Genetic Processes

GENETIC PROCESSES

Through complex processes of transcription and translation, the genetic code is made into proteins. Transcription involves the copying of the DNA segment into ribonucleic acid or RNA. Translation is when the RNA message gets turned into a protein segment. Most recently, the human genome has been sequenced and the polymerase chain reaction was developed, which is a technique for amplifying and identifying DNA sequences.

The patterns of inheritance can be determined by doing a genetic pedigree. This is a drawing that looks and the male and female parents, their offspring, and their own offspring. This can be done through many generations, highlighting which individuals in the pedigree carry the trait or disease state. Often, the pedigree will show the different patterns of inheritance of a particular trait or disease.

Some genetic traits are the result of multiple genes, such as the height of one s offspring. These are called polygenic traits. This makes it more difficult to draw an actual pedigree chart. In many cases, scientists do not know exactly which genes get involved in the determination of the end result in the offspring. Remember that humans, and many multicellular eukaryotic organisms reproduce sexually so there are traits that can be passed on from both parents to each of their offspring.

There are also complex traits that not only involve many genes but yield offspring that have features on a continuum between the two parents. This is true of human skin color. It doesn t mean that these are not heritable but rather that the actual inheritance of the feature is more complicated than can be explained by one gene. Heritability is the degree to which genetic factors determine a specific trait. Human height is an example of something that isn t 100 percent heritable. This is because the environment, such as nutrition, also plays a role in the end result.

As mentioned, DNA is what makes up genes. DNA is itself made of nucleotides, which are a type of molecule that can easily form chains. There are four different nucleotides that make up DNA. These are adenine, guanine, cytosine, and thymine. The arrangement of the nucleotides determines the protein made by the gene. Figure 4 shows what the nucleotides look like:

Figure 4.

As you can see by the figure, adenine combines with thymine and cytosine combines with guanine. When these are on opposite strands of DNA, they create a sort of ladder with rungs that makes up the DNA structure. This means that one strand can be used to copy its opposite strand.

The longest human chromosome is about 250 million base pairs or nucleotides long, with hundreds or thousands of genes per chromosome. There are proteins that support and organize the DNA so that the whole chromosome structure is called chromatin.

Most organisms are considered diploid, which means that they contain two copies of each chromosome. This is not the case with haploid organisms like bacteria, that have just one copy of the chromosome. Gametes, or sex cells, are haploid because, when they come together in the offspring, the offspring will be diploid. Each separate gene on one chromosome is called an

allele. For traits that are passed on with a single gene, one allele is given by each parent to make two alleles in a diploid organism.

An allele can be dominant, recessive, or codominant. Some are sex-linked or X-linked and affect primarily boys, because they have just one X chromosome so they cannot have a normal allele on a second X chromosome. This is the case with girls that have two X chromosomes.

Organisms can divide sexually and asexually. The process of mitosis is when a cell divides itself into two daughter cells that are identical to the parent cell. These daughter cells are called clones. Mitosis represents asexual reproduction of the cell. Bacteria do this as part of their reproductive process, although it is called binary fission.

Eukaryotic organisms may or may not be involved with sexual reproduction. This involves the making of a haploid sex cell that combines with the haploid sex cell of the opposite gender to make offspring that have mixed patterns of DNA and that are diploid like the parent cells. A gamete is a sex cell, which can be either a sperm cell or an egg cell.

Bacterial cells can take up new genetic material in ways that are not the same thing as sexual reproduction. Conjugation involves taking up a piece of DNA from another bacterium, while transformation involves the uptake of DNA from the environment. This is what is meant by horizontal gene transfer.

You should know that chromosomes in the sex cell do not get transferred unchanged from parent child. If this was the case, all offspring would be identical in form and appearance. There is genetic recombination that takes place, which basically shuffles the DNA on homologous chromosomes. This is also referred to as chromosomal crossover. It happens in the process of meiosis, when a diploid germ cell becomes haploid in order to make a sex cell. If two genes are far apart, their crossover rate is higher. With genetic linkage, two genes are so close together that they get inherited together.

As we talked about, genes and DNA serve to provide a template used to make proteins. The genetic code is made from a set of three nucleotides in a row, called a triplet code or codon. Each codon makes up the code for a certain amino acid. There are 20 amino acids that make up the structure of proteins. It takes a messenger RNA molecule to turn the DNA into a message that is read in order to make proteins.

We will talk later about mutations, which are mistakes in the DNA structure. Mistakes can be substitutions in the nucleotide sequence, deletions of a gene, or additions to the gene. Some can make very little difference to the organism, while others can be extremely deleterious. Genetic

diseases often happen because of mutations in one or more genes. Sickle cell anemia, cystic fibrosis, and muscular dystrophy are all genetic diseases that come from gene mutations.

While most phenotypes in an organism are inherited, there is some contribution of the environment in determining the actual phenotype. This is true for diseases like phenylketonuria, which is a problem in the metabolism of the amino acid called phenylalanine. Without phenylalanine in the diet, the person does not have characteristics of the disease. Even identical twins do not have 100 percent heritability as it applies to getting certain diseases. This is why twin studies are used to determine just how heritable a disease process is.

While every cell of a multicellular organism contains the same DNA in their genome, not all cells are the same in the organism. This is because of differences in gene regulation. There are certain transcription factors that determine what genes get expressed and those that do not. This process of regulation can be complex and can involve factors that promote a gene s expression and those that inhibit a gene s expression. There are signals within a cell and around the cell that determine what the cell s function is in a multicellular organism.

Mutations play a role in natural selection. Most mutations either have no effect or are detrimental to the organism. Some, however, can be beneficial to the organism. This is where natural selection plays a role in genetics. If a mutation is beneficial and confers an advantage to the host, the mutation could be selected for and the frequency of the mutated gene would increase in the population.

There are certain organisms studied more frequently than others in the study of genetics. Most of these organisms are easy to grow, have short generation times, known genotypes, and easy manipulability when it comes to their genome. Some of these include Escherichia coli, baker s yeast or Saccharomyces cerevisiae, certain plants, mice, fruit flies, and some nematodes.