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The Structure of RNA 
All cells contain ribonucleic acid, also known as RNA, which is a second nucleic acid. RNA is a 
polymer of nucleotides, just like DNA. Each RNA nucleotide contains a nitrogenous base, a sugar with five 
carbons, and a phosphate group. The five-carbon sugar in RNA is ribose rather than deoxyribose. Ribose has a 
hydroxyl 
group at the 2' carbon, unlike deoxyribose, which has only a hydrogen atom (Figure 39). 
Figure 39.
The presence of a hydroxyl group at the 2' carbon distinguishes ribose, which is 
found in RNA, from deoxyribose, which is found in DNA. 
The nitrogenous bases adenine, cytosine, and guanine are found in RNA nucleotides. They do not, 
however, contain thymine; instead, uracil, denoted by a "U," is present. Instead of being a double-stranded 
helix, RNA is a single-stranded molecule. According to their intended use, RNA types have been given names 
by molecular biologists. These include messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA 
(rRNA)—molecules that are involved in the production of proteins from the DNA code

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3.2 Reading Resource #2:
DNA Replication 
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(credit a: modification of work by Jerome Walker, Dennis Myts)
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Fowler, S., Roush, R. & Wise, J. (2017) 
Concepts in Biology, 
Chapter 9, -Pp. 200-202. OpenStax, 
https://openstax.org/details/books/concepts-biology
 

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70 
It is crucial that each daughter cell receives an identical copy of the DNA when a cell divides. DNA 
replication is the process that makes this happen. Before the cell enters mitosis or meiosis, during the synthesis 
phase, also known as the S phase, of the cell cycle, DNA replication takes place. An indication of how DNA is 
copied was given by the clarification of the double helix's structure. Recall that adenine nucleotides pair with 
thymine nucleotides, and cytosine with guanine. This means that the two strands are complementary to each 
other. For example, a strand of DNA with a nucleotide sequence of AGTCATGA will have a complementary 
strand with the sequence TCAGTACT (Figure 40). Having one strand makes it possible to recreate the other 
strand because the two strands complement one another. According to this replication model, the double helix's 
two strands split apart during the process and each one acts as a template for the copying of the new 
complementary strand. 
Each of the two strands that make up the double helix acts as a template from which new strands are 
copied during DNA replication. The parental or "old" strand and the new strand will complement one another. 
One new daughter strand and one new parental strand make up each new double strand. Semiconservative 
replication is what is meant by this. When two DNA copies are created, they are equally divided into two 
daughter cells and have an identical nucleotide base sequence.
Figure 40.
Because the two strands of DNA are complementary, the correct base sequence for one strand can 
be generated from the base sequence for the other strand.