F cell II- Gene expression, protein synthesis and antibiotics and fate of newly synthesised proteins Flashcards
- Outline cellular components required for protein synthesis - Describe key structural features of DNA and RNA - Outline the key steps involved in DNA replication - Outline the basic steps involved in transcription and mRNA processing - Describe the main types of RNA found in mammalian cells and their functions - Outline how the genetic code functions to enable protein synthesis, including the concept of reading frames - List the steps in the biosynthesis of proteins - Explain how antibiotics
What are the cellular components required for protein synthesis?
- Ribosomes
- Nucleus
- DNA
- DNA strand
- DNA nucleotides
- RNA nucleotides
- tRNA
- mRNA
- rRNA
- mRNA processing enzymes
Describe the key structural features of DNA
- Double stranded (double helix)
- Polymer comprised of nucleotides.
- Each nucleotide has 3 components:
- Sugar molecule deoxyribose, attached to 1 of the bases
- Phosphate (phosphate forms backbone with deoxyribose due to alternating deoxyribose and phosphate subunits in chain)
- Base (A, G , T ,C). Bases join complimentary bases on opposite strand via formation of hydrogen bonds, holding 2 strands together
Adenine + thymine = 2 H bonds
Guanine + cytosine = 3 H bonds
Describe the key structural features of RNA
- Single stranded
- Ribose sugar (has extra hydroxyl group), instead of deoxyribose
- Uracil (lack of methyl group present in thymine) instead of thymine
Why is DNA replication semiconservative?
- Due to fact that one of strands of newly synthesised DNA is from one of the strands from the parent DNA. Each copy contains one original strand and one newly synthesised strand
Outline the key steps in semi-conservative DNA replication
- 2 strands of DNA ‘melt’ (The 2 strands split). Enzyme DNA helicase breaks down H bonds between DNA strands, double helix unzips
- Nucleotides on daughter strands used as a template to incorporate the complementary bases.
- New strands formed by complementary base pairing. The nucleotides of the new strands are joined by enzyme DNA polymerase, forms the sugar-phosphate backbone. H bonds form between the complementary bases on original and new strand, strands twist to reform double helix.
- This daughter DNA is then distributed into the daughter cells during cell division
Outline the basic steps involved in transcription
- DNA strands melt
- RNA strand form complementary base pairs with DNA antisense template stand
- Once transcription complete, RNA strand dissociates from DNA strand, now a pre-mRNA strand (or primary transcript), ready for processing
What is the antisense strand?
Begins with 3’ ends with 5’
What is the sense strand?
Begins with 5’ and ends with 3’
Outline the basic steps in mRNA processing
- RNA splicing- RNA units called introns are removed
RNA units that remain are exons, but exons are not synonymous with the coding sequence, this depends on where the start and stop codons are. Because of this, some exons contain both coding and non-coding regions - 5’ capping- An extra molecule is added to the 5’ end of transcript, this is 7-methyl-guanosine (derived from GTP) , and once this has been attached, the guanine bases are methylated and often additional methyl groups are attached to the first and second base of the transcript
- 3’ polyadenylation - This is attachment of 50-200 copies of adenine nucleotides to the VERY END of the transcript. This is essential for transcription termination, release of mRNA from the site of transcription, and export to cytoplasm
What are introns?
Segment of a DNA or RNA molecule which does not code for proteins and interrupts the sequence of genes, hence they’re removed
What are exons?
- Region of genome that end up within an mRNA molecule, some exons coding, others are not
What is the part of pre-mRNA before translation begins called?
5’UTR (untranslated region), which are the exons that are not translated. This region is all the exons before the start codon (AUG) initiates translation
What is the part of the mRNA strand that hasn’t been translated after translation has been completed?
- 3’ UTR, this comes after the stop codon has terminated translation
What is the stop codon?
AUG
What additions in mRNA processing are not coded for in the gene?
- 5’ capping
- 3’ polyadenylation
Describe the main types of RNA found in mammalian cells and their functions?
- rRNA- 80-85% abundance, nucleotides 4800, 1900, 160 + 120. This is a structural component of ribosomes or other large enzymes that catalyse translation. They both have protein and RNA subunits the make up the whole enzymes therefore rRNA catalyses protein synthesis
- tRNA- 10-15% cellular abundance, nucleotide 75. Essential in translation, central to protein synthesis as adaptors between mRNA and amino acids
- mRNA- 2-5%, nucleotides highly variable, only mRNA encodes proteins
What is the function of small nuclear RNAs (snRNAs)?
Function in variety of nuclear processes, including splicing of pre-mRNA
What is the function of small nucleolar RNAs (snoRNAs)?
- Used to process and chemically modify rRNAs
What is the function of small canal RNAs (scaRNAs)?
Small canal RNAs, used to modify snoRNAs and snRNAs
What is the function of microRNAs (miRNAs)?
- Regulate gene expression typically by blocking translation of selective mRNAs
What is the function of small interfering RNAs (siRNAs)?
- Turn off gene expression by directing degradation of selective mRNAs and the establishment of compact chromatin structures
What is the leading strand?
The sense strand (5’ -> 3’)
What is the lagging strand?
- The antisense strand (3’-> 5’)
What are genes?
- Stretches of DNA that contain information for making RNA, mostly protein-encoding mRNA
What is the process of gene expression?
The process by which the information encoded in a gene is turned into a function, primarily via transcription of RNA molecules and translation of mRNA into an amino acid chain
Describe the genetic code
- Amino acids of final protein determined by triplet of bases (codons) in nucleotide sequence
- 64 codons that code for amino acids in body