3.A.1 DNA and RNA Part III:
Transcription
DNA, and in some cases RNA, is the primary source of heritable information.
DNA, and in some cases RNA, is the primary source of heritable information.

DNA and RNA molecules have structural similarities and differences that define function.

DNA and RNA are nucleic acids .
The monomer of a nucleic acid is the nucleotide .
Structure: pentose sugar , nitrogenous base , phosphate group
DNA has a negative charge due to the negatively charged phosphate groups.

In RNA, pentose sugar is ribose .
In DNA, pentose sugar is deoxyribose .

There are two types of nitrogenous bases: pyrimidines
(single-ring) purines
(double-ring)

• Purines (G and A) have a double ring structure.
• Pyrimidines (C, T and U) have a single ring structure.
Adenine Guanine
Cytosine
Thymine Uracil
Pyrimidines

DNA is a double helix .

Covalent Bonds
Hydrogen Bonds

RNA is a single strand.

Both DNA and RNA exhibit specific nucleotide base pairing that is conserved through evolution.

In transcription , a strand of mRNA is made from the DNA. The sequence of base pairs in the mRNA molecule is determined by the gene on the DNA molecule.
DNA molecule
Gene 2
Gene 1
Gene 3
DNA strand
(template)
3
A C C A A A C C G A G T
5
TRANSCRIPTION mRNA
TRANSLATION
5
U G G U U U G G C U C A
3
Codon
Protein Trp
Amino acid
Phe Gly Ser

mRNA synthesis is catalyzed by RNA polymerase , which pries the DNA strands apart and hooks together the RNA nucleotides.

mRNA synthesis follows the same basepairing rules as DNA, except that in RNA uracil substitutes for thymine.

The stages of transcription are:
• Initiation
• Elongation
• Termination

RNA polymerase reads the template
DNA strand in the 3’ 5’ direction.
New mRNA is assembled in the 5’ 3’ direction.

The sequence of the RNA bases, together with the structure of the RNA molecule, determines RNA function.

mRNA carries information from the DNA to the ribosomes , which are in located in the cytoplasm.