Review of Processes

Let's review the basic themes of nucleic acid processes before considering variations on those themes.

Protecting Genetic Information

DNA is found in the nucleus of cells. We presume that there's a protection factor involved in this. The nucleus is a safer place because there is not so much going on in the nucleus as in other parts of the cells. A parallel would be that in a military situation, headquarters is not placed on the front lines. Similarly, in a business situation, money is kept in a safe or a vault or a cash register rather than being out on the counter.

The double strand of DNA seems to be an additional protection factor. The double-strand encloses the genetic code (sequence of bases), so that it's not out where it can react with other chemicals quite so easily.

Another protection factor might be that missing OH from the second carbon atom of deoxyribose. The absence of that OH on every sugar molecule in the DNA would cut down on the mobility of the DNA (solubility in water) and keep it in place, in a safe place.

The DNA is maintained by a set of an unknown number of repair enzymes that repair any deterioration of the DNA that may take place.

Using Genetic Information

The replication of DNA provides a complete identical copy of the DNA for each new cell, and this allows for both growth of the organism and also for new generations. Transcription of DNA provides a copy of the code using RNA nucleotides. Now, RNA is, although it's basically similar to DNA, it is different from DNA in several important factors. One is that it employs a single strand rather than a double strand. It also uses ribose instead of deoxyribose and it uses uracil instead of thymine.

After being created in the transcription process, the messenger RNA moves or is moved out of the nucleus to the ribosomes where it is translated into proteins with a particular amino acid sequence.

There are generally four bases used in DNA (adenine, guanine, cytosine and thymine) and there are four comparable bases used in RNA (adenine, guanine, cytosine and uracil instead of thymine).

Variations

However, there are variations that can occur in this theme.

One is that modifications of the four bases are sometimes found in certain kinds of specialized RNA.

Another variation is that, not every cell has a nucleus, and not all DNA is found in the nucleus even if the cell has one.

In addition, it seems to me that I have heard somewhere, or read about the existence of single stranded DNA instead of double stranded occurring in some particular organisms.

Some things, such as some viruses, seem to have DNA without a cell and other viruses seem to have RNA without a cell, and also RNA without DNA.

Another variation is that transcription does not solely involve DNA being copied into RNA. For example, some viruses seem to be able to use RNA to create a DNA copy of it. Another thing to keep in mind is that all three of these processes, the replication, the transcription, and also the translation do not always produce perfect products. There are changes in the sequence that are incorporated into the processes as mistakes occur during them.

Practice

As a quick test of your understanding of the structure of nucleic acids and the processes of replication, transcription and translation, take a moment to answer the following questions. (These are also given in Exercise 16 in your workbook.) You will need to refer to a chart of codons. You can check your answers below.

What is the sequence of bases in a single stranded RNA that would be complementary to the strand with bases A-A-C-G-C-A?

Using "B" for base, "S" for sugar and "P" for phosphate, sketch a single strand of DNA, three or more bases long, that shows the arrangement of these molecules in the strand.

Identify each of the following descriptions as being "translation," "replication," or "transcription."

	A complete copy of a DNA double helix is produced.
	A single stranded copy of a gene is made out of RNA.
	A protein is synthesized from single-stranded mRNA

How many amino acids would there be in a protein made from a length of mRNA that contained 24 bases, including the termination codon?

Briefly describe the function of each of these molecules.

	the ribosome
	tRNA
	ATP

Answers

What is the sequence of bases in a single stranded RNA that would be complementary to the strand with bases A-A-C-G-C-A?

U-U-G-C-G-U

Using "B" for base, "S" for sugar and "P" for phosphate, sketch a single strand of DNA, three or more bases long, that shows the arrangement of these molecules in the strand.

        -P-S-P-S-P-S-P-S-
           |   |   | |
           B   B   B B

Identify each of the following descriptions as being "translation," "replication," or "transcription."

	A complete copy of a DNA double helix is produced. - Replication
	A single stranded copy of a gene is made out of RNA. - Transcription
	A protein is synthesized from single-stranded mRNA - Translation

How many amino acids would there be in a protein made from a length of mRNA that contained 24 bases, including the termination codon?

Seven amino acids, because there are eight codons and the termination codon does not have an amino acid associated with it.

Briefly describe the function of each of these molecules.

	the ribosome - Catalyzes the synthesis of protein using mRNA to determine the sequence and tRNA to provide the amino acids.
	tRNA - Transports amino acids to the ribosome and lines them up in order according to the codons in mRNA.
	ATP - Provides energy for a variety of biochemical reactions through its exothermic hydrolysis to become ADP.

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