The nucleus, DNA and genes

Nucleic acids are polymers of nucleotides. Each nucleotide has a phosphate, a five-carbon sugar and a nitrogenous base.

• Amino acids:
• Correct: Nucleotides:
• Monosaccharides:
• Fatty acids:

An enzyme lowers the activation energy, so the reaction runs faster. It comes out of the reaction unchanged and does not change how much energy the reaction releases.

• Correct: It lowers the activation energy, which speeds the reaction:
• It supplies the energy the reaction releases:
• It becomes one of the products:
• It raises the activation energy:

Rough ER is studded with ribosomes, which gives it its rough look. Its membranes connect to the outer layer of the nuclear envelope.

• Golgi apparatus:
• Smooth ER:
• Correct: Rough ER:
• Lysosome:

Guanine pairs with cytosine. A purine (guanine) pairs with a pyrimidine (cytosine), and the two line up to form three hydrogen bonds.

• Adenine: Adenine is a purine, like guanine. Two purines together would make a rung too wide for the helix, and they cannot line up hydrogen bonds. Adenine pairs with thymine.
• Thymine: Thymine is a pyrimidine, so the width would fit, but its hydrogen-bonding groups do not line up well with guanine's. Thymine pairs with adenine.
• Correct: Cytosine: Correct. Guanine and cytosine form three hydrogen bonds and make a rung of standard width.
• Another guanine: Two guanines are two purines: too wide for the helix, and their bonding groups do not match.

Thymine pairs with adenine, so A = T = 22%, and A + T = 44%. That leaves 56% for G + C. Guanine pairs with cytosine, so G = C, and C = 56% ÷ 2 = 28%.

• 22%: 22% is the thymine value, and also the adenine value, since A pairs with T. Cytosine pairs with guanine, not thymine.
• 56%: 56% is guanine and cytosine together. Split it in half, because every guanine has one cytosine partner.
• Correct: 28%: Correct. A + T = 44%, G + C = 56%, and cytosine is half of that.
• 44%: 44% is adenine plus thymine together, not cytosine.

Copying doubles the DNA but not the chromosome count. Each of the 46 chromosomes now consists of two sister chromatids joined at the centromere, so there are 46 chromosomes and 92 chromatids.

• 92 chromosomes and 92 chromatids: This counts each chromatid as a separate chromosome. Sister chromatids stay joined, and together they are still one chromosome.
• 46 chromosomes and 46 chromatids: This is the cell before copying, when each chromosome is a single DNA molecule. Copying adds a second chromatid to each.
• 23 chromosomes and 46 chromatids: 23 is the count in an egg or sperm cell, which carries one set. Your body cells carry two sets, 46 chromosomes.
• Correct: 46 chromosomes and 92 chromatids: Correct. 46 chromosomes, each made of two sister chromatids.

Replication is semiconservative. Each new double helix keeps one old strand, which served as the template, and pairs it with one new strand. The old strands do not rejoin.

• Helicase separates the two strands by breaking the hydrogen bonds between paired bases.: This step is right. Helicase breaks the hydrogen bonds between bases, which separates the strands without breaking either backbone.
• Free nucleotides pair with the exposed bases: A with T and G with C.: This step is right. Free nucleotides pair with the template by the base-pairing rule.
• DNA polymerase links the new nucleotides into a strand and removes most wrongly placed bases.: This step is right. DNA polymerase joins the nucleotides with covalent bonds and proofreads as it goes.
• Correct: Each new double helix is made of two new strands, and the two old strands rejoin each other.: This is the error. Each copy keeps one old strand and gains one new strand (semiconservative replication).

Each phosphate group in DNA's backbone carries a negative charge. Opposite charges attract, so the negative backbone clings to the positive histone surface.

• DNA's bases form hydrogen bonds with the histones: The bases are tucked into the middle of the helix, paired with each other. The histones contact the outside of the helix, which is the backbone.
• Correct: The negative phosphate backbone is drawn to the positive histones: Correct. The attraction between opposite charges holds DNA on the histone core.
• DNA's sugars are hydrophobic and hide inside the histone core: Deoxyribose is a polar sugar, and the backbone faces the watery nucleus. The binding comes from charge, not from avoiding water.
• Histones cut the DNA into short pieces that fit each core: Histones package DNA without cutting it. Each chromosome stays one continuous DNA molecule.

Almost every cell with a nucleus carries the same genome, copied from the same first cell. Cells differ because each uses a different subset of genes.

• The liver cell carries only the genes for liver work: Cells do not discard the genes they do not use. A liver cell still carries the genes for skin proteins.
• Correct: They carry the same genome, but each uses a different set of genes: Correct. Same DNA, different genes in use.
• The skin cell has fewer chromosomes than the liver cell: Both have 46 chromosomes. Chromosome number does not differ between body cell types.
• Each carries a different sequence of bases in the same genes: Every copy of your DNA comes from the same original through accurate replication, so the sequences are the same apart from rare copying errors.