Protein Synthesis Protein Synthesis DNA is in the

Protein Synthesis

Protein Synthesis • DNA is in the form of specific sequences of nucleotides along the DNA strands • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • The process by which DNA directs protein synthesis, gene expression includes two stages, called transcription and translation

*Transcription and Translation* • Cells are governed by a cellular chain of command – DNA RNA PROTEIN • Transcription (DNA to RNA) – Is the synthesis of RNA under the direction of DNA – Produces messenger RNA (m. RNA) • Translation (RNA to PROTEIN) – Is the actual synthesis of a polypeptide, which occurs under the direction of m. RNA – Occurs on ribosomes

*Transcription and Translation in Eukaryotes* • In a eukaryotic cell the nuclear envelope separates transcription from translation Nuclear envelope DNA TRANSCRIPTION Pre-m. RNA PROCESSING m. RNA Ribosome TRANSLATION Polypeptide (b) Eukaryotic cell. The nucleus provides a separate compartment for transcription. The original RNA transcript, called pre-m. RNA, is processed in various ways before leaving the nucleus as m. RNA.

*Transcription (Part 1 of Protein Synthesis)* • Transcription is the DNAdirected synthesis of RNA synthesis Is catalyzed by? RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides How does it work? Follows the same base-pairing rules as DNA, except that in RNA, Uracil substitutes for Thymine

*RNA* • • RNA is single stranded, not double stranded like DNA RNA is short, only 1 gene long RNA uses the sugar ribose instead of deoxyribose in DNA RNA uses the base uracil (U) instead of thymine (T) Table 17. 1

*Synthesis of an RNA Transcript* • The stages of transcription are – Initiation – Elongation – Termination Promoter Transcription unit 5 3 Start point RNA polymerase 5 3 Unwound DNA 3 5 DNA 1 Initiation. After RNA polymerase binds to the promoter, the DNA strands unwind, and the polymerase initiates RNA synthesis at the start point on the template strand. 3 5 Template strand of DNA transcript 2 Elongation. The polymerase moves downstream, unwinding the DNA and elongating the RNA transcript 5 3 . In the wake of transcription, the DNA strands re-form a double helix. Rewound RNA 5 3 3 5 RNA transcript 3 Termination. Eventually, the RNA transcript is released, and the polymerase detaches from the DNA. 5 3 3 5 5 Completed RNA transcript 3

*Synthesis of an RNA Transcript – Initiation* • Promoters signal the initiation of RNA synthesis • Transcription factors help eukaryotic RNA polymerase recognize promoter sequences 1 Eukaryotic promoters TRANSCRIPTION DNA RNA PROCESSING Pre-m. RNA TRANSLATION Ribosome Polypeptide 5 3 Promoter 3 5 T A A AA AT AT T TATA box Start point Template DNA strand Several transcription factors 2 Transcription factors 5 3 3 Additional transcription 3 5 factors RNA polymerase II 5 3 Transcription factors 3 5 5 RNA transcript Transcription initiation complex

Synthesis of an RNA Transcript - *Elongation* • RNA polymerase synthesizes a single strand of RNA against the DNA template strand, adding nucleotides to the 3’ end of the RNA chain • As RNA polymerase moves along the DNA it continues to untwist the double helix, exposing about 10 to 20 DNA bases at a time for pairing with RNA nucleotides Non-template strand of DNA Elongation RNA nucleotides RNA polymerase 3 C C C A A T 3 end T U C A T E A G G C G A T Newly made RNA U A A T Direction of transcription (“downstream”) 5 T G A A C G A Template strand of DNA

*Synthesis of an RNA Transcript – Termination* • • Specific sequences in the DNA signal termination of transcription When one of these is encountered by the polymerase, the RNA transcript is released from the DNA and the double helix can zip up again.

Transcription Overview

*Post Termination RNA Processing* • Transcription of RNA processing occur in the ____. After this, the messenger RNA moves to the _____ for translation. • The cell adds a protective cap to one end, and a tail to the other end. Why? • **Most of the genome consists of non-coding regions called introns – Non-coding regions may have specific chromosomal functions or have regulatory purposes – Introns also allow for alternative RNA splicing • **Thus, an RNA copy of a gene is converted into messenger RNA by doing 2 things: – Add protective bases to the ends – Cut out the introns

*Alteration of m. RNA Ends* • Each end of a pre-m. RNA molecule is modified in a particular way – The 5 end receives a modified nucleotide cap – The 3 end gets a poly-A tail A modified guanine nucleotide added to the 5 end TRANSCRIPTION RNA PROCESSING 50 to 250 adenine nucleotides added to the 3 end DNA Pre-m. RNA 5 m. RNA Protein-coding segment Polyadenylation signal 3 G P P P AAUAAA AAA…AAA Ribosome TRANSLATION 5 Cap Polypeptide 5 UTR Start codon Stop codon 3 UTR Poly-A tail

RNA Processing - Splicing • The original transcript from the DNA is called pre-m. RNA. • It contains transcripts of both introns and exons. • The introns are removed by a process called splicing to produce messenger RNA (m. RNA)

RNA Processing • Proteins often have a modular architecture consisting of discrete structural and functional regions called domains • In many cases different exons code for the different domains in a protein DNA Gene Exon 1 Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 Figure 17. 12 Polypeptide

Homework Worksheet on Transcription