From DNA to Protein: Each gene is the information to build one protein (or polypeptide chain) that the organism needs. The first step in producing the protein is making a mRNA copy of the gene. This process is called transcription. Transcription and Translation

Differences between DNA and mRNA While DNA is a double helix, mRNA is a single strand of nucleotides. mRNA nucleotides have the sugar ribose instead of deoxyribose mRNA has the nitrogen base uracil instead of thymine.

The Process of Transcription The process of transcription is similar to DNA replication in that the DNA is unwound and complementary nucleotides are added. Differences: Only a gene is copied, not the whole chromosome. RNA nucleotides are added instead of DNA nucleotides. –Uracil is paired with adenine instead of thymine. Transcription occurs on an ongoing basis as proteins needed, replication only occurs prior to cell division.

Transcription The enzyme RNA polymerase unwinds the DNA double helix and adds the complementary RNA nucleotides. If the sequence of a gene is CGGCAT what mRNA transcript will be produced? DNA CGCATA mRNA GCG UAU In RNA, the nucleotide uracil is used in place of thymine.

DNA Transcription Video Click once on image to start

Transcription Animation imat/molgenetics/transcription.swf

Modification of mRNA in Eukaryotic Cells Genes in eukaryotic organisms contain non-coding segments called introns. The coding portions of the gene are called exons. The noncoding introns need to be removed before the mRNA leaves the nucleus.

Prokaryotic Cells In prokaryotic cells, there is no nuclear membrane. So mRNA transcripts can not be modified before being translated. So in prokaryotes, transcription of mRNA and translation to produce the protein can occur simultaneously