Bài giảng Basic Food Chemistry - Chapter 2+3: Protein biosynthesis

Nội dung text: Bài giảng Basic Food Chemistry - Chapter 2+3: Protein biosynthesis

1. The Central Dogma of Life replication 12/31/2022 by TDLV 2
2. Transcription and Translation • Transcription – the synthesis of RNA under the direction of DNA – Produces messenger RNA (mRNA) • Translation – the actual synthesis of a polypeptide under the direction of mRNA – Occurs on ribosomes 12/31/2022 by TDLV 4
3. Transcription and Translation • In a eukaryotic cell, the nuclear envelope separates transcription from translation • Extensive RNA processing occurs in the nucleus Nuclear envelope TRANSCRIPTION DNA Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION (b) Eukaryotic cell. The nucleus provides a separate Polypeptide compartment for transcription. The original RNA transcript, called pre-mRNA, is processed in various ways before leaving the nucleus as mRNA. 12/31/2022 by TDLV 6
4. TRANSCRIPTION - OVERVIEW ❑ Transcription - when a cell requires a particular protein → specific mRNA synthesized - first, a section of DNA containing the gene unwinds - only one of the DNA strands copied (at the initiation point: sequence TATAAA) - RNA polymerase: moves along DNA template in the 3’-5’direction → replicates DNA sequence into a complementary sequence of mRNA - mRNA synthesized using complementary base pairing with uracil (U) replacing thymine (T) → moves out of the nucleus to ribosomes in the cytoplasm, acts as the template for protein biosynthesis (translation) and the DNA re-winds → released at the termination point
5. Synthesis of an RNA Transcript • The stages of transcription: – Initiation – Elongation – Termination 12/31/2022 by TDLV 10
6. Synthesis of an RNA Transcript - Elongation • RNA polymerase synthesizes a single strand of RNA against the DNA template strand (anti-sense 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 Elongation Non-template strand of DNA RNA nucleotides RNA polymerase T C C A A A 3 3 end U A E G 5 C A T A G G T T Direction of transcription 5 (“downstream”) Template strand of DNA Newly made RNA 12/31/2022 by TDLV 12
7. Post Termination RNA Processing • Most eukaryotic mRNAs aren’t ready to be translated into protein directly after being transcribed from DNA. mRNA requires processing. • RNA processing occur in the nucleus. After this, the messenger RNA moves to the cytoplasm for translation. • The cell adds a protective cap to one end, and a tail of A’s to the other end. These both function to protect the RNA from enzymes that would degrade • 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 12/31/2022 by TDLV 14
8. RNA Processing - Splicing • The original transcript from the DNA is called pre- mRNA. • It contains transcripts of both introns and exons. • The introns are removed by a process called splicing to produce messenger RNA (mRNA) 12/31/2022 by TDLV 16
9. RNA Processing - by spliceosomes • RNA Splicing can also be carried out by spliceosomes 12/31/2022 by TDLV 18
10. Protein – modular structure • 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 Gene DNA Exon 1 Intron Exon 2 Intron Exon 3 Transcription RNA processing Translation Domain 3 Domain 2 Domain 1 12/31/2022 by TDLV Polypeptide 20
11. The Genetic Code • Genetic information is encoded as a sequence of nonoverlapping base triplets, or codons • The gene determines the sequence of bases along the length of an mRNA molecule 12/31/2022 by TDLV 22
12. The Genetic Code • A codon in messenger RNA is either translated into an amino acid or serves as a translational start/stop signal Second mRNA base U C A G UUU UCU UAU UGU U Phe Tyr Cys UUC UCC UAC UGC C U Ser UUA UCA UAA Stop UGA Stop A Leu UUG UCG UAG Stop UGG Trp G end) end) CUU CCU CAU CGU U His CUC CCC CAC CGC C C Pro Arg CUA Leu CCA CAA CGA A Gln CUG CCG CAG CGG G AUU ACU AAU AGU U Asn AUC lle ACC AAC AGC Ser C A Thr AUA ACA AAA AGA A Met or Lys Arg AUG start ACG AAG AGG G First mRNA (5 base mRNA First Third mRNA (3 base mRNA Third GUU GCU GAU GGU U GUC GCC GAC Asp GGC C G Val Ala Gly GUA GCA GAA GGA A Glu GUG GCG GAG GGG G 12/31/2022 by TDLV 24
13. Transfer RNA • 3 dimensional tRNA molecule is roughly “L” shaped 12/31/2022 by TDLV 26
14. Ribosome • The ribosome has three binding sites for tRNA – The P site – The A site – The E site 12/31/2022 by TDLV 28
15. Building a Molecule of tRNA • A specific enzyme called an aminoacyl-tRNA synthetase joins each amino acid to the correct tRNA 12/31/2022 by TDLV 30
16. Initiation of Translation • The initiation stage of translation brings together mRNA, tRNA bearing the first amino acid of the polypeptide, and two subunits of a ribosome 12/31/2022 by TDLV 32
17. Termination of Translation • When the ribosome reaches a STOP codon, there is no corresponding transfer RNA. → a small protein called a “release factor” attaches to the stop codon. → causes the whole complex to fall apart: messenger RNA, the two ribosome subunits, the new polypeptide. • The messenger RNA can be translated many times, to produce many protein copies. 12/31/2022 by TDLV 34
18. Comparing Gene Expression In Prokaryotes And Eukaryotes 12/31/2022 by TDLV 36
19. Targeting Polypeptides to Specific Locations • Completed proteins are targeted to specific sites in the cell • Two populations of ribosomes are evident in cells: free ribsomes (in the cytosol) and bound ribosomes (attached to the ER) (endoplasmic reticulum) – Free ribosomes mostly synthesize proteins that function in the cytosol – Bound ribosomes make proteins of the endomembrane system and proteins that are secreted from the cell • Ribosomes are identical and can switch from free to bound 12/31/2022 by TDLV 38
20. Mutation Causes and Rate • The natural replication of DNA produces occasional errors. DNA polymerase has an editing mechanism that decreases the rate, but it still exists • Typically genes incur base substitutions about once in every 10,000 to 1,000,000 cells • Since we have about 6 billion bases of DNA in each cell, virtually every cell in your body contains several mutations • Mutations can be harmful, lethal, helpful, silent • However, most mutations are neutral: have no effect • Only mutations in cells that become sperm or eggs—are passed on to future generations • Mutations in other body cells only cause trouble when they cause cancer or related diseases 12/31/2022 by TDLV 40