Cycling Through Life Trivia Quiz

Answer: The two daughter cells are genetically identical to the parent cell.

There is no genetic mixing in mitosis; instead, it's a sort of cell-cloning process. Have you ever wondered how your body gets new cells, for growth or healing or simple replacement? Mitosis is the answer: a cell destroys its own nuclear membrane, unwinds the two identical copies of its genome and then constructs two new cell nuclei to keep the copies in. In a phase called cytokinesis, the cell then divides, resulting in two new daughter cells, each with one copy of the original genome.

Ideally, the parent cell and the two daughter cells are all exactly genetically identical, but there's always a possibility of errors (that is, mutations) in the copying process. Descendants of the mutated daughter cell will inherit the same error, and this can lead to serious problems. In particular, some mutations change the timing of mitosis and can cause runaway growth: cancer.

Answer: Interphase

The name - "interphase" - implies that this process isn't very important; it's just what comes between cytokinesis (the two daughter cells splitting apart) and mitosis (the creation of two daughter nuclei right before cytokinesis). But this is misleading: interphase is tremendously important. It's divided into three further stages. First comes Gap 1 (another misleading name), in which the cell synthesizes proteins, performs its normal functions, builds more organelles, and grows in size. The second stage is Synthesis, in which the cell copies its genetic material in preparation for mitosis, but leaves the two genomes in its own cell nucleus. Interphase ends in Gap 2, another growth period immediately before mitosis. The length of these stages varies greatly depending on the organism and system of the cell, but a typical human cell spends around 90% of its life in interphase.

Prophase, anaphase, and telophase are all stages of mitosis.

Answer: Carbon dioxide

The basic equation for oxygenic photosynthesis, canceling a factor of 2, is CO2 + 2H2O + photons -> CH2O + O2 + H2O. H2O is, of course, water: two hydrogen atoms and an oxygen atom. O2, or two oxygen atoms, is an oxygen molecule. CH2O (one carbon, two hydrogen, one oxygen) is a simple sugar, and CO2 (one carbon, two oxygen) is carbon dioxide. You can see from the equation that photosynthesis removes carbon dioxide from the atmosphere and puts oxygen in; this is why forests have been called the lungs of the world, though there are plenty of photosynthetic bacteria, too.

Photosynthesis is a two-stage process. In the first stage, sunlight is absorbed and converted to chemical energy; naturally, this can't happen at night. In the second stage, which can happen at any time, this temporarily stored chemical energy is transferred to a more stable form. In general, the whole process converts less than 10% of the incident energy of sunlight into long-term stored energy, about a factor of two less than the best commercial solar panels - but that's enough to make possible modern life on Earth.

Answer: Chloroplast

Plant cells usually contain quite a lot of chloroplasts - it isn't uncommon for one cell to have dozens of them. Each chloroplast has its own two-layer phospholipid membrane (much like a cell membrane); inside are stacks of disks called thylakoids, where photosynthesis actually happens. The light-absorbing pigment chlorophyll, which is responsible for the green color of most plants, is stored in the thylakoid membranes, along with the rest of the protein apparatus for performing photosynthesis.

In bacteria, any photosynthesis happens within the cell membrane; specialized chloroplasts do not exist. The membrane isn't necessarily smooth, though, and can be bunched into thylakoid structures in order to pack more photosynthetic capability into a single cell. It is thought that chloroplasts may have evolved from photosynthetic bacteria that were absorbed by eukaryotic cells.

Answer: Carbon fixation

Also called the Calvin-Benson-Bassham cycle (after all three of its discoverers) or the C3 cycle, the Calvin cycle uses captured energy from sunlight to take carbon from CO2 gas molecules and "fix" it in organic compounds, i.e. sugars. The process is mediated by the enzyme RuBisCo, which itself contains five carbon atoms. Each turn of the Calvin cycle uses up one CO2 molecule and fixes six carbon atoms, but five of these must be used to re-make a RuBisCo molecule for the next turn, so there is a net gain of one fixed carbon atom every time the cycle comes around.

A six-carbon sugar, like glucose, thus needs six turns of the Calvin cycle. Glucose isn't always the goal of the Calvin cycle, of course; the cell might make cellulose or starch, instead.

Answer: Whether the process uses oxygen

The end result of cellular respiration is always ATP, or adenosine triphosphate: a pentose sugar, an adenine ring, and three phosphate groups. The bonds that hold this molecule together contain chemical energy that powers an incredible number of cellular processes, including protein synthesis, the production of DNA, the contraction of filaments in muscle cells, and transport of materials into and out of cells. In using up its share of ATP, each of these processes breaks the molecular bonds, but the raw materials are soon recycled into more ATP through additional cellular respiration or fermentation.

Cellular respiration - a process which produces ATP - uses an electron transport chain to power a proton pump, which in turn creates the conditions necessary to synthesize ATP. At one end of the electron transport chain, an electron donor - in this case, a chemical derived from sugar in an earlier stage of the process - loses an electron to a molecule that attracts that electron more. Then that molecule loses the electron to an even more electronegative one, and on down the chain. Oxygen, an extremely electronegative element, is the "final acceptor" in aerobic respiration, ending the chain; it's eventually bound up in water or carbon dioxide. Anaerobic organisms must make do with less efficient final acceptors, such as sulfur, sulfate (SO4), or nitrate (NO3).

Answer: Citric acid cycle

This is actually a cycle of many names: Krebs Cycle, Szent-Györgyi-Krebs cycle, and tricarboxylic acid cycle, are also useful names to check if you're paging through an index. The acetyl-CoA is produced using pyruvate, itself the result of the breakdown of glucose via glycolysis.

The citric acid cycle begins when an acetyl group (two carbons) from acetyl-CoA is transferred to oxaloacetate (four carbons) to make citrate (six carbons). A long series of chemical reactions releases energy in the form of NADH and reduced ubiquinone, which are then used in the electron transport chain (another part of aerobic respiration) to make ATP.

At the end, the oxaloacetate molecule has been reconstructed, and a new molecule of acetyl-CoA will begin the cycle again. From the input of one glucose molecule, the citric acid cycle is responsible for around twenty ATP molecules, more than half the ATP output of the entire process of aerobic respiration.

Answer: Oxygen gas

Fermentation, like other cellular digestion processes, begins with glycolysis, which gives a net gain of two ATP molecules per glucose molecule. Glycolysis uses the molecule NAD+ as an electron acceptor, making NADH in the process, and deriving pyruvate from glucose. For glycolysis to continue, however, the cell's supply of NAD+ must be regenerated.

In the anaerobic process of fermentation, no oxygen is available to accept electrons and re-make NAD+ from NADH. Instead, the hydrogen and electrons from NADH attach themselves to parts of the pyruvate molecule, producing either ethanol (in alcoholic fermentation) or lactic acid (in lactic acid fermentation). Humans harness the first process to make alcoholic beverages, and the second process to make cheeses. And, while yeast and bacteria rely on fermentation for their basic energy needs, human cells use the process too: when your muscles are working very hard, as in a sprint, your muscle cells use lactic-acid fermentation to supplement ATP production from aerobic cellular respiration. You can partly blame fermentation for your soreness afterward!

Answer: An abnormal number of sex chromosomes

If everything goes right, a person is supposed to have two copies of each chromosome. A deviation from that number - one copy or three - is usually fatal, with only a few exceptions. People with Down Syndrome, for example, have three copies of chromosome 21 (out of 23 chromosomes), and can lead meaningful lives well into adulthood despite mental retardation and a high likelihood of other health problems. Human life is much more tolerant of abnormal numbers of sex chromosomes. You're supposed to have two sex chromosomes, typically XX for a female and XY for a male. People with Turner Syndrome, however, have only one sex chromosome - an X - while people with Triple X Syndrome have (wait for it) three X chromosomes. Extra chromosomes are also possible in genetic males: someone with a Y and two or more Xes has Klinefelter Syndrome, while someone with two Ys has XYY Syndrome. Symptoms range from very minor (people with Triple X or XYY are likely to be taller than average) to reduced fertility or sterility (people with Klinefelter or Turner) to increased likelihood of heart disease (Turner) or breast cancer (Klinefelter).

Answer: Haploid: one copy of each chromosome

In sexual reproduction, the offspring gets one copy of each chromosome from each parent, thereby incorporating the genes of both. (The offspring is still related to all four grandparents, though: in an early phase of meiosis, the parent's genes are shuffled and recombined, so that each chromosome represents a mix of genes from both of the parent's parents.) The resulting haploid cells may be spores if the parents are, say, fungi, or gametes if the parents are animals.

The gamete from the father is called a sperm cell; the gamete from the mother is called an egg cell; and the diploid fusion of the two is a zygote that may well develop into a new organism. If that organism is destined to be multicellular, it will grow by mitosis, a form of cell division that preserves the genetic makeup of the original cell.

The cycle continues!