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DNA and RNA

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Biology

The two macromolecules that are essential for heredity in all living cells are DNA, deoxyribonucleic acid and RNA, ribonucleic acid. Both DNA and RNA are nucleic acids, and they perform vital functions in the continuation of life.

Functions of DNA

The chief function of DNA is to store genetic information in structures called chromosomes. In eukaryotic cells, DNA can be found in the nucleus, the mitochondria and the chloroplast (in plants only). Meanwhile, prokaryotes carry DNA in the nucleoid, which is a region in the cytoplasm, and plasmids.

Functions of RNA

RNA transfers genetic information from the DNA found in the nucleus to the ribosomes, specialized organelles comprised of RNA and proteins. The ribosomes are especially important as translation (the final stage of protein synthesis) occurs here. There are different types of RNA, such as messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA) , each with its specific function.

mRNA is the primary molecule responsible for carrying genetic information to the ribosomes for translation, tRNA is responsible for carrying the correct amino acid to the ribosomes and rRNA forms ribosomes. Overall, RNA is vital in the creation of proteins, such as enzymes.

In eukaryotes, RNA is found in the nucleolus, an organelle within the nucleus, and ribosomes. In prokaryotes, RNA can be found in the nucleoid, plasmids and ribosomes.

What are the nucleotide structures?

DNA and RNA are polynucleotides , meaning they are polymers made of monomers. These monomers are called nucleotides. Here, we will explore their structures and how they differ.

DNA nucleotide structure

A single DNA nucleotide is comprised of 3 components:

  • A phosphate group
  • A pentose sugar (deoxyribose)
  • An organic nitrogenous base

DNA and RNA structure of a DNA nucleotide StudySmarterThe diagram shows the structure of a DNA nucleotide.

Above, you'll see how these different components are organized within a single nucleotide. There are four different types of DNA nucleotides as there are four different types of nitrogenous bases: adenine (A), thymine (T), cytosine (C) and guanine (G). These four different bases can be further divided into two groups: pyrimidine and purine.

Pyrimidine bases are the smaller bases as these are composed of a 1 carbon ring structure. The pyrimidine bases are thymine and cytosine. Purine bases are the larger bases as these are 2 carbon ring structures. The purine bases are adenine and guanine.

RNA nucleotide structure

An RNA nucleotide has a very similar structure to a DNA nucleotide and like DNA, it is comprised of three components:

  • A phosphate group
  • A pentose sugar (ribose)
  • An organic nitrogenous base

DNA and RNA [+] structure of an RNA nucleotide [+] StudySmarterThe diagram shows the structure of an RNA nucleotide.

You'll see the structure of a single RNA nucleotide above. An RNA nucleotide can contain four different types of nitrogenous bases: adenine, uracil, cytosine or guanine. Uracil, a pyrimidine base, is a nitrogenous base that is exclusive to RNA and cannot be found in DNA nucleotides.

Comparing DNA and RNA nucleotides

The main differences between DNA and RNA nucleotides are:

  • DNA nucleotides contain a deoxyribose sugar, while RNA nucleotides contain a ribose sugar
  • Only DNA nucleotides can contain a thymine base, while only RNA nucleotides can contain a uracil base

The main similarities between DNA and RNA nucleotides are:

  • Both nucleotides contain a phosphate group

  • Both nucleotides contain a pentose sugar

  • Both nucleotides contain a nitrogenous base

DNA and RNA structure

DNA and RNA polynucleotides are formed from condensation reactions between individual nucleotides. A phosphodiester bond is formed between the phosphate group of one nucleotide and the hydroxyl (OH) group at the 3 'pentose sugar of another nucleotide. A dinucleotide is created when two nucleotides are joined together by a phosphodiester bond. A DNA or RNA polynucleotide occurs when many nucleotides are joined together by phosphodiester bonds. The diagram below shows where the phosphodiester bond is positioned between 2 nucleotides. A hydrolysis reaction must take place to break phosphodiester bonds.

A dinucleotide is built of only 2 nucleotides whereas a polynucleotide consists of MANY nucleotides!

DNA and RNA [+] phosphodiester bond [+] StudySmarterThe diagram illustrates the phosphodiester bond.

DNA structure

The DNA molecule is an anti-parallel double helix formed of two polynucleotide strands. It is anti-parallel as the DNA strands run in opposite directions to each other. The two polynucleotide strands are joined together by hydrogen bonds between complementary base pairs, which we will explore later. The DNA molecule is also described as having a deoxyribose-phosphate backbone - some textbooks may also call this a sugar-phosphate backbone.

RNA structure

The RNA molecule is a little different to DNA in that it is made of only one polynucleotide which is shorter than DNA. This helps it carry out one of its primary functions, which is to transfer genetic information from the nucleus to the ribosomes - the nucleus contains pores that mRNA can pass through due to its small size, unlike DNA, a larger molecule. Below, you can visually see how DNA and RNA differ from each other, both in size and the number of polynucleotide strands.

DNA and RNA [+] structure of DNA and RNA molecules [+] StudySmarterThe diagram shows the structure of DNA and RNA.

What is base pairing?

The bases can pair up together by forming hydrogen bonds and this is termed complementary base pairing . This keeps the 2 polynucleotide molecules in DNA together and is essential in DNA replication and protein synthesis.

Complementary base pairing requires the joining of a pyrimidine base to a purine base via hydrogen bonds. In DNA, this means

  • Adenine pairs with thymine with 2 hydrogen bonds

  • Cytosine pairs with guanine with 3 hydrogen bonds

In RNA, this means

  • Adenine pairs with uracil with 2 hydrogen bonds

  • Cytosine pairs with guanine with 3 hydrogen bonds

DNA and RNA, complementary base pairing with hydrogen bonds, StudySmarterThe diagram shows complementary base pairing.

The diagram above helps you to visualize the number of hydrogen bonds formed in complementary base pairing. Although you do not need to know the chemical structure of the bases, you will need to know the number of hydrogen bonds formed.

Due to complementary base pairing, there are equal quantities of each base in a base pair. For example, if there are approximately 23% guanine bases in a DNA molecule, there will also be approximately 23% cytosine.

DNA stability

As cytosine and guanine form 3 hydrogen bonds, this pair is stronger than adenine and thymine which only form 2 hydrogen bonds. This contributes to the stability of DNA. DNA molecules with a high proportion of cytosine-guanine bonds are more stable than DNA molecules with a lower proportion of these bonds.

Another factor that stabilizes DNA is the deoxyribose-phosphate backbone. This keeps the base pairs inside the double helix, and this orientation protects these bases which are highly reactive.

Differences and similarities between DNA and RNA

It's important to know that while DNA and RNA work closely together, they also differ. Use the table below to see how these nucleic acids are different and similar.

DNA
RNA
Function
Stores genetic information
Protein synthesis - transfers genetic information to the ribosomes (transcription) and translation
Size
2 large polynucleotide strands
1 polynucleotide strand, relatively shorter than DNA
Structure
Anti-parallel double helix
Single-stranded chain
Location in cell (eukaryotes)
Nucleus, mitochondria, chloroplast (in plants)
Nucleolus, ribosomes
Location in cell (prokaryotes)
Nucleoid, plasmid
Nucleoid, plasmid, ribosomes
Bases
Adenine, thymine, cytosine, guanine
Adenine, uracil, cytosine, guanine
Pentose sugar
Deoxyribose
Ribose

DNA and RNA - Key takeaways

  • DNA stores genetic information while RNA transfers this genetic information to the ribosomes for translation.
  • DNA and RNA are made of nucleotides that are made of 3 main components: a phosphate group, a pentose sugar and an organic nitrogenous base. The pyrimidine bases are thymine, cytosine and uracil. The purine bases are adenine and guanine.
  • DNA is an anti-parallel double helix made of 2 polynucleotide strands while RNA is a single-chain molecule made of 1 polynucleotide strand.
  • Complementary base pairing occurs when a pyrimidine base pairs with a purine base via hydrogen bonds. Adenine forms 2 hydrogen bonds with thymine in DNA or uracil in RNA. Cytosine forms 3 hydrogen bonds with guanine.

DNA and RNA

DNA and RNA work together because DNA stores genetic information in structures called chromosomes while RNA transfers this genetic information in the form of messenger RNA (mRNA) to the ribosomes for protein synthesis.

DNA nucleotides contain deoxyribose sugar, while RNA nucleotides contain ribose sugar. Only DNA nucleotides can contain thymine, while only RNA nucleotides can contain uracil. DNA is an anti-parallel double helix made of 2 polynucleotide molecules while RNA is a single-stranded molecule made of only 1 polynucleotide molecule. DNA functions to store genetic information, while RNA functions to transfer this genetic information for protein synthesis.

A DNA molecule is made of 2 polynucleotide strands that run in opposite directions (anti-parallel) to form a double helix. The 2 polynucleotide strands are kept together by hydrogen bonds found between complementary base pairs. DNA has a deoxyribose-phosphate backbone which is kept together by phosphodiester bonds between individual nucleotides.

DNA is described as a polynucleotide as it is a polymer made of many monomers, called nucleotides.

The three basic parts of DNA and RNA are: a phosphate group, a pentose sugar and an organic nitrogenous base.

The three different types of RNA are messenger RNA (mRNA), transfer RNA (tRNA) and ribosomal RNA (rRNA). mRNA carries genetic information from the DNA in the nucleus to the ribosomes. tRNA brings the correct amino acid to the ribosomes during translation. rRNA forms the ribosomes.

Final DNA and RNA Quiz

Question

Both DNA and RNA are _____ acids.

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Answer

Nucleic.

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Question

Compare the functions of DNA and RNA.


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Answer

DNA stores genetic information while RNA transfers this genetic information for protein synthesis.

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Question

Where is DNA found in the cells of eukaryotes and prokaryotes?


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Answer

In eukaryotes, DNA is in the nucleus, mitochondria and chloroplast (in plants). 


In prokaryotes, DNA is in the nucleoid and plasmids.

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Question

Where is RNA found in the cells of eukaryotes and prokaryotes?


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Answer

In eukaryotes, RNA is in the nucleolus and ribosomes. 


In prokaryotes, RNA is in the nucleoid, plasmids and ribosomes.

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Question

Identify the three different types of RNA.


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Answer

Messenger RNA (mRNA), transfer RNA (tRNA) and ribosomes RNA (rRNA).

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Question

What nitrogenous bases can DNA nucleotides have?


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Answer

Adenine, thymine, cytosine or guanine.

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Question

What nitrogenous bases can RNA nucleotides have?


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Answer

Adenine, uracil, cytosine and guanine.

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Question

Identify the pyrimidine and purine bases. 


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Answer

The pyrimidine bases are cytosine, uracil and thymine. 


The purine bases are adenine and guanine.

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Question

DNA contains the _____ pentose sugar while RNA contains the _____ pentose sugar. 


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Answer

Deoxyribose. 

Ribose.

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Question

________ reactions form polynucleotides while _____ reactions break polynucleotides. 

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Answer

Condensation. 

Hydrolysis.

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Question

What are phosphodiester bonds and how are they formed?


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Answer

Phosphodiester bonds link nucleotides together. They are formed between the phosphate group of one nucleotide and the hydroxyl group at the 3 'pentose sugar of another nucleotide.

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Question

Compare the structure of a DNA and RNA molecule. 


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Answer

DNA is an anti-parallel double helix made of 2 polynucleotide strands while RNA is a single-stranded molecule made of 1 polynucleotide strand.

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Question

What is complementary base pairing? 


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Answer

Complementary base pairing is the joining of a pyrimidine base to a purine base via hydrogen bonds.

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Question

In complementary base pairing, how many hydrogen bonds are formed between the base pairs?


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Answer

Adenine forms 2 hydrogen bonds with thymine in DNA, or uracil in RNA. 


Cytosine forms three hydrogen bonds with guanine.

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Question

DNA and RNA are polynucleotides. The monomers that make up these polynucleotides are called _____.

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Answer

Nucleotides.

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