1.4 Dna and protein synethesis

• The structure of a nucleotide consists of a pentose sugar, a phosphate group, and a nitrogenous base.
• The pentose sugars in DNA & RNA are deoxyribose and ribose respectively.
• Polynucleotide strands form through condensation reactions between nucleotides, which form strong phosphodiester bonds (sugar-phosphate backbone).
• The structure of DNA consists of a double helix of 2 deoxyribose polynucleotide strands, with H-bonds between complementary base pairs on opposite strands (AT & GC).
• The purine bases in DNA are adenine and guanine, which are two-ring molecules.
• The pyrimidine bases in DNA are thymine and cytosine, which are one-ring molecules.
• The complementary base pairs in DNA are adenine ( A ) + thymine ( T ), guanine ( G ) + cytosine ( C ), with 2 H-bonds between adenine ( A ) + thymine ( T ) and 3 H-bonds between guanine ( G ) + cytosine ( C ).
• The complementary base pairs in RNA are adenine ( A ) + uracil ( U ), guanine ( G ) + cytosine ( C ), with 2 H-bonds between adenine ( A ) + uracil ( U ) and 3 H-bonds between guanine ( G ) + cytosine ( C ).
• DNA replication is described as semiconservative because the strands from the original DNA molecule act as templates and the new DNA molecule contains 1 old strand & 1 new strand.
• DNA helicase plays a role in semiconservative replication by unwinding DNA during replication.
• Sickle cell anaemia is a genetic condition that results in abnormal haemoglobin, impaired ability to transport oxygen, rapid heart rate, fatigue, dizziness, and sickle shaped red blood cells that 'stick' in vessels.
• A mutation is any change in the base sequence of DNA, often arise spontaneously during DNA replication.
• AUG on mRNA codes for the amino acid Met and initiates translation of a polypeptide.
• Exons are regions of DNA that code for amino acid sequences, separated by one or more introns.
• A deletion mutation is when a nucleotide in the DNA sequence is lost, leading to a frame shift, significant since entire amino acid sequence downstream of mutation will be different.
• An insertion mutation is when addition of one or more base pairs to DNA sequence, often in microsatellite regions, causes frameshift, significant since entire amino acid sequence downstream of mutation will be different.
• A substitution mutation is when one nucleotide in the DNA sequence is replaced by another, often a silent mutation which does not change amino acid sequence.
• Sickle cell anaemia in humans is caused by a missense point mutation in the gene that codes for 𝛽 strand in haemoglobin, changing DNA from CTC (Glut) to CAC (Val), resulting in a change in primary structure and different tertiary structure, leading to the formation of abnormal haemoglobin molecules that form strands that make red blood cells sickle shaped.
• Introns are the majority of DNA consists of non-coding regions within and between genes.
• Nucleotide triplets UAA, UAG, UGA on mRNA do not code for an amino acid and terminate translation.
• The sense strand of DNA is the strand with the same base sequence as mRNA, but with thymine instead of uracil.
• ATP hydrolysis provides energy to form peptide bonds during translation.
• The function of mRNA is to transfer the genetic code from DNA in the nucleus to ribosomes for translation into a specific polypeptide.
• The genetic code is universal, using the same bases and sequences used by all species.
• mRNA moves out of the nucleus via nuclear pore and attaches to ribosome.
• The genetic code is degenerate, meaning more than one triplet codes for the same amino acid, with 64 possible triplets for 20 amino acids.
• The genetic code is non-overlapping, meaning each triplet is only read once.
• The antisense strand of DNA is the template strand which is transcribed.
• Translation produces proteins and occurs in the cytoplasm on ribosomes.
• DNA triplets are sequences of 3 bases that code for a particular amino acid.
• After a strand of mRNA is transcribed, RNA polymerase detaches at the terminator region, allowing H-bonds to reform and DNA to rewind.
• Splicing removes introns from pre-mRNA in eukaryotic cells.
• The process of translation involves the mRNA moving along ribosome until the ‘start’ codon, tRNA anticodon attaching to complementary bases on mRNA, condensation reactions between amino acids on tRNA forming peptide bonds, and the process continuing to form a polypeptide chain until the ‘stop’ codon is reached.
• DNA ligase joins adjacent nucleotides to form phosphodiester bonds.
• Transcription produces mRNA and occurs in the nucleus.
• H-bonds reform after DNA replication.
• The process of transcription involves RNA polymerase binding to a promoter region on a gene, causing a section of DNA to uncoil into 2 strands with exposed bases, the antisense strand acting as a template, free nucleotides being attracted to their complementary bases, and DNA polymerase joining adjacent nucleotides on the new strand in a 5’ → 3’ direction via condensation reactions to form phosphodiester bonds.
• DNA polymerase joins adjacent nucleotides on new strand in a 5’ → 3’ direction via condensation reactions to form phosphodiester bonds.
• During semi-conservative replication, a new strand is formed by attaching free nucleotides to exposed bases by complementary base pairing.
• A gene is a sequence of bases on a DNA molecule that codes for a specific sequence of amino acids to make a polypeptide.