1A DNA molecule is a polymer built from many repeating units called nucleotides. Which three components make up ONE nucleotide?
Every nucleotide has exactly three parts: a phosphate group, a deoxyribose sugar and ONE nitrogenous base (A, T, C or G). Option B names ribose, which is not the sugar found in DNA. Option C describes two bases on one sugar, which does not exist. Option D confuses components of a nucleotide with a base pair (which needs nucleotides on two different strands) and with a hydrogen bond, which is a bond, not a building block.
Interphase (G1, S and G2) occupies most of the cell cycle and is the busiest part of it: the cell grows, makes organelles and proteins, and replicates its DNA. It is not a 'rest' stage. It is also not part of mitosis at all - it is the part of the CELL CYCLE that comes before M phase. Chromosomes stay as long, uncondensed chromatin threads during interphase; condensation happens in prophase.
（分裂）间期 (interphase) 包含 G1 期、S 期、G2 期，占细胞周期 (cell cycle) 的绝大部分时间，而且是最忙的阶段：细胞生长、合成细胞器 (organelle) 和蛋白质，并完成 DNA 复制 (DNA replication)。它绝不是"休息期"（这是最常见的错误说法）。它也不属于有丝分裂 (mitosis)，而是细胞周期中位于 M 期 (M phase) 之前的阶段。间期里 DNA 以细长的染色质 (chromatin) 形式存在，只有到了前期 (prophase) 才会螺旋缩短成显微镜下可见的染色体 (chromosome)。
3State the outcome of mitosis followed by cytokinesis in a human somatic cell.
Mitosis divides the nucleus and cytokinesis divides the cytoplasm, producing TWO daughter cells that are genetically identical to each other and to the parent cell, and that keep the parent's chromosome number (diploid, 46 in humans). The other three options import meiosis logic - four cells, halving of chromosome number, genetic variation - none of which happens in mitosis.
4Which statement about complementary base pairing in DNA is correct?
The pairing rule is fixed: A-T and C-G. The two bases of a pair are held together by weak hydrogen bonds (2 between A and T, 3 between C and G). Option B breaks the pairing rule. Option C names the wrong bond - covalent bonds hold the sugar-phosphate backbone together, not the base pair; if the bases were covalently bonded the helix could not unzip. Option D is the 'it just has to fit' misconception: pairing is specific, not merely a matter of size.
5DNA replication is described as semi-conservative. What does this mean?
'Semi-conservative' means half of each new double-stranded molecule is conserved (kept) from the parent molecule: one whole original strand acts as a template and pairs with one whole new strand. Option B splits each individual strand into old and new halves, which is not what happens. Option C is the conservative model, which Meselson and Stahl's evidence ruled out. Option D discards the original strands entirely, which contradicts the word 'semi-conservative'.
6Which statement best describes what happens during S phase of the cell cycle?
S phase is the DNA synthesis (replication) phase of interphase. After it, every chromosome is still ONE chromosome but is now made of two identical sister chromatids joined at a centromere. Option B describes prophase, option C describes G1, and option D describes anaphase. Note that replication is finished before mitosis starts - it never happens 'during prophase'.
7Which statement correctly contrasts cell division in bacteria with mitosis in a human cell?
Prokaryotes do not undergo mitosis. They divide by binary fission: the single circular chromosome is copied, the copies move apart, and the cell divides - with no nucleus, no nuclear envelope to break down and no spindle fibres. Option B repeats the common error that bacteria 'do mitosis'. Option C invents four daughter cells; binary fission gives two, normally genetically identical. Option D is wrong because the chromosome IS copied first - otherwise the daughter cells would not both get a full set of genes.
8One strand of a short section of DNA has the base sequence: T - A - C - G - G - C - A - T. What is the base sequence of the complementary strand?
Apply A-T and C-G base by base: T pairs A, A pairs T, C pairs G, G pairs C, G pairs C, C pairs G, A pairs T, T pairs A, giving A-T-G-C-C-G-T-A. Option B is simply the same sequence copied, the classic 'copy not complement' error. Option C leaves C opposite C and G opposite G at positions 3 and 4. Option D breaks the rule at the last two positions (A opposite A, T opposite T).
9During DNA replication the double helix 'unzips'. Which bonds are broken when this happens?
Only the weak hydrogen bonds between paired bases are broken, which is why the two strands can separate while each strand stays intact. The sugar-phosphate backbone is held by strong covalent bonds and is NOT broken - if it were, the strand would fall apart and could not act as a template. Option C would remove bases from the strand, destroying the sequence. Option D describes a structure at the centromere, which has nothing to do with unzipping the helix.
10A micrograph of an onion root-tip cell shows the following: no nuclear envelope is visible; each chromosome is clearly made of two chromatids; all the chromosomes lie in a single line across the middle of the cell; spindle fibres run from each pole and attach to the chromosomes at their centromeres. Identify the stage of mitosis shown.
The decisive clue is that the chromosomes are aligned in a single line on the equator (metaphase plate) with spindle fibres attached at their centromeres - this is metaphase. In prophase the chromosomes are condensing but are scattered and the spindle is still forming. In anaphase the centromeres have already divided and the separated chromatids are moving towards the poles, so nothing is lined up at the equator. In telophase the chromosomes are at the poles, decondensing, and nuclear envelopes are reforming.
11Which option places these structures in order from SMALLEST to LARGEST?
A base pair is a single rung of the DNA ladder; a gene is a length of DNA made of many base pairs; a chromosome is one very long DNA molecule (with its associated proteins) carrying many genes; the genome is the complete set of an organism's DNA - in humans, all 46 chromosomes. The other options invert the gene/base-pair or gene/chromosome relationship, which is the common error of treating 'gene' and 'chromosome' as interchangeable or assuming one chromosome carries one gene.
12A student examines a stained garlic root-tip squash under the microscope. Of the 250 cells counted, 40 show condensed chromosomes and spindle fibres, indicating they are undergoing mitosis. Calculate the mitotic index of this tissue.
Mitotic index = (number of cells in mitosis ÷ total number of cells counted) × 100 = (40 ÷ 250) × 100 = 16%. Option B is the result of dividing the wrong way round (250 ÷ 40). Option C simply reports the raw count as a percentage. Option D is the percentage of cells NOT in mitosis (which, usefully, tells you that most cells are in interphase at any moment).
13A human somatic cell has just completed S phase and is now in G2. How many chromosomes and how many chromatids does this cell contain?
Replication in S phase does not change the chromosome NUMBER. Each of the 46 chromosomes is now made of two identical sister chromatids joined at a centromere, so there are 46 chromosomes but 92 chromatids. Counting chromatids as chromosomes (option B) is the single most common error in this topic: a replicated chromosome is still ONE chromosome. Option D describes a haploid number, which is not what a somatic cell has.
14Which statement BEST explains why the two daughter cells produced by mitosis are genetically identical to each other?
The identity comes from COPYING followed by equal separation, and a full-mark answer must name both steps. Option B is the answer markers penalise: sharing DNA evenly without copying it first would give each cell only half the genetic information, not a complete identical set. Option C imports meiosis - homologous chromosomes are not paired or separated in mitosis. Option D is simply false: mitosis conserves the chromosome number.
"完全相同"来自两步：先复制、再均分，缺一不可，满分答案必须两步都写出来。选项 B 正是评分老师会扣分的写法——只"平均分配"而没有先复制，每个子细胞只会拿到一半的遗传信息，根本不可能相同。C 把减数分裂 (meiosis) 的情节搬了进来：有丝分裂中同源染色体并不配对，也不被分开。D 是错误陈述：有丝分裂维持染色体数目不变，并不减半。
15A drug completely blocks DNA replication, but a cell nevertheless enters mitosis. Predict the most likely consequence for the two daughter cells.
Mitosis only SEPARATES material that already exists; it does not copy DNA. Without S phase, every chromosome is a single unreplicated chromatid, so there is no identical partner to send to the opposite pole and the daughter nuclei cannot each receive a complete set of chromosomes. Option B is the opposite of blocking replication. Option C describes meiosis, which is not what mitosis does under any circumstances. Option D confuses cell size with genetic content - the problem is missing genetic information, not volume.
Write full sentences and use the proper terms — that is where the marks are.
1Describe the structure of a DNA molecule. (3 marks)3 marks
Full-mark answer
DNA is a polymer made of many nucleotides, and each nucleotide consists of a phosphate group, a deoxyribose sugar and one nitrogenous base (adenine, thymine, cytosine or guanine). The nucleotides of one strand are joined through their sugars and phosphates by strong covalent bonds, forming a sugar-phosphate backbone. Two such strands run in opposite directions (antiparallel) and are wound into a double helix, held together by weak hydrogen bonds between complementary bases: adenine always pairs with thymine (two hydrogen bonds) and cytosine always pairs with guanine (three hydrogen bonds).
2Explain why it is important that the sugar-phosphate backbone is held together by strong covalent bonds while the two strands are held together by weak hydrogen bonds. (3 marks)3 marks
Full-mark answer
Hydrogen bonds between paired bases are individually weak, so they can be broken relatively easily and the two strands can separate, or 'unzip', when the DNA needs to be replicated. Because the sugar-phosphate backbone is held by strong covalent bonds, each separated strand stays intact and its base sequence is preserved, so it can act as a template for building a new complementary strand. At the same time, a DNA molecule contains an enormous number of hydrogen bonds, so collectively they are strong enough to hold the double helix together and keep the molecule stable when it is not replicating.
得分点 3：数量效应——一个 DNA 分子中氢键数目极多，合起来足以维持双螺旋 (double helix) 的稳定。Explain 题的每一句都要用 because / so that / therefore 把因果串起来，只罗列"氢键弱、共价键强"而不说后果，最多得 1 分。
3Outline the events that occur in a eukaryotic cell from the beginning of G1 phase through to the completion of cytokinesis. (4 marks)4 marks
Full-mark answer
During G1 the cell grows in size, carries out normal metabolism and synthesises organelles and proteins. In S phase the DNA is replicated, so that each chromosome comes to consist of two identical sister chromatids joined at a centromere. In G2 the cell grows further and makes the proteins needed for division, including the components of the spindle, and checks that the DNA has been correctly copied. The cell then enters M phase: mitosis divides the nucleus through prophase, metaphase, anaphase and telophase, and this is followed by cytokinesis, in which the cytoplasm divides to produce two separate daughter cells.
4Justify the statement that DNA replication must be completed before mitosis begins. (4 marks)4 marks
Full-mark answer
DNA replication occurs during S phase of interphase, and it is the only stage at which the genetic material is copied. Mitosis itself does not copy DNA; it only separates chromatids that already exist, because at anaphase the centromeres divide and one chromatid of each pair is pulled to each pole. Therefore, if replication had not already produced two identical sister chromatids on every chromosome, there would be no identical copy to send to each pole, and the DNA content of the daughter cells would be halved at every division so that each daughter cell would receive an incomplete set of genetic instructions. Replication before mitosis is therefore essential for producing two complete, genetically identical daughter cells, and this is why the statement is valid.
得分点 Marking points：
得分点 1：指出复制发生在（分裂）间期 (interphase) 的 S 期，这是唯一复制遗传物质的阶段。
5Compare cytokinesis in a plant cell with cytokinesis in an animal cell. (3 marks)3 marks
Full-mark answer
In both plant and animal cells, cytokinesis is a separate process that follows mitosis and divides the cytoplasm of the parent cell, producing two separate daughter cells. In an animal cell the plasma membrane is pulled inwards at the equator to form a cleavage furrow, which deepens until the cell is pinched into two. In a plant cell no furrow can form because of the rigid cell wall; instead, vesicles collect at the equator and fuse to form a cell plate, which develops into a new cell wall and membrane separating the two daughter cells.
6Explain how mitosis produces two genetically identical daughter cells. Use correct biological terminology in your answer. (5 marks)5 marks
Full-mark answer
Before mitosis begins, the DNA is replicated semi-conservatively during S phase of interphase: the double helix unzips as the hydrogen bonds between complementary bases break, each original strand acts as a template, and free nucleotides pair complementarily with it, so that each new DNA molecule contains one original and one new strand and is an exact copy of the original. As a result, each chromosome now consists of two identical sister chromatids joined at a centromere. In prophase the chromosomes condense and become visible, the nuclear envelope breaks down and spindle fibres form; in metaphase the chromosomes line up on the equator with spindle fibres attached at their centromeres. At anaphase the centromeres divide and the spindle fibres pull one chromatid of each pair to each pole, so each pole receives one complete copy of every chromosome. In telophase the chromosomes decondense and a nuclear envelope reforms around each set, and cytokinesis then divides the cytoplasm, producing two diploid daughter cells that are genetically identical to each other and to the parent cell.
得分点 Marking points：
得分点 1：复制这一步 — 说明在（分裂）间期 (interphase) 的 S 期进行半保留复制 (semi-conservative replication)：氢键 (hydrogen bond) 断裂使双螺旋解开，原有链作模板链 (template strand)，游离核苷酸按碱基互补配对 (complementary base pairing) 结合，新分子=一旧链+一新链，因而是精确拷贝。缺这一步的答案（只说"平均分配 DNA"）会被判为关键遗漏。
得分点 5：收尾 — 末期 (telophase) 染色体解螺旋、两个新核膜重建，随后胞质分裂 (cytokinesis) 分开细胞质，形成 2 个二倍体 (diploid)、彼此及与亲代细胞遗传物质完全相同 (genetically identical) 的子细胞。Explain 题要求因果成链：用 so that / because / therefore 把"复制→相同的姐妹染色单体→着丝粒分裂→各得一份→完全相同"一路串到底。
Part C · Name the Term
12 definitions
Read the definition and write the term it describes.
1
DNA is the molecule found inside the nucleus of a cell that stores the genetic instructions for building and running an organism.
DNA
2
Deoxyribonucleic acid is the full chemical name for DNA: a nucleic acid built from many repeating nucleotide units, each containing a deoxyribose sugar.
deoxyribonucleic acid
3
A nucleotide is the repeating building block (monomer) of DNA, made of a phosphate group, a deoxyribose sugar and one nitrogenous base.
nucleotide
4
The cell cycle is the ordered sequence of events in which a cell grows, copies its DNA and then divides to produce two new cells.
cell cycle
5
Interphase is the longest part of the cell cycle, made up of G1, S and G2, during which the cell grows, carries out its normal functions and replicates its DNA — it is not a resting phase.
interphase
6
G1, the first gap phase, is the part of interphase after division in which the cell grows, makes proteins and new organelles, and prepares for DNA replication.
G1 phase
7
DNA replication is the process in which a DNA molecule is copied to produce two identical molecules; it happens during S phase of interphase, before mitosis begins.
DNA replication
8
Semi-conservative replication means each new DNA molecule keeps one original (parent) strand and one newly built strand.
semi-conservative replication
9
A template strand is an original strand of DNA whose base sequence is read to determine the sequence of the new complementary strand.
template strand
10
Prophase is the first stage of mitosis, in which chromatin condenses into visible chromosomes (each already made of two sister chromatids), the spindle begins to form and the nuclear envelope breaks down.
prophase
11
Metaphase is the stage in which the chromosomes are lined up along the equator of the cell, attached to spindle fibres at their centromeres.
metaphase
12
Anaphase is the stage in which the centromeres split and the spindle fibres pull the sister chromatids to opposite poles, so each pole receives an identical set.