GENETICS - BIOCHEMISTRY - The Handy Chemistry Answer Book (2014)

The Handy Chemistry Answer Book (2014)

BIOCHEMISTRY

GENETICS

What is a nucleotide?

Nucleotides are one of the basic units for another class of biomolecules: the ones that join together to form your DNA and RNA (the long names for these are deoxyribonucleic acid and ribonucleic acid). DNA and RNA are the macromolecules that store the genetic information in your body. A nucleotide consists of three parts: a nitrogenous base, a sugar, and a phosphate group. The identity of the nitrogenous base determines which “letter” in the genetic code is associated with the nucleotide, while the type of sugar determines whether the nucleotide is a ribonucleotide or a deoxyribonucleotide (which essentially just tells us whether it’s going to be a part of RNA or a part of DNA).

What is a nucleic acid?

Nucleic acids are the polymers formed by a series of linked nucleotides. DNA and RNA are the two most important types of nucleic acids for life as these are the molecules that carry and transmit our genetic information. In humans, DNA is found in the nucleus of our cells, and a complete copy of the genetic information is actually contained within each individual cell. That means we each have about fifty trillion copies of our genetic information in our body! RNA is made by enzymes called RNA polymerases, which are responsible for transcribing the information from DNA into a strand of RNA, where it can move about the cell to carry out its function.

How many types of nucleotides are found in humans?

Nucleotides are named for the nitrogenous (nitrogen-containing) base incorporated into the nucleotide, and there are only five types of bases used: adenine, guanine, cytosine (all found in both DNA and RNA), and thymine (DNA only) or uracil (RNA only). The corresponding nucleosides are called deoxyadenosine (DNA)/adenosine (RNA), deoxyguanosine (DNA)/guanosine (RNA), deoxycytidine (DNA)/cytidine (RNA), deoxythymidine (DNA), and uridine (RNA). All of the genetic information in our body is stored in our DNA using sequences of only these four nucleotides. Imagine trying to describe how to make and operate an entire machine using only four letters—that’s just what our genetic information does.

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DNA looks like a ladder that has been twisted. The rungs of the ladder consist of pairs of nucleotides, and the way they are arranged creates a genetic code of instructions that tell our body how to grow and function.

What is the structure of DNA?

DNA is well known for its double helical structure. Two scientists named James Watson and Francis Crick first discovered this structure in 1953 using data from X-ray diffraction experiments.

The two strands of the helix are held together by hydrogen bonding as well as stacking interactions between the aromatic rings of the nitrogenous bases. These strong interactions make the double helical structure very stable. It’s also true, however, that the double helical structure has to be temporarily pulled apart when it’s time to transcribe the DNA into RNA. This is accomplished by an enzyme called helicase.

What is a phosphodiester bond?

Phosphodiester bonds are what make up the backbone of DNA and RNA molecules. These bonds provide the linkage that holds together the individual nucleotides or nucleosides in DNA and RNA. Enzymes called polymerases catalyze their formation (DNA polymerases for DNA and RNA polymerases for RNA). Take a look at the picture below to see the chemical structure of a phosphodiester bond.

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How is information encoded in RNA and DNA?

The sequence of nucleotides in DNA in can be read only by first separating the DNA double helix (performed by the enzyme helicase). The information is “read” by RNA polymerase, which then transcribes a corresponding piece of RNA. If the purpose of the RNA is to encode a protein, it can then be read by a structure called a ribosome, which creates proteins according to the specific ordering of the nucleotides in the RNA strand.

The ordering of the nucleotides in DNA (and thus the nucleosides in RNA) is extremely important in determining the function of a protein. Nucleotides are read in groups of three. These groups of three are called codons, and each codon tells the ribosome to incorporate a specific amino acid into the peptide/protein it creates. There are also specific codons that tell the ribosome to start or stop creating a peptide. Any mistake in the copying of DNA or RNA can potentially lead to major problems, so it’s important that the cellular machinery that carries out these processes is extremely accurate.

What can go wrong if errors are present in DNA?

If errors do occur during DNA replication, they can have serious biological and physiological effects. Many diseases are believed to be associated with errors in genetic sequences, including cystic fibrosis, sickle cell anemia, hemophilia, Huntington’s disease, Tay-Sachs disease, and a number of other genetic disorders. Other diseases or disorders may be linked to your genetics in a more complicated manner, such that you are more or less likely to suffer from them than the rest of the population. Some such diseases/disorders are cancers, mental/mood disorders, asthma, heart disease, and diabetes. Fortunately we have several DNA repair mechanisms that are constantly on the lookout for damaged DNA.

What are genes?

Genes are the basic unit of heredity. A gene is a sequence of nucleotides containing information on specific traits that shape the characteristics of an organism. Each person actually has two copies of each gene; one comes from each parent. There’s not really a “typical size” for a gene, since the number of base pairs in a gene can range from just a couple hundred base pairs to millions of base pairs. Most of the genetic information in each of our genes is actually the same from person to person. In fact, differences in less than 1% of the DNA in our genes accounts for all of the physical differences between people.

What are chromosomes?

Chromosomes are a bundle of DNA and proteins packaged together. They are how your DNA is stored in your cells when it isn’t being “read” by any enzymes. Each chromosome contains a large number of genes. Humans each have forty-six chromosomes, all of which are present in each cell in the body. Chromosomes need to be “unpacked” by enzymes before their DNA can be read by RNA polymerase or other enzymes.

What is gene therapy?

Gene therapy is a method of medical treatment that tries to correct defective or disease-causing genes. There are a few ways that this may be attempted, all of which are centered around getting a normal, functional copy of the gene inserted into the genome. Some methods involve repairing the mutated gene, while others involve just adding a working copy of the gene into a nonspecific location in the genome.

What is genetic engineering?

Genetic engineering is the process of changing the genetic makeup of cells or organisms to produce versions with specific traits or to produce new organisms altogether.

What is the difference between eukaryotes and prokaryotes?

Prokaryotes are organisms whose cells do not have a nucleus, while eukaryotes are organisms whose cells do have a nucleus.

Prokaryotes are most often, but not exclusively, unicellular (single-celled) organisms. In addition to lacking a nucleus, they also lack other separate membrane-bound organelles. All of the proteins, DNA, and other molecules in a prokaryotic cell float around within the cell membrane but are not separated into different compartments.

Eukaryotes do typically have separate membrane-bound organelles within their cells. They may be single-celled or multicellular organisms. Every large organism (animals, plants, fungi) is eukaryotic, and many small and single-celled organisms are in this category as well.