DNA Structure

Deoxyribose and phosphate groups

The backbone of each DNA strand alternates deoxyribose sugars and phosphate groups.

Purines and deoxyribose: Purines are nitrogenous bases that connect the two backbones; they aren't part of the backbone.

Pyrimidines and purines: Purines and pyrimidines are bases that form the 'rungs' between the backbones.

Phosphate groups and pyrimidines: Pyrimidines are bases that connect the backbones; they aren't part of the backbone.

They contain DNA composed of the same four types of nucleotides.

All living organisms use DNA made of the same four nucleotides (A, T, G, C).

They are prokaryotic organisms.: Trees, cats, and amoebas are all eukaryotes.

They are classified as members of the same kingdom.: Trees, cats, and amoebas are in different kingdoms.

They have genomes that consist of the same number of genes.: The number of genes varies among species.

3 and 5 only

Adenine bonds to the deoxyribose sugar on its own strand and to thymine, its complementary base, on the other strand.

1, 3, 5, and 6 only: Adenine doesn't pair with another adenine, and phosphate bonds to the sugar, not to the base.

2 and 4 only: Ribose and uracil are found in RNA, not double-stranded DNA.

3, 4, 7, and 8 only: Guanine and cytosine don't pair with adenine, and uracil is found in RNA.

Cytosine and guanine

Cytosine and guanine are nitrogenous bases in DNA, the genetic code shared by all living organisms.

Estrogen and testosterone: These hormones are found only in vertebrate animals.

Hemoglobin and lymphocytes: These blood components are found only in animals.

Cellulose and chlorophyll: These are found mainly in plants, not animals.

5′ TCCAGTCCA 3′

Pair each base: A–T and G–C. AGGTCAGGT pairs with TCCAGTCCA, running 5′ to 3′.

5′ AGGTCAGGT 3′: This is the same sequence, not the complement. A pairs with T and G pairs with C.

5′ ACCUGAGGU 3′: DNA has no uracil (U), and the pairings don't follow the base-pairing rules.

5′ TGGACTGGA 3′: The pairings don't follow the rules: G must pair with C, not G.

Part J

Part J marks the nitrogenous base pairs. The sequence of these bases encodes genetic information.

Part F: Phosphate groups help form the backbone but do not encode the genetic sequence.

Part G: A complete structural unit is not the specific information-coding component; the base sequence is.

Part H: Deoxyribose sugar forms part of the backbone, not the genetic code.

The cells contain DNA composed of adenine, thymine, guanine, and cytosine.

All living cells use DNA containing the four bases adenine, thymine, guanine, and cytosine.

The cells are surrounded by a phospholipid bilayer and a cell wall made of cellulose.: Not all cells have a cellulose cell wall.

The cells have chromosomes that are located inside a membrane-bound nucleus.: Prokaryotic cells do not have membrane-bound nuclei.

The cells rely on mitochondria to carry out aerobic cellular respiration.: Prokaryotes lack mitochondria, and not all cells use aerobic respiration.

Hydrogen bond

X points to the weak attractions between complementary base pairs holding the two DNA strands together: hydrogen bonds.

Nucleotide: A nucleotide contains sugar, phosphate, and a base; X marks an attraction between bases.

Phosphate group: Phosphate groups are part of each strand's backbone, not the link labeled X.

Deoxyribose: Deoxyribose is a sugar in the backbone, not the hydrogen bond between strands.

Differences in the nucleotide sequences of genes

Inherited variation results from differences in the nucleotide sequences of alleles that affect coat color.

Differences in the numbers of chromosomes in cells: Normal inherited coat-color variation does not require different chromosome numbers.

Differences in the diets of individual cattle: Diet is environmental and is not the genetic basis of inherited variation.

Differences in the environmental conditions of different geographic areas: Environment can affect expression, but inherited variation originates in gene sequences.

The missing component is a nitrogenous base. The sequence of nitrogenous bases in DNA determines an organism's traits.

The full response identifies a nitrogenous base and explains that the sequence of adenine, cytosine, guanine, and thymine determines traits. One point is awarded for a partially correct response.

Nucleotides in DNA

Genes are located primarily in DNA nucleotides. Each nucleotide contains phosphate, a deoxyribose sugar, and a nitrogenous base. The sequence of nucleotides determines the gene.

Ribose sugars in RNA: The genetic code is not stored in RNA's ribose sugars. The sequence of DNA nucleotides stores inherited information.

Phosphates in DNA: Phosphates form part of the DNA backbone; their sequence does not encode the gene.

Exons in RNA: RNA exons code for proteins, but the genetic code is primarily located in DNA nucleotides.

From top to bottom: Base pair, Nucleotide, Phosphate

From top to bottom, T–A is a base pair, the circle–pentagon–G group is a nucleotide, and the lone circle represents phosphate. Two correct responses earn one point; all three earn two points.

Structure W is a chromosome and Structure Z is a gene.

Structure W is made of multiple genes, so it is a chromosome. Structure Z is made of many base pairs, so it is a gene.

Structure W is a gene and Structure Z is a chromosome.: Structure W is made of multiple genes, so it is a chromosome, and Structure Z is a stretch of base pairs, so it is a gene.

Structure W is an amino acid and Structure Z is a nucleic acid.: The diagram does not show any amino acids. Structures W and Z are both nucleic acids.

Structure W is a nucleic acid and Structure Z is an amino acid.: The diagram does not show any amino acids. Structures W and Z are both nucleic acids.

3’ T A G C G A T C 5’

The complementary DNA strand runs from 3’ to 5’ and pairs A with T and C with G, giving 3’ T A G C G A T C 5’.

5’ T A C G G A T C 3’: The complementary strand would run from 3’ to 5’.

5’ A T C G C T A G 3’: This repeats the original strand; the complementary strand runs from 3’ to 5’ with paired bases.

3’ G A T C G C T A 5’: This strand runs 3’ to 5’, but its base pairs are incorrect.