3 minute read
101 DNA and RNA
Key Idea: DNA and RNA are nucleic acids made up of long chains of nucleotides, which store and transmit genetic information. DNA is double-stranded. RNA is single-stranded.
DNA
When long chains of nucleotides are joined together, they form nucleic acids. Deoxyribonucleic acid (DNA) is a nucleic acid. DNA consists of a two strands of nucleotides linked together, forming a double helix. A double helix is like a ladder twisted into a corkscrew shape around its length. It is ‘unwound’ in the diagram on the right to show how the bases pair up. ` The DNA backbone is made up of alternating phosphate and sugar molecules. ` Each "rung" of the DNA molecule is made up of two nitrogen bases, joined together by hydrogen bonds between the bases (a hydrogen bond is a bond between hydrogen and an electronegative atom such as oxygen). ` Each DNA strand has a direction. Each single strand runs in the opposite direction to the other.
This gives the DNA molecule an asymmetrical (uneven) structure. ` The ends of a DNA strand are labelled the 5' (five prime) and 3' (three prime) ends. The 5' end has a terminal phosphate group (off carbon 5), the 3' end has a terminal hydroxyl group (off carbon 3).
5'
Carbon no. 5
T
A
3'
G A
C T
3'
Carbon no. 3
C
G
Hydrogen bond
3D
5'
T G
H2O Nucleotides are linked together by a condensation reaction (left). It is called a condensation reaction because water is produced. The link occurs between the phosphate group of one nucleotide and the sugar group of another.
RNA
Ribonucleic acid (RNA) is a type of nucleic acid. Like DNA, the nucleotides are linked together through condensation reactions. RNA is single stranded. Its functions include protein synthesis and cell regulation. There are 3 types of RNA: ` Messenger RNA (mRNA) ` Transfer RNA (tRNA) ` Ribosomal RNA (rRNA)
In RNA, uracil (U) replaces thymine in the code.
G U C A
The presence of two OH groups on the ribose sugar stops it from forming a double stranded helix.
1. The diagram on the right shows a double-stranded DNA molecule. Label the following: (a) Sugar group (d) Purine bases (b) Phosphate group (e) Pyrimidine bases (c) Hydrogen bonds (f) 5' and 3' ends
2. If you wanted to use a radioactive or fluorescent tag to label only the RNA in a cell and not the DNA, what compound(s) would you label?
3. If you wanted to use a radioactive or fluorescent tag to label only the DNA in a cell and not the RNA, what compound(s) would you label?
A
T C T
G A G
C
Messenger RNA (above) is transcribed (written) from DNA. It carries a copy of the genetic instructions from the DNA to ribosomes in the cytoplasm, where it is translated into a polypeptide chain. Transfer RNA (above) carries amino acids to the growing polypeptide chain. One end of the tRNA carries the genetic code in a three-nucleotide sequence called the anticodon. The amino acid links to the 3' end of the tRNA. Ribosomal RNA (above) forms ribosomes from two separate ribosomal components (the large and small subunits) and assembles amino acids into a polypeptide chain.
4. What is the purpose of the hydrogen bonds in double-stranded DNA?
5. Briefly describe the roles of RNA:
(a) mRNA:
(b) tRNA:
(c) rRNA:
6. (a) Why do the DNA strands have an asymmetrical structure?
(b) What are the differences between the 5' and 3' ends of a DNA strand?
7. Complete the following table summarising the differences between DNA and RNA molecules:
