Module 3

Question 1

true breeding

Answer 1

when a purple-flowered parent is self-fertilized, all of its offspring will be purple and never some other color.

Question 2

monohybrid cross

Answer 2

Mendel chose to study the inheritance of several traits such as plant height, pod color, flower color, etc. (Fig. 7-7) in peas, and somehow he was smart enough to know to study just one trait (characteristic) at a time. He also noticed that there were always two forms of the trait, i.e. the pea plants were either tall or short, their flowers were either white or purple, etc. Let’s use plant height as an example. Refer to Fig. 7-8 which uses flower color as an example. The first cross involves the “parent” plants, so it is called the parental or P generation. All of these crosses involve only one trait (one gene), so they are called monohybrid crosses.

Question 3

Autosomal Recessive

Answer 3

(i.e. the gene is found on any chromosome OTHER than a sex chromosome, and the person has to be homozygous recessive for the trait to have the disorder) 1. Cystic fibrosis ,2. Sickle-cell disorder 3. PKU4. Albinism

Question 4

allele (form of the gene)

Answer 4

In modern terminology, Mendel’s factors that code for each trait we now know as genes; the two alternative forms of each gene (tall vs. short, purple vs. white, etc.) are called alleles

Question 5

homozygous

Answer 5

Since offspring inherit one letter (one allele) from each parent, a true-breeding tall pea plant can be represented by TT, the F1 tall hybrids are Tt, and the recessive short plants are tt. If both letters are the same (TT or tt), this condition is called homozygous (p. 271; homo = same), whereas if the letters are different, this condition is called heterozygous (hetero = other).

Question 6

heterozygous

Answer 6

Since offspring inherit one letter (one allele) from each parent, a true-breeding tall pea plant can be represented by TT, the F1 tall hybrids are Tt, and the recessive short plants are tt. If both letters are the same (TT or tt), this condition is called homozygous (p. 271; homo = same), whereas if the letters are different, this condition is called heterozygous (hetero = other).

Question 7

test cross

Answer 7

One other principle Mendel devised is the test cross (see the white alligator example in Fig. 7-14) to reveal an unknown genotype. Can you tell if a tall pea plant is TT or Tt just by looking at it? Right, you can’t! But you can use a test cross to determine this information. When the dominant phenotype (tall) of unknown genotype (TT or Tt) is crossed with the homozygous recessive, here’s what happens:

If the tall parent is TT, TT x tt = all the offspring will still be tall (all will be Tt).
If the tall parent is Tt, Tt x tt = ½ Tt and ½ tt. So, basically if any of the offspring are short, you will know that the tall parent was Tt.

Question 8

P generation

Answer 8

Thus, a cross between a homozygous dominant (tall) plant with a homozygous recessive (short) plant (aka the P generation) produces all tall offspring that are heterozygous (Tt). We can represent this monohybrid cross of the P generation

Question 9

F1 generation

Answer 9

Every time he crossed a tall with a short pea plant, all of the offspring were tall. These offspring (children) are called the F1 generation (F for “filial” which is Latin for son or daughter. The genetics discipline uses several gender references for reasons unknown to me!)

Question 10

F2 generation

Answer 10

He did these experiments thousands of times, and every time all individuals of the F1 generation were tall (unless there was a rare mutation). He then came up with terminology to describe his results: the form of the trait that was present in the F1 generation was called dominant, while the form of the trait that was absent in the F1 he called recessive. Thus, in pea plants, tall is dominant to short. When Mendel then made crosses with the F1 generation (F1 x F1), approx. ¾ of the offspring (grandchildren of the original parents) were tall and ¼ were short. So, the short factor re-appeared in this F2 generation. This same phenomenon happened whether he was studying flower color, pea shape or pea color. In the flower color example (Fig. 7-8), a true-breeding plant with purple flowers crossed with one with white flowers always produced purple-flowered offspring (F1generation), so Mendel determined that purple flowers were dominant to white flowers.

Question 11

genotype

Answer 11

The combination of alleles (letters) is called the genotype

Question 12

phenotype

Answer 12

while the physical appearance (tall, purple, etc.) is called the phenotype

Question 13

incomplete dominance

Answer 13

In peas, tall is dominant to short, purple flowers are dominant to white flowers. But it actually is quite common for traits to be incompletely dominant, especially when colors are involved. In a cross between a red and a white snapdragon (a kind of flower; see Fig. 7-17, and please substitute a little r for the w in the figure), if red were completely dominant to white, all of the offspring would be red, right? . But guess what - all of the F1 plants are pink! In this case, red is incompletely dominant to white. In incomplete dominance, the heterozygote has a phenotype intermediate to those of the homozygous parents. The capital R allele instructs the plant to make red pigment, while the little r allele tells the plant not to make any pigment. It’s sort of like buckets of paint: R is a bucket of red paint while r is a bucket of white paint (no color). It takes two doses (buckets) of the R (red) allele to make the flower look red. The offspring are all Rr (pink): when you mix a bucket of red paint and a bucket of white paint, you get pink paint, right? Only one dose of red pigment results in a pink petal coloration; two doses are needed to produce red. And, of course, rr is like mixing 2 buckets of white paint; in this case no red pigment is made by the plant, so the petals appear white.

Question 14

multiple alleles

Answer 14

Although a genotype is always designated by two alleles, sometimes the body has more than two alleles to choose from, an example of which is human blood types. Do you know your blood type? Every person on the planet has either Type A, B, AB or O blood type, and now you’ll learn a little about the genetics behind your blood type.As an example, if you have type A blood, your RBCs are covered with a particular type (A) of recognition protein/surface antigen (remember learning about recognition proteins in Module 1?). The RBCs can be completely covered with the A antigen, in which case your genotype would be AA, or they can be only partially covered, in which case your genotype would be AO (the O basically means zero or no antigens, at least of this type). A person with type O blood has neither the A nor B antigen on their RBC surfaces. The body has the A, B & O alleles (multiple alleles) to choose from to determine the blood type. Since both the A and B alleles are expressed (seen or readily apparent) in the heterozygote (aka type AB blood), both the A and B alleles are codominant to the recessive O allele. You may also know that you have “A-positive” or “A-negative” blood. The “positive” and “negative” are separate antigens from the A and B antigens. They are the Rh antigens (p. 282) and you either have them (“A positive” blood) or you don’t (“A negative” blood).

Some of you may know that type O blood is the most common blood type (at least in this country and probably worldwide.) So, you may ask, if this blood type is so common, why is it recessive? Having freckles is dominant to not having freckles, and having a widow’s peak is dominant to no widow’s peak (Fig. 7-5), etc. I don’t know about you, but I know many more people who do NOT have freckles or a widow’s peak than those who do have these traits. I know it’s difficult to understand, but the terms dominant and recessive simply refer to how a particular trait is inherited; these terms have nothing to do with the frequency with which a trait occurs in the population.

Question 15

Polygenic inheritance

Answer 15

Height in pea plants is controlled by just one gene, so the tall and short phenotypes are very distinct phenotypes. The VAST majority of our traits are not controlled by just one gene but rather the interactions between many (at least 3) genes, each of which has an additive effect. Examples are your IQ, height, weight, and the color of your eyes, hair and skin (Fig. 7-22). While there are only two heights of pea plants, human heights, along with many of our other traits, fall into a series of overlapping phenotypic classes. Hair, skin and eye color is determined by the amount of a pigment called melanin; there are red, brown, yellow and black melanins. The more melanin a person has, the darker their hair, skin and eyes. It’s similar to the doses of color/buckets of paint analogy: dominant alleles code for the production of melanin (darker color) while recessive alleles take color away. To a certain extent these genes are linked (p. 91), for it is not common (though not impossible) to see a person who has very dark skin to have blue eyes and naturally blonde hair. In other words, most people with dark skin have dark hair and dark eye color as well (i.e. lots of melanin). Melanin protects skin and eyes against the harmful effects of UV light; people with dark skin usually have a natural SPF of 8 or more and, thus, a reduced chance of getting skin cancer.

Question 16

how the environment affects gene expression.

Answer 16

Many of our traits are also, to greater or lesser degrees, controlled or influenced by the environment. Certainly one’s IQ, height, weight, etc. are all heavily influenced by cultural and environmental effects. The fur of Arctic animals turns white in winter for protective coloration. Actually, many alleles for coat color in animals are temperature-sensitive (Fig. 7-26). A Siamese cat has a silvery gray body with dark fur on the tips of its ears, tail and paws. The gene for dark coat color is switched on only where temperatures are a bit cooler than the body temperature (i.e. at the extremities such as the paws and tail); the torso (main body) is too warm for the dark coat color gene to be expressed. So, if a Siamese cat is left outside on a cold, snowy day, would it turn all brown??

Question 17

Autosomal Dominant Disorders

Answer 17

With these disorders, sadly enough, people that are homozygous dominant (HH, as an example) or heterozygotes (Hh) are affected; oftentimes the homozygous dominant embryos die1. Huntington’s disease

Question 18

X-linked Recessive Disorders

Answer 18

These disorders are found only on the X chromosome; there is no corresponding allele on the Y chromosome. So, a woman who is HH for a particular X-linked disease is normal, she’s usually an unaffected carrier if she’s Hh, and she has the disease only if she’s hh (i.e. she inherited diseased X alleles from each of her parents). A male who is HY is normal but is affected if his genotype is hY. Thus, these diseases are far more common in males than in females, the latter of whom had to have had a diseased father and at least a carrier mom.1. Hemophilia2. Red-Green Color Deficiency

Question 19

Conditions Due To Chromosomal Variations

Answer 19

The following conditions (discussed back in Chapter 6) are NOT due to faulty genes but rather are due to mistakes made during meiosis. The embryo ends up with too few or too many chromosomes, some conditions of which account for at least half of all miscarriages. Some people can lead normal lives with 45, 47 or 48 chromosomes, but others are quite affected1. Trisomy 21 (Down Syndrome (caused by autosomes) Trisomy X (caused by sex chromosomes) Turner Syndrome
These XO females (they have only one X chromosome) don’t develop secondary sex characteristics (very little breast development or body hair), they don’t ovulate or menstruate and are sterile. They also have some physical abnormalities such as short stature, extra folds of skin on the neck and a broad chest, but otherwise they usually lead relatively normal lives.

3. Klinefelter Syndrome
   These XXY men have underdeveloped testes and are therefore infertile. They may have feminine-type features such as scant body hair and some breast development, but most appear perfectly normal and lead full lives with the exception of their inability to father children (very low sperm count).
4. XYY
   This “supermale” syndrome is linked to tallness (over 6 feet), acne and possibly antisocial behavior. The frequency of XYY males in the general population is 0.1%, whereas it represents about 4.5% of men in prisons. It was once thought that the extra Y chromosome imparted aggressive behavior (several serial killers have tried unsuccessfully to use it as a defense), but there is no strong evidence to support a link between the XYY condition and violent/criminal behavior. The higher incidence of the XYY condition in prisoners quite possibly is due to defensive behavior learned as an adolescent rather than aggressiveness imparted by the extra Y chromosome.

Question 20

Telomeres

Answer 20

As your book points out, most of the cells in our bodies replace themselves when they get old. Each cell is capable of dividing about 50 times before it dies. Our cells can tell how many times they’ve divided with the help of “odometers” (like a car has) called telomeres, unique DNA sequences that occur at the tips of each of our chromosomes (Fig. 6-1). A telomere is a protective cap at the end of the chromosome, sort of like the plastic piece at the end of shoelaces. Each time the cell’s chromosomes divide, the telomere gets a bit shorter; after about 50 cell divisions the protective telomere is about gone, and the cell dies soon thereafter. Certain cells can rebuild telomeres after each division, so the cells don’t know when to stop dividing – these are cancer cells. Interestingly enough, in 2009 three American researchers won the Nobel Prize in Medicine for their discovery of telomeres and their ongoing research on how diet, smoking, pollutants, etc. can affect telomeres and influence the aging of our cells – cool stuff:

Question 21

mitosis

Answer 21

for growth and repair; it’s the normal way your cells divide every day

Question 22

meiosis

Answer 22

a specialized cell division for the sole purpose of making eggs or sperm

Question 23

The purpose of mitosis is to produce genetically identical body cells (as opposed to egg and sperm cells). Cells undergo mitosis for 3 main reasons

Answer 23

1) for growth of the organism (Fig. 6-7). A zygote (the first cell produced after the sperm fertilizes an egg) undergoes mitosis to become an embryo, then a fetus, then the baby is born, and cells continue to be added to its body (at least in height) until the person is approx. 20 years old. Are your hands still growing? Is your head still growing? The answer to all of those questions is - hopefully not! Once the person is an adult, mitosis usually only occurs for the next reason, which is:
2) for repair. If you get a cut, if your kidney is injured, etc., these tissues will undergo mitosis to repair the injury.
3) for asexual reproduction. Plants and a few animals can reproduce without involving egg and sperm. When you plant a potato you actually plant a piece of the spud that has an “eye”, which will sprout up and grow into a new potato plant identical to the parent. It’s the same thing when you see “suckers” growing up around the base of a tree. The offspring are all clones, offspring that are genetically identical to the parent.

Question 24

All of the cells in your body (except for eggs and sperm) contain____chromosomes in their nucleus.

Answer 24

46

Question 25

Your cells have a life (or cell) cycle

Answer 25

sort of like we all have a life cycle. Most of the cells in your body usually are NOT dividing; they divide only when they need to. Some of your cells never divide: red blood cells (RBCs) cannot divide because they lack a nucleus (so they do not have chromosomes), and nerve cells usually do not divide either, unfortunately (otherwise, spinal cord injuries like the one suffered by the late actor Christopher Reeve would not be such a problem). The only cells that regularly (approx. once a day) divide are the epithelial cells lining your GI tract and your hair follicle cells. Most chemotherapeutic drugs used to treat cancers target ALL regularly dividing cells, so that’s why a person may vomit and their hair may fall out when they’re undergoing “chemo”. And that is why it is so important for cancer researchers to develop drugs that target only the bad, cancerous cells, not the healthy ones as well. Your skin cells divide somewhat regularly as well but not as often as the above-mentioned ones.

Question 26

stages if life cycle

Answer 26

The first stage in a cell’s life cycle is G1 (Gap 1), during which time the cell is actively growing, metabolizing, making new proteins, and just plain hanging out. All of the cells in your body stay in G1 unless they are stimulated to divide. RBCs and nerve cells remain in G1 permanently. Now, you may have seen pictures of chromosomes in magazines (and top of p. 223), and the chromosomes appeared to look like the letter X, right? Well, chromosomes look like this only some of the time. If you look at Fig. 6-5, you’ll see that chromosomes occur in what I like to call their “single-rod” form right after mitosis and in G1. So, you have 46 single-rod chromosomes in your cells during G1; each looks like a stick or the letter “I” and not an “X”. Are you with me so far? If you get a cut, etc., the cell is stimulated to proceed forward to S (DNA Synthesis) phase, during which time the DNA is duplicated/replicated. Notice that by the end of S phase the chromosomes are in their “double-rod” form; now the chromosomes look like an X. You still have 46 chromosomes (THAT NUMBER WILL NEVER CHANGE IN MITOSIS), but now we say that each chromosome consists of 2 sister chromatids which are held together by a tiny button of protein called a centromere. Each sister chromatid is identical to its partner in each chromosome; this is where the duplicate DNA comes in. During G2 phase the cell makes final preparations to divide. G1, S and G2 phases collectively are called interphase, a stage when the cell is not dividing. Again, most of your cells stay in G1 of interphase for much of the time and proceed on to actually divide (mitosis) only when necessary. (Please don’t be mad at me; I know that genetics terminology can be confusing!!)

Cell division actually consists of two main stages, mitosis (division of the nucleus; Fig. 6-10) and cytokinesis (division of the cytoplasm; p. 233 bottom right). You do not have to memorize the various stages of mitosis (Fig. 6-11). However, notice that during metaphase the double-rod chromosomes all line up individually, and in anaphase the sister chromatids separate. Once they separate they are once again called single-rod chromosomes. The cytoplasm then divides (cytokinesis), resulting in two daughter cells, each containing 46 single-rod chromosomes in a human. If cytokinesis did not occur, then how many chromosomes would be in a nucleus after one round of mitosis?

To review and to put this differently, for most of the time (G1) your cells contain 46 single-rod chromosomes. During S phase, each chromosome makes a copy of itself and then attaches itself to that copy, resulting in 46 double-rod chromosomes (each copy is now called a chromatid). During mitosis the chromatids separate; half (46) go into one new cell, and the others go into the other newly-created cell, thus restoring the original number (46) and original type (single-rod) of chromosomes in the two daughter cells.

Question 27

Cancer

Answer 27

Cancer occurs as a result of uncontrolled mitosis (cell division). As indicated earlier, the vast majority of the cells in your body remain in G1 of interphase unless they are stimulated to divide for some reason (if you get a cut or injure an organ). Exceptions to this are the epithelial cells lining your GI tract and hair follicle cells; these cells divide (undergo mitosis) quite regularly, roughly approx. once every 24 hours. Chemotherapeutic drugs target regularly-dividing cells like cancer cells, but an unfortunate side effect is that they also kill off some healthy epithelial and hair follicle cells, so that’s why a person undergoing chemo may lose her hair and experience nausea/vomiting/diarrhea from the chemo. In cancerous cells, the normal checks and balances that regulate cell division have been mutated, so that cancer cells undergo mitosis every day, and unless chemotherapeutic drugs or the immune system kills them off, cancer cells can become immortal (Fig. 6-10). Cancer cells may also exhibit other weird characteristics (some can strip off their recognition proteins so the immune system can’t recognize them); they can basically change from a normal “Dr. Jekyll” skin cell, as an example, to a “Mr. Hyde” cell which bears no resemblance at all to the original skin cell!

Question 28

Benign vs. malignant??

Answer 28

A benign tumor has experienced some uncontrolled growth, but usually it stops growing eventually and can be surgically removed (or left alone). Plus, a benign tumor does not have the ability to spread or metastasize to other parts of the body (Fig. 6-11). A mole on your skin is a good example (see skin cancer link on next-to-last page). Another example of a benign tumor is a wart – it is caused by a virus that convinces some of your cells to grow into that warty bump, but then the bump stops growing, and the wart can be gotten rid of. Malignant tumors have the capacity to invade the lymph and blood vessels and use them as a means to spread to other organs in the body, sort of like a cancer cell highway. Patients die from cancer because the cells impede the normal function of the invaded organ(s) (Fig. 6-12). It is fairly common for many cancers to spread to the liver, bone and brain after originating in a different organ or tissue such as the breast, ovary or colon.

Question 29

explanation of the molecular basis of cancer

Answer 29

Imagine everyone’s surprise and the uproar when it was announced approx. 40 years ago that we all contain dozens of sort of “pre-cancer” genes – no one believed those wacky molecular biologists! These perfectly normal, healthy genes are called proto-oncogenes; they code for growth factors (proteins) that tell the cell to divide. An example is epidermal growth factor. When your cat or dog cuts their paw, they lick it continuously, right? They have epidermal growth factor in their saliva which they are depositing on the wound to stimulate mitosis and repair of the cut. We also have this growth factor, but not in our saliva and, no, our bodies will not recognize these growth factors from other species. At any rate, if a proto-oncogene gets mutated or moved, it then becomes an oncogene which then constantly, continuously, and inappropriately floods the cells with growth factors, causing the cells to constantly divide and become cancerous. It would be as if the accelerator on your car were stuck – the car is going full speed ahead and you can’t control it.

Mutations in tumor suppressor genes also are usually involved in the development of cancer. These genes code for proteins that slow or stop tumor growth (they are the good guys; they’re like the brakes in your car), so if they are mutated they cannot inhibit cell division. We have dozens of these good-guy genes. A mutated tumor suppressor gene is like failed brakes in your car: the car is out-of-control, not because the accelerator is stuck but because you have no brakes! The end result is the same: out-of-control and continual cell division in the cell.

It is important for you to understand that cancer is a multi-step process that, with a few exceptions, usually takes decades to develop. We know the exact scenario/ mutations that have to occur for a person to develop colon cancer and some other cancers. It’s a multi-hit process; usually several proto-oncogenes and tumor suppressor genes have to be mutated in order for the cells to be cancerous. You may have heard that there can be pre-cancerous changes in the skin (keratoses) and pre-cancerous polyps in the colon (to name a few examples) - these cells are not quite normal any more and are along the path leading to full-blown cancer (but are not there yet).

Question 30

Meiosis

Answer 30

Meiosis is a “reduction division” whose purpose is to make haploid gametes (eggs and sperm), meaning they contain half of the normal chromosome number contained in body cells. All of the cells in your body (except egg or sperm) are diploid, meaning that they each contain 2 sets (aka 2 pairs) of 23 chromosomes (Fig. 6-14). The 2 chromosomes of each pair are said to be homologous because they each contain basically the same genes, like the gene for eye color. You inherit one chromosome in each homologous pair from your mom and the other chromosome in each pair from your dad. Chromosome pairs #1-22 are called autosomes; they code for all of your characteristics except your sex. The 23rd pair of chromosomes are your sex chromosomes, XX in females and XY in males.

Notice our life cycle in Fig. 6-14. Meiosis occurs in the ovaries of women and in the testes of men to produce haploid eggs and haploid sperm, respectively. As with mitosis, you don’t have to memorize the stages, but you might want to check out Fig. 6-16 to help you understand what’s going on. In meiosis I, homologous pairs of chromosomes pair up and then separate into different cells, thereby reducing the chromosome number to 23 in a human. Meiosis II is simply mitosis which is needed to restore the chromosomes to their single-rod state. When a sperm (containing 23 chromosomes) fertilizes an egg (also containing 23 chromosomes), the nuclei of the sperm and egg actually fuse, so that’s why the zygote (1st cell of the new life) has the proper number of chromosomes (46). If sperm and eggs each contained 46 chromosomes like the rest of your body cells, do you see that the cells of the new developing baby would contain 92 chromosomes (which is not consistent with human life).

Babies occasionally are born with cells containing different numbers of chromosomes. In some cases the condition is not noticeable, while in other cases the other-than-46 number of chromosomes causes drastic physical and/or mental changes in the person, some of which are fatal.

Question 31

Structure of DNA

Answer 31

DNA (deoxyribonucleic acid) is a nucleic acid, a large molecule that stores information. It is made up of monomers called nucleotides which have three components: a molecule of sugar, a phosphate group, and a nitrogen-containing molecule called a base. Check out Fig. 5-4 to review the structure of DNA. It is a double helix consisting of an alternating sugar-phosphate backbone (the uprights or hand-holds of a ladder) with paired nucleotide bases – Adenine (A), Guanine (G), Cytosine (C) and Thymine (T) - joined together inside the helix, equivalent to the rungs of a ladder. The bases are always paired a certain way (forming base pairs) – A always bonds with T and G always bonds with C (unless a mistake/mutation occurs). If you could magnify DNA with an electron microscope, it would look like a twisted rope ladder.

Question 32

genome

Answer 32

an organism’s (such as yourself) complete set of DNA. The human (our) genome is found on 46 chromosomes that occur in the nucleus of almost every cell in your body.

Question 33

gene

Answer 33

a specific sequence of DNA found in a certain location on a chromosome. It is made up of 3,000 or so nucleotide bases and it contains the information to produce a specific protein. This protein results in one of your characteristics or traits such as your hair color or eye color. So, in a nutshell, your genes code for your traits. (You learned about alleles in Ch. 7). Look over Fig. 5-6 to understand this concept. DNA is an amazing molecule, for it contains the instructions for building virtually any living thing on the planet! It is sort of like a universal language, the letters of which are the bases A, T, G and C (the sugar-phosphate backbone just holds the bases in place, like a book binding). The English language consists of 26 letters (A-Z) which are strung together in a particular sequence to make words and sentences with which we use to communicate information. The same holds true for DNA.

Question 34

central dogma of molecular biology

Answer 34

genetic information flows from DNA to RNA to protein. In other words, genes (which, as you know, code for your characteristics) are in the form of DNA (true for everything on the planet except for viruses and viroids, neither of which is a living organism), a messenger RNA copy of the gene gets made when needed, and that RNA gets translated into protein in the ribosomes. Thus, an organism’s genotype (genes) codes for a protein (usually an enzyme or a pigment) that produces the phenotype (tall, brown skin color, green eye color, nose shape, etc.).DNA makes RNA, RNA makes protein, and proteins do nearly all the real work of biology. Geneticists have long focused upon just the very small part of our DNA that contains blueprints for proteins. Only about 2 % of our DNA actually codes for proteins; the rest (98% of our DNA) is that “junk” mentioned above! A few years ago it was discovered that many times after DNA gets transcribed into RNA, the buck stops there and the RNA does NOT get translated into protein! The RNA acts like a gene to do many things, including controlling the activity of regular DNA genes. So, instead of these introns being useless “junk”, these hidden “RNA-only genes” are proving to play major roles in the health and development of plants and animals.

Question 35

RNA

Answer 35

RNA is nearly identical in structure to DNA except that it has ribose as a sugar instead of deoxyribose. See p. 71. Also, RNA usually is single-stranded (except in those pesky viruses!), and instead of the base thymine (T) it has uracil (U).

Question 36

Making protein from our genes is a two-step process.

Answer 36

The first step in the flow of genetic information is called transcription, during which a complementary copy of the gene (DNA) is made into RNA (Fig. 5-12). The resulting RNA is called messenger RNA (mRNA) because it is a messenger or “middleman” delivering the message of the gene to the ribosome (where proteins are fashioned, you may remember from Module 1). Now, recall that the paired bases inside of the DNA helix are complementary; in other word, they are paired in a specific manner: “A” always pairs with “T”, “G” always pairs with “C”. You will just have to memorize “A pairs with T, G pairs with C” (doesn’t it have a ring to it??). So, getting back to Fig. 5-12, when transcription is about to take place, the gene (DNA) unwinds locally, there is a molecular marker that tells an enzyme that the gene begins in that certain place, and a particular enzyme then hops on and begins to “read” the gene (which is on ONE strand of the DNA), putting in an A where there is a T on the DNA and a C where there is a G. Where there is an A on the DNA the enzyme puts in a U (NOT a T) because uracil replaces thymine in RNA. When the entire gene has been copied into RNA, the RNA dislodges itself, snakes its way out of the nucleus and goes to the ribosomes in the cytoplasm. That’s all the detail you need to know!The second step in the genetic flow of information is called translation. During translation the mRNA is translated into protein. Most of you probably remember from high school that the genetic code is a triplet code, in other words each group of 3 RNA bases (called a codon) codes for an amino acid – see Fig. 5-13 and 5-14. So, looking at Fig. 5-14, you can see that during translation the mRNA codons get translated into amino acids, and a whole string of amino acids makes up a protein.

Question 37

introns (short for intervening sequences).

Answer 37

genes of eukaryotic organisms (including humans) contain internal sequences of “junk” that do not code for a protein. These “junk” DNA sequences

Question 38

The actual parts of a gene that code for something (those parts of the gene are “expressed” in the phenotype)

Answer 38

exons

Question 39

Epigenetics

Answer 39

So, you know what a genome is – all the genes in each of your cells that give you your characteristics. It has recently been discovered that we all also have an epigenome (“epi” means above, upon, attached), which is a series of chemical tags (molecular markers frequently made up of methyl groups) on our DNA. These chemical tags react to signals from the environment (our diet, stress, behaviors such as smoking or drinking, etc.) and adjust gene expression (activate or silence genes) in response to these environmental stimuli. The genes (letters of the DNA) stay the same, but environmental factors and our behaviors can alter their expression (turn them on or off). As an example, certain toxins in cigarette smoke react with the epigenome to turn on genes that lead to lung cancer.

Question 40

mutation.

Answer 40

Any change in the correct nucleotide sequence of DNA