Nucleic acids and protein synthesis Flashcards
What is DNA?
- Deoxyribose nucleic acid
- Present in. the nucleus of all cells in all living organisms
- Controls all chemical changes which take place in cells
What are the functions of DNA?
- Sequence of bases in DNA codes for the sequence of amino acids in a protein, which determines all the characteristics of an organism
- Must be replicated in order to make a new cell
What is DNA made up of?
Very large molecules made up of long sub strands called nucleotides
Each nucleotides is made up of:
- a sugar called deoxyribose
- a phosphate group -PO4-3
- a nitrogenous base
What are the pyrimidine bases?
- Cytosine
- Thymine
- Uracil
What are the purine bases?
- Adenine
- Guanine
What are the bases held together by?
Hydrogen bonds
Thymine + Adenine = 2 H bonds
Cytosine + Guanine = 3 H bonds
What is semi-conservative replication?
- Before a cell divides, the DNA strands unwind and separate
- Each strand makes a new partner by adding the appropriate nucleotides
- The result is that there are now two double-stranded DNA molecules in the nucleus
- So that when the cell divides, each nucleus contains identical DNA
What are the stages of DNA replication?
- DNA helicase unwinds the double strand of DNA and separates them by breaking the hydrogen bonds between the bases - each separated strand is now a template for the new strands
- DNA polymerase causes free nucleotides to bind to the template strands, the free nucleotides form H bonds with their complementary base pairs on the template strand
- DNA strands rewind to produce a new DNA molecule which is identical to the initial one
What is the evidence of semi-conservative replication of DNA?
- Grow bacteria for many generations on medium with 15NH4 -(centrifuge)-> DNA is all heavy
- Return to 14NH4 medium for 20 mins -(centrifuge)–> DNA is exactly half way between heavy and light
- Continue to grow on 14NH4 for 40 mins -(centrifuge)–> DNA is either light or half-and-half
What is RNA made up of?
- Single stranded polymer
- Contains ribose
- Contains organic bases adenine, guanine, cytosine and uracil
What is mRNA?
Messenger RNA
- Manufactured in the nucleus and carries the genetic code from the DNA in the nucleus to the ribosome
What is rRNA?
Ribosomal RNA
- Ribosomes are made up of rRNA and protein
What is tRNA?
Transfer RNA
- Molecules bring amino acids to the ribosome so that proteins can be synthesised
What is the genetic code?
The sequence of bases in DNA forms the genetic code
What is a triplet?
A group of 3 bases, controls the production of a particular amino acid in the cytoplasm of the cell
What is a gene?
A length of DNA that codes for a polypeptide
What is transcription and why?
DNA is copied into a complementary strand of mRNA
WHY?
- DNA cannot leave the nucleus
- Proteins are made in the cytoplasm
What are the steps of transcription?
- A gene unwinds and unzips, hydrogen bonds between complementary bases break
- Activated RNA nucleotides bind with H bonds to their exposed complementary base - catalysed by RNA polymerase
- mRNA produced is complementary to the nucleotide base sequence on the template strand of the DNA
What are the steps of translation?
- mRNA attaches to a ribosome and tRNA collects amino acids from the cytoplasm and carries them to the ribosome, tRNA is a single stranded molecule with one binding site on the end so can only carry one type of amino acid and a triplet base at the other end
- 2 tRNA molecules attaches to mRNA by complementary base pairing
- The amino acids attached to two tRNA molecules joined by a peptide bond and then detach themselves
- Process is repeated thus leading to the formation of a polypeptide chain until a stop codon is reach on mRNA and end the process of protein synthesis
What is an introns and exons?
- Exons are regions of DNA that code for proteins
- Eukaryotes also have regions of non coding DNA between exons that are called introns
- The introns must be cut out of the mRNA before the mRNA leaves the nucleus - called RNA splicing
What is the difference between eukaryotic and prokaryotic DNA?
Prokaryotes (bacteria mostly) do not have introns in their DNA, the entire gene is transcribed and translated without editing or splicing
