work with bacteria showed that a chemical from dead cells could permanently change the traits of living cells, a change called transformation. When the DNA in that chemical mix was destroyed, the change stopped, so DNA was the cause.
- Work with viruses that infect bacteria pointed the same way. A virus's DNA goes into the host cell and directs it to build new viruses, while the protein coat stays outside. Radioactive sulfur tags protein and radioactive phosphorus tags DNA, which is how the two parts were told apart.
- Chargaff's rules: the mix of bases differs from one species to another, but in double-stranded DNA the amount of A matches T and the amount of G matches C.
- DNA is a double helix. Two sugar-phosphate backbones run along the outside, and the bases point inward and pair up through hydrogen bonds: A with T (2 bonds) and G with C (3 bonds). The two strands are antiparallel, so one runs 5′→3′ and its partner runs 3′→5′.
- Each pair joins a two-ring purine (A or G) to a one-ring pyrimidine (T or C). That keeps the helix the same width everywhere, which matched the X-ray images Rosalind Franklin made.
- Pairing rules fix which base sits across from which, but not the order of bases along a strand. That order is where genetic information lives, and because each strand predicts its partner, the structure hints at how DNA gets copied.
Key terms (12)
- genetic material
- The molecule that carries hereditary instructions from parent to offspring. In every cell it's DNA; some viruses use RNA instead.
- transformation
- A lasting change in a cell's genes and traits after it takes in DNA from its surroundings. Bacteria can gain new traits this way.
- bacteriophage
- A virus that infects bacteria, often just called a phage. Its basic parts are a nucleic acid genome packed inside a protein coat.
- radioactive label
- An unstable isotope built into a molecule so you can follow where that molecule goes. Sulfur tags protein and phosphorus tags DNA.
- Chargaff's rules
- Two findings about DNA's bases: their proportions vary between species, and in double-stranded DNA, A equals T and G equals C.
- double helix
- DNA's shape: two strands twisted around each other like a spiral staircase, with paired bases as the steps.
- sugar-phosphate backbone
- The outer rail of each DNA strand, made of sugars and phosphates linked in a chain. The bases stick out from it.
- antiparallel
- Running in opposite directions. In DNA, one strand goes 5′→3′ and the strand paired with it goes 3′→5′.
- purine
- A base with a double-ring structure: adenine (A) or guanine (G).
- pyrimidine
- A base with a single ring: cytosine (C), thymine (T) or, in RNA, uracil (U).
- complementary base pairing
- The rule that A pairs with T (or U in RNA) and G pairs with C. Knowing one strand tells you the other.
- X-ray crystallography
- A method that shines X-rays through a crystal or fiber of a molecule and uses the scattering pattern to work out its shape.
Check yourself: 16.1 How we know genes are DNA, and the double helix
4 questions on 16.1 How we know genes are DNA, and the double helix. Pick an answer to see if you got it, and why.
Which feature of DNA keeps the double helix the same width along its entire length?
Cytosine makes up 21% of the nucleotides in the double-stranded DNA of a species of moss. What percent of its nucleotides are adenine?
A newly discovered virus has a genome with this base makeup: A 31%, T 18%, G 27%, C 24%. What kind of genome does the virus most likely have?
One strand of a DNA segment reads 5′-ATGCCA-3′. Which sequence is its complementary strand, written 5′ to 3′?
0 of 4 answered
16.2 Copying DNA: replication, proofreading and repair
pp. 311–319
Topic 6.2 tests semiconservative copying, the 5′→3′ rule, leading versus lagging strands, RNA primers and what helicase, topoisomerase, DNA polymerase and ligase do. Polymerase numbers, repair pathways and telomeres are background, and errors that slip through are the mutations of Topic 6.7.
In the course: Topic 6.2 DNA Replication, Topic 6.7 Mutations, Topic 4.5 Cell Cycle (notes, videos and more questions)
Key points
- Each strand holds the information to rebuild the other, so when DNA is copied the two strands come apart and each one serves as a template. The result is semiconservative: each new double helix has one old strand and one new strand.
- Experiments that tracked heavy and light forms of nitrogen through rounds of copying found DNA of in-between density after one round. That ruled out a model where the old helix stays whole, and later rounds ruled out a model where old and new DNA are mixed along every strand.
- Copying starts at origins of replication: one on a circular bacterial chromosome, many along each eukaryotic chromosome. Two forks move away from each origin in opposite directions, opening a bubble.
- At each fork, helicase unzips the strands, single-strand binding proteins keep them apart, and topoisomerase cuts and rejoins DNA ahead of the fork to release the extra twisting.
- DNA polymerase can only add nucleotides to an existing 3′ end, so new DNA always grows 5′→3′, and an RNA primer made by primase starts each new stretch. The incoming nucleotides carry three phosphates and lose two as they join, which powers the reaction.
- Because the templates are antiparallel, the leading strand is built in one continuous piece toward the fork, while the lagging strand is built away from the fork in short Okazaki fragments. The RNA primers are later swapped for DNA, and ligase seals the pieces into one strand.
- Polymerase proofreads each new base, and other enzymes fix mismatches and damage later, often by cutting out a stretch and refilling it using the other strand as a guide. Rare errors that slip through become mutations, the raw material for evolution. Linear chromosomes lose a little DNA at their tips each round, which telomeres buffer and telomerase restores in germ cells.
Key terms (15)
- semiconservative replication
- The way DNA is copied: each new double helix keeps one strand from the original and pairs it with one newly built strand.
- origin of replication
- A specific DNA sequence where copying begins. Forks head out from it in both directions, forming a replication bubble.
- helicase
- The enzyme that unzips DNA at a replication fork by breaking the hydrogen bonds between paired bases.
- single-strand binding protein
- A protein that coats unzipped DNA strands so they don't snap back together before they're copied.
- topoisomerase
- An enzyme that relieves the overwinding ahead of a replication fork by cutting DNA, letting it untwist and rejoining it.
- RNA primer
- A short stretch of RNA, laid down by the enzyme primase, that gives DNA polymerase a 3′ end to build from.
- DNA polymerase
- The enzyme that builds new DNA by adding nucleotides that match the template, always onto a 3′ end, so the strand grows 5′→3′.
- leading strand
- The new strand that's built in one continuous piece, moving in the same direction the fork is opening.
- lagging strand
- The new strand that's built in short pieces, each started near the fork and extended away from it.
- Okazaki fragment
- One of the short DNA pieces of the lagging strand. Each starts with its own RNA primer.
- DNA ligase
- The enzyme that seals gaps in a DNA backbone, joining Okazaki fragments into one unbroken strand.
- proofreading
- DNA polymerase checking each base it adds and removing it right away if it doesn't pair correctly.
- nucleotide excision repair
- A repair system that cuts out a damaged stretch of one strand and fills the gap by copying the intact strand.
- telomere
- A run of short repeated sequences at the end of a linear chromosome. It has no genes and gets shorter as copying trims the tips.
- telomerase
- An enzyme that lengthens telomeres. It's active in germ cells and many cancer cells but not in most body cells.
Check yourself: 16.2 Copying DNA: replication, proofreading and repair
4 questions on 16.2 Copying DNA: replication, proofreading and repair. Pick an answer to see if you got it, and why.
Yeast cells are grown for many generations in a medium containing only a heavy isotope of carbon (¹³C), so all of their DNA is heavy. They are then moved to a medium containing only normal carbon (¹²C) and allowed to copy their DNA three times. Assuming replication is semiconservative, what fraction of the double helices contain any heavy DNA?
Fluoroquinolone antibiotics block a topoisomerase in bacteria. In a treated bacterium, which problem would most directly arise during DNA replication?
A replication fork is opening toward the right. The top template strand runs 3′ on the left to 5′ on the right. How is the new strand paired with this template made?
A researcher adds a drug that completely blocks primase to dividing cells. Which effect on DNA replication is most likely?
0 of 4 answered
16.3 Packing DNA into chromosomes
pp. 320–322
Topic 6.1 covers histones and circular bacterial versus linear eukaryotic chromosomes, and Topic 6.5 covers how tighter or looser chromatin turns genes down or up. You won't need fiber widths or the names of the histone types.
In the course: Topic 6.1 DNA and RNA Structure, Topic 6.5 Regulation of Gene Expression, Topic 4.5 Cell Cycle (notes, videos and more questions)
Key points
- Most bacteria have one circular chromosome with a modest amount of protein. It's folded and supercoiled into a region called the nucleoid, which has no membrane around it. Many bacteria also carry plasmids, small extra circles of DNA.
- A eukaryotic cell has several linear chromosomes. Each one is a single long DNA double helix plus a lot of protein, and this DNA–protein mix is called chromatin. Stretched end to end, the DNA in one human cell would be about 2 meters long.
- Histones are small proteins packed with positively charged amino acids, which grip DNA's negatively charged phosphates. DNA wraps around a core of eight histones to form a nucleosome, and a row of nucleosomes joined by linker DNA looks like beads on a string.
- Nucleosomes fold into thicker fibers, and those form loops held on a protein framework. Before mitosis, chromatin condenses the most, into short, thick chromosomes that can be pulled apart without tangling.
- Between divisions, most chromatin is loosely packed euchromatin, where genes can be read. Some stays tightly packed as heterochromatin, such as near centromeres and telomeres, and its genes are mostly silent.
- Packing is not fixed. Chemical tags on histone tails and on DNA loosen or tighten chromatin, which turns genes up or down without changing the sequence. That's the basis of epigenetic regulation in Topic 6.5.
Key terms (10)
- chromatin
- The mix of DNA and proteins that makes up eukaryotic chromosomes. It can be loosely or tightly packed.
- histone
- A small, positively charged protein that DNA winds around. Wrapping DNA around histones is the most basic stage of packing it in eukaryotes.
- nucleosome
- A unit of DNA packing: a short length of DNA wrapped around a core of eight histone proteins.
- linker DNA
- The short stretch of DNA that runs between one nucleosome and the next.
- histone tail
- The end of a histone that sticks out from the nucleosome. Chemical tags added here change how tightly chromatin packs.
- nucleoid
- The region of a bacterial cell where its chromosome is packed. It isn't wrapped in a membrane.
- plasmid
- A small, circular DNA molecule separate from the main chromosome, common in bacteria. It often carries genes like antibiotic resistance.
- euchromatin
- Loosely packed chromatin. Its DNA is easy for the transcription machinery to reach, so its genes can be active.
- heterochromatin
- Tightly packed chromatin that stays condensed even between divisions. Its genes are mostly switched off.
- condensed chromosome
- The compact, thick form chromatin takes before mitosis or meiosis, which lets chromosomes be moved without tangling.
Check yourself: 16.3 Packing DNA into chromosomes
4 questions on 16.3 Packing DNA into chromosomes. Pick an answer to see if you got it, and why.
In a lab, a histone is made in which its positively charged amino acids are replaced with negatively charged ones. What would most likely happen when this histone is mixed with DNA?
Researchers insert the same gene for a glowing protein into two places in the genome of a fruit fly. Copy 1 lands in a region that stays tightly packed throughout the cell cycle. Copy 2 lands in a loosely packed region. Which result is most likely?
A drug blocks the enzymes that remove acetyl groups from histone tails, so acetyl groups build up. Adding acetyl groups to histones cancels some of their positive charge. What is the most likely effect on the cell?
Which statement correctly compares the main genetic material of a typical bacterium with that of a typical animal cell?
0 of 4 answered