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Chapter 12: Molecular Genetics

Chapter Worksheet

Ch. 12.1 DNA: The Genetic Material

DNA Structure


In 1920s biochemist P.A. Levene determined the basic structure of nucleotides that make up DNA.
Nucleotides are the subunits of nucleic acids; consisting of a 5-carbon sugar, a phosphate group, and a nitrogenous base.

  1. DNA: contains the sugar deoxyribose, a phosphate, and a one of four bases (adenine, guanine, cytosine, thymine)
  2. RNA: contains the sugar ribose, a phosphate, and one of four bases ( adenine, guanine, cytosine, uracil)

Purines- bases with double rings; guanine, adenine
Pyrimidine- bases with single rings; thymine, cytosine, and uracil


Erwin Chargaff analyzed the DNA of various species. His findings are known as Chargaff's rule: C=G, T=A.

X-ray Diffraction

Rosalind Franklin and Maurice Wilkins shot X-rays at DNA in a method called X-ray diffraction. Franklin took a photo that indicated DNA was a double helix. Double helix- twisted ladder shape, formed by two strands of nucleotides twisted around each other.

Watson and Crick

Using Franklin's X-ray diffraction picture and Chargaff's rule, Watson and Crick built a model of the double helix.

DNA Structure

DNA is often compared to a twisted ladder; the rails of the ladder are the deoxyribose and phosphate while the base pairs form the rungs.
Purines always attach to Pyrimidines. A to T, C to G; referred to as complementary base pairing.


The two strands of the DNA molecule run in opposite directions. This is referred to as antiparallel. One strand is oriented 5' to 3' while opposite side is oriented 3' to 5'.

The Announcement

In 1953, Watson and Crick published their findings suggesting the structure for DNA and hypothesizing a method of replication.

Chromosome Structure

In prokaryotes, DNA is contained in the cytoplasm and consists mainly of a ring. Eukaryotic DNA is organized into individual chromosomes. The length of a human chromosome ranges from 51 million to 245 million base pairs.
DNA tightly coils around a group of beadlike proteins called histones. Phosphates have a create a negative charge which attracts positively charged histone proteins to form a nucleosome. Nucleosomes group together to form chromatin fibers, which supercoil to make chromosomes.

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Ch. 12.2 Replication of DNA

DNA replicates by making a strand that is complementary to each original strand.

Semiconservative Replication

Watson and Crick suggested a possible method of replication. Semiconservative replication- parental strands of DNA separate, serve as templates, and produce DNA molecules that have one strand of parental DNA and one strand of new DNA.
DNA replication occurs during interphase of mitosis and meiosis and occurs in three main stages.


An enzyme, DNA helicase, unwinds and unzips the double helix breaking the hydrogen bonds between the bases. Poteins called single-stranded binding proteins keep the strands from rejoining. While unwinding, the enzyme RNA primase adds a short segment of RNA (RNA primer) on each DNA strand.

Base Pairing

The enzyme DNA polymerase adds appropriate nucleotides to the new DNA strand; starting at 3' end. Following base pairing the templates create identical copies of the original DNA.

The two new strands are created differently. One strand, the leading strand, is elongated continuously. The other strand, the lagging strand, elongates away from the replication fork. Okazaki fragments- small segments created by DNA polymerase in the 3' to 5' direction. The fragments are then connected by the enzyme DNA ligase.


Replication occurs in multiple sites on the DNA molecule. When DNA polymerase comes to an RNA primer, it removes it and replaces it with DNA nucleotides. DNA ligase then links the sections.

Comparing DNA Replication in Eukaryotes and Prokaryotes

Eukaryotic DNA has multiple areas of replication occurring at the same time. In prokaryotes,th circular DNA strand is opened at one origin of replication. Replication then occurs in two directions.

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Ch. 12.3 DNA, RNA, and Protein

Central Dogma

DNA codes for RNA, which guides protein synthesis


RNA is a nucleic acid similar to DNA. Has the sugar ribose instead of deoxyribose, the base uracil replaces thymine, is a single strand.

  1. Messenger RNA (mRNA)- formed complementary to one strand of DNA.
  2. Ribosomal RNA (rRNA)- forms ribosomes.
  3. Transfer RNA (tRNA)- transport amino acids to the ribosome.


Transcription- the synthesis of mRNA from DNA in the nucleus. DNA is unzipped; RNA polymerase- an enzyme that regulates RNA synthesis, binds to DNA where mRNA is to be made; RNA polymerase moves along template DNA strand; mRNA is made complementary to DNA strand; new mRNA then leaves nucleus through nuclear pores into the cytoplasm

RNA Processing

mRNA in nucleus is pre-mRNA. Small pieces called introns are removed so that the final mRNA to leave is composed of exons.

The Code

The only way DNA varies among organisms is in the sequence of the bases. Proteins are made from 20 amino acids. A group of three bases, a codon, codes for a amino acid; there are 64 possible codons. Three codons are "stop" codons, while the codon AUG codes for the amino acid methionine and is the "start" codon.


Once mRNA leaves the nucleus it travels to the ribosome. Here it is read and translated into a protein through translation. tRNA is folded into a cloverleaf shape that attaches a specific amino acid, and has a three-base sequence called the anticodon. Each anticodon is complementary to a codon on the mRNA.

The Role of the Ribosome

The ribosome consists of two subunits, the two parts come together and attach to the mRNA. A tRNA with the anticodon CAU carrying a methionine move in and bind to the mRNA start codon of AUG at the P-site. A second tRNA moves in next at the A-site that is complimentary to the second codon of the mRNA. A bond forms between the two amino acids causing them to shift forward. The first amino acid is now at the E-site to exit. The ribosome continues along mRNA strand linking amino acids that are complimentary until the A site contains a "stop" codon signaling the end of protein synthesis.

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Ch. 12.4 Gene Regulation and Mutation

Recall, multicellular eukaryotes develop from a zygote. The zygote undergoes mitosis to produce all the different cells needed by an organism. Differentiation then specializes the cells in structure and function.

Hox Genes

Hox genes are a group of genes that important for determining body plans of organisms. These genes are transcribed at specific times, and located in specific places on the genome to control what body part will develop in a given location. Clusters of Hox genes have bee found in all animals.


Cells sometimes make mistakes during replication but cells have repair mechanisms that can repair some damage.
a permanent change in a cell's DNA.

Types of Mutations

Can range from a single base pair to the deletions of large pieces of chromosomes.

  • Point mutation- a change in a single base pair.
  • Substitution mutation- when a single base gets exchanged for another base.

Most substitutions are missense mutations, where the DNA code is altered so that it codes for the wrong amino acid. The change of on amino acid for another can change the sequence of amino acids enough to change a protein. Sickle-cell disease is the result of a point mutation in which the codon for glutamic acid (GAA) is changed to valine (GUA) in the protein, causing a change in structure of hemoglobin.

Nonsense mutations are when a substitution codes for a stop codon and terminates translation early. Nearly all nonsense mutations lead to proteins that do not function normally.

  • Frameshift mutation- mutations that change the "frame" of the amino acid sequence by changing the multiples of three in nucleotide sequence.
  • Deletion- the loss of a nucleotide.
  • Insertion- the addition of a nucleotide.

Large portions of DNA can be involved in mutations. A piece of an individual chromosome containing one or more genes can be deleted or moved to a different location on the chromosome, or to a different chromosome.

Causes of Mutation

Certain chemicals and radiation can damage DNA. Mutagens- substance which cause mutations. High-energy forms of radiation such as X-rays, gamma rays, UV rays are examples.

Body-cell vs. Sex-cell Mutation

Mutations in body cells (somatic cells) will be passed on to future daughter cells but not future offspring.

  1. Neutral mutations are mutations with no affect.
  2. Mutation might lead to cell death.
  3. Mutations might lead to cancer.

Mutations in sex cells (germ cells) are passed on to offspring and will be present in every cell of the offspring.

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Page last updated January 2, 2017.