Thyroid for Dummies (36 page)

Mendel knew that pea plants form new seeds when pollen in the male part of the plant (the anther) manages to attach to the female part (the stigma) and get down to the ovaries, which house the eggs which it fertilizes. The result is a seed, which grows into a plant.

Mendel carefully controlled the fertilization of his pea plants so he knew exactly which plant provided the pollen and which plant provided the ovary.

He took, for example, the pollen of short pea plants that never produce anything but short pea plants, and used it to fertilize the ovaries of tall pea plants that never produce anything but tall pea plants. Then he took the pollen from tall pea plants and fertilized the ovaries of short pea plants. The result, in both cases, was always tall pea plants.

Mendel then took the tall offspring from this first fertilization (called the first cross) and fertilised them with each other. The offspring of the second cross were not all tall: Three-fourths were tall, and one-fourth was short. When Mendel crossed (or ferilised) the short plants from the second cross with other short plants from the second cross, the resulting plants were always short. But if he crossed the short ones with the tall plants from the second cross, the new plants were usually, but not always, tall. The same pattern held true for the other characteristics that Mendel studied.

On the basis of these studies, Mendel made the following observations in the pea plant:

ߜ A feature of the pollen and the egg determines whether a plant is tall or short. (This feature is now called a gene; Mendel did not use this term.) ߜ When the gene from the tall plant combines with the gene from the short plant, they do not mix to form an average plant.

ߜ If a plant has two characteristics for the same gene such as tallness and shortness, one is found more often than the other when they are crossed.

(The gene that produces the trait found more often is the
dominant
gene, and the gene producing the trait found less often is the
recessive
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ߜ There are two different genes for a trait. (Two genes that determine the same trait are called
alleles.
)

ߜ When plants with two different traits such as height and texture of the seed are bred, these traits pass to the offspring independently of one another. For example, a tall plant isn’t always found with a smooth seed or always with a wrinkled seed. Mendel concluded that genes (which he called
atoms of inheritance
) follow the principle of
independent assortment:
Each trait is inherited separately from all other traits.

Mendel’s work received little attention when it announced in 1865, but luck-ily, it was rediscovered in 1900, and he got posthumous credit for his discoveries (for what that was worth).

Talk the talk

Using Mendel’s research as a basis, scientists created the new language of genetics which makes it difficult to understand what they are talking about.

First, if a person (or a plant, dog, or chimpanzee) has two copies of the same allele, he or she is
homozygous
for that gene. If he or she has one of each allele, the person is
heterozygous
for that gene. (We now know that, within a population, there are more than two different alleles for each gene. However, any given person, animal, or plant has only two alleles for each gene.) The appearance of the trait due to particular genes is called the
phenotype,
while the genes that make up that phenotype are called the
genotype.
For instance, two tall pea plants with the same appearance (phenotype) can have a different genotype. A plant with only tall genes is tall, but so is a plant with a tall gene and a short gene (because tallness is the dominant gene).

A quick quiz: Based on your in-depth knowledge of genetics, can two short pea plants have different genotypes? The answer is no, because shortness is the recessive gene. If a tallness gene is thrown into the mix, the plant is tall.

Therefore, all short pea plants can only have the genes for shortness.

The great divide

While the world was ignoring Mendel’s work, great things were happening under the microscope. Scientists saw that tissues are made up of cells and that new cells come from the division of old cells. As two new cells form, the old cell produces two copies of everything so that each new cell contains exactly what the old cell had.

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One particular area of the cell, which looks like a cell within the cell, is especially intriguing to scientists. This area is called the
nucleus
of the cell. As two new cells are formed, some substances in the nucleus double and separate so that each new cell gets a complete set of these substances, which are called
chromosomes
.

Scientists now know that each plant and animal has a set of chromosomes, but the numbers of chromosomes often differs between species. For instance, humans have 23 pairs or 46 chromosomes, while chimpanzees have 24 pairs or 48 chromosomes. (But chimpanzee chromosomes look more like human chromosomes than ape chromosomes, so don’t start thinking you’re so smart.) The whole process in which one cell becomes two is called
mitosis
.

Through examining the division of egg cells and sperm cells (the so-called
germ cells
), scientists know that each of these cells contains only half the normal number of chromosomes. In humans, for example, each egg cell and each sperm cell has one set of 23 chromosomes (while other human cells have 46 chromosomes). When these cells divide to form more sperm or egg cells (through a process called meiosis), the result is again 23 chromosomes per cell. When the egg and the sperm join together in fertilization, the combination, called a
zygote
, has the normal number of 46 chromosomes.

When a zygote is formed, one set of its chromosomes comes from the female (mother) and one set of chromosomes comes from the male (father). In the zygote cell, these chromosomes pair up two-by-two. The members of each chromosome pair are called
homologous chromosomes.

As is always the case, there’s an exception to this rule. But like the French say,

‘vive le difference’. Very loosely translated, that means ‘Thank goodness for this particular set of chromosomes.’ This unique set is the sex chromosomes that determine whether you are a boy or a girl. All other pairs of chromosomes have matched genes so that if there’s a gene for a given characteristic on one of the chromosomes of the pair, the other chromosome also has a gene for that characteristic. While a female has two matched sex chromosomes (called X

chromosomes), a male has two different sex chromosomes (called an X chromosome and a Y chromosome).

Genes, chromosomes, and

the traits they create

You are probably thinking, ‘Two genes for every trait, two sets of chromosomes . . . genes and chromosomes are the same.’ The problem 24_031727 ch17.qxp 9/6/06 10:44 PM Page 211

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is that any given plant, animal, or human has far more traits than the number of chromosomes they own. Recognizing this, scientists realized that each chromosome contains many genes, not just one.

Each gene for a particular trait is found on a single chromosome. Each chromosome is passed down to its daughter cells – the cells they create when that cell divides. Therefore, some genes (and the traits they create) get passed down together from generation to generation. (They don’t follow the principle of independent assortment.) Geneticists described genes on the same chromosome as linked.

Even though some genes are linked, they sometimes do get inherited independently, just as Mendel predicted. This happens because
crossing over
takes place. What is crossing over? During the process of meiosis, which produces sperm and egg cells, as the sets of chromosomes line up close together, genes on one chromosome can cross over to the other while the alleles they replace cross over in the other direction. In this way,
recombinant chromosomes
form –

new combinations that help to make your offspring different from you.

The discovery of crossing over means that scientists can now map chromosomes. That is, they can determine which genes are on which chromosomes and where, because the closer two genes are, the less likely they are separated by a cross-over, while the farther they are from one another, the more likely they are to separate.

Another way that a new trait replaces an old one is when a
mutation
takes place. As a result of faulty copying of the chromosome or an outside influence such as radiation, chemicals, or the sun, a slightly different code appears in the genetic material, so that a new gene replaces an old one. A gene provides all the information needed for a cell to make a particular protein. If the code changes due to a mutation, this means a different amino acid sequence is inserted into that protein. Usually mutation isn’t noticed, either because the different amino acid sequence doesn’t affect the function of the protein, or because the mutation kills the individual so that it isn’t reproduced. Once in a while, a mutation is good for the animal or plant in which it occurs, producing a useful trait such as blue-coloured roses instead of white.

The secret lives of genes

Genes are made up of long, long, long chains of nucleic acids. Nucleic acids have three components: a sugar called deoxyribose; a phosphate attached at one end of the sugar; and a base (one of adenine, cytosine, guanine, or thymine) attached to the sugar at another point. The long chemical structure from which genes are made is known as deoxyribonucleic acid or DNA.

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DNA

Within the DNA, the number of adenine molecules is always equal to the number of thymine molecules, while the cytosine equals the guanine. In 1952, biochemists James Watson and Francis Crick showed that this equality is because each gene contains two chains of nucleic acids. The adenine on one chain is always paired with the thymine on the other, while cytosine on one chain is always paired with guanine on the other. As DNA has a helical structure, Watson and Crick called the structure a double helix – a shape that looks like a spiral staircase.

One of the best things about the identification of the double helix is that it became clear how genes copy themselves or replicate. The helix simply breaks apart into two strands. Each of the two strands then act as a template so that a new strand forms on it in the only way it can, so that a nucleic acid containing adenine connects to a nucleic acid containing thymine, and a nucleic acid containing cytosine connects with a nucleic acid containing guanine. The result is two new double helixes.

Next, DNA acts as a blueprint to control the creation of the animal or plant and the ongoing processes that allow it to live. This function is carried out using ribonucleic acid, or RNA.

RNA

RNA is made up of nucleic acids just like DNA, but the sugar in RNA is ribose and the bases are adenine, cytosine, and guanine, with uracil replacing thymine.

RNA forms against the DNA template in the same way that the double helix breaks apart to reproduce itself. When a gene is active, the DNA containing that gene splits apart and a strand of RNA is made with the gene providing the template. This copying process is called transcription. The RNA molecule is essentially a copy of the gene but made up of RNA instead of DNA. The RNA remains as a single strand, not a double helix, and is called messenger RNA (mRNA) because it carries the message from the DNA to the next level of control, the
ribosome
. Ribosomes are small dumb-bell-shaped factories found in each cell which take in the strip of RNA instructions and read it, rather like ticker-tape, churning out a protein at the other end. This process is called translation.

Proteins

Proteins are made up of amino acids. Every three bases in the messenger RNA, called a
triplet
, causes one particular amino acid to line up opposite them within the ribosome. Each group of three bases is called a
codon
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each codon codes for a specific amino acid. Scientists use artificial messenger RNA containing the same codon again and again to determine which amino acid is selected by each codon.

With four different bases, each of which can occur at any of the three sites in a codon, the maximum number of codon combinations that are possible is 4 × 4 × 4 = 64. But, there are only 20 amino acids, not 64. Researchers now know that several different codons select the same amino acid, and that some codons act as the code for the end of a protein, without selecting an amino acid. They form the genetic equivalent of a full stop at the end of a messenger RNA sentence.

Just to complicate things a little further, amino acids don’t actually line up opposite the codons but are carried at one end of another RNA molecule called transfer RNA. At its other end, transfer RNA has the bases that are complementary to the codon. So the transfer RNA lines up neatly against the messenger RNA, within the ribosome, while the amino acids are lined up next to one another at the other end. A series of other steps then bind the amino acids into a protein.

Shall I form a liver or a brain?

As we mentioned (see ‘The great divide’ earlier in this chapter) a fertilized egg reproduces itself in a process called mitosis, which creates two identical cells. These cells divide and divide again until a complex human being develops. So how do these identical cells transform themselves into a thyroid cell, a liver cell or even a brain cell which are very different from each other?

Even though each cell contains an identical set of genes, not all these genes are active. In some cells, certain genes are switched on, so that certain proteins are made, while other genes are switched off. A thyroid cell differs from a brain cell as a result of the particular genes that are
expressed
(active) in each cell. Many different factors determine whether a gene is expressed or not, such as the presence of various hormones or growth factors that are allowed to enter some cells but not others.