Topic 8 - DNA, genes and protein synthesis

• A gene is a section of DNA that contains the coded information for making polypeptides and functional RNA. This coded information is in the form of a specific sequence of bases along the DNA molecule.
• Polypeptides make up proteins (e.g. enzymes), so genes determine the proteins of an organism, which affect the organism's development and activities.
• A gene is a section of DNA located at a particular position, called a locus, on a DNA molecule.
• The gene is a base sequence of DNA that codes for the amino acid sequence of a polypeptide, or a functional RNA (e.g. ribosomal RNA or tRNA).
• Only 20 different amino acids regularly occur in proteins, and each amino acid has it's own code of bases on the DNA. Each amino acid is coded for with a minimum of three bases to allow for many combinations.
• Only four different bases exist in DNA : adenine (a), cytosine (c), guanine (g), and thymine (t). If each base coded for an amino acid, there would only be 4 coded amino acids. If two bases coded for an amino acid, there would only be 16 coded amino acids. However, three base combinations can code for 64 different amino acids, which is suitable because it exceeds 20.
• As the genetic code has 3 bases for each amino acid, the 3 bases are referred to as a triplet. Since there are 64 possible triplets, but only 20 amino acids, many amino acids can be coded for using more than one triplet.
• The DNA code is degenerate, since many amino acids can be coded for using more than one triplets.
• The start of a DNA sequence is always coded for using the same start triplet (which codes for methionine). There are three triplets which don't code for any amino acid, which are known as stop triplets. These mark the end of the polypeptide chain.
• The DNA code is non-overlapping, because each base in the sequence is only read once (i.e. only as part of one triplet).
• The DNA code is universal, because every triplet codes for the same amino acid in all organisms. This provides evidence for evolution.
• Lots of the DNA in eukaryotes is 'junk DNA', and doesn't code for polypeptides. Coding sequences for polypeptides are known as exons, which are separated by non-coding sequences known as introns.
• Prokaryotic DNA and eukaryotic DNA is very different. Prokaryotic DNA is shorter and spherical, forming a DNA loop, and isn't associated with histones. Therefore, prokaryotic DNA doesn't have chromosomes.
• Eukaryotic DNA has longer, linear DNA and is associated with proteins called histones. These allow the eukaryotic DNA to form chromosomes. Eukaryotic DNA is contained within the nucleus, as well as the mitochondria and chloroplasts. However, the DNA is these structures is more like prokaryotic DNA.
• Chromosomes are only visible when they are dividing. They initially appear as two threads, which are referred to as sister chromatids, since the DNA has already replicated to form two genetically identical DNA molecules.
• DNA is a highly coiled double helix shape. To achieve this, the helix winds around histone proteins to produce a DNA-histone complex which is coiled. This coil is further condensed into a chromosome.
• Humans have 46 chromosomes (23 pairs of chromosomes) in every cell.
• In sexually produced organisms (e.g. humans), one chromosome in each pair comes from the mother (maternal) and one comes from the father (paternal). These are known as homologous pairs, and the total number is known as the diploid number.
• A homologous pair is always two chromosomes that carry the same gene, but not always the same alleles of this gene.
• During meiosis, the number of chromosomes is halved so that each daughter cell receives one chromosome from each homologous pair (each cell gets one gene for each characteristic).
• An allele is an alternative form of a gene. Each allele has a unique base sequence, hence a unique amino acid sequence, which produces a unique polypeptide. Sometimes, an allele may code for a different polypeptide, which may not function properly.
• An enzyme has a very specific tertiary shape. This allows it to have a unique active site shape. If a different polypeptide is produced from a different allele, the active site shape might change. This means that the substrate may not longer fit the active site, reducing the ability of the enzyme to function.
• Messenger RNA (mRNA) is a type of RNA which transfers the DNA code from the nucleus to the cytoplasm. It is small enough to leave the nucleus through the nuclear pores, and is able to transfer coded information used to determine the sequence of amino acids when producing a polypeptide.
• A codon is a sequence of three bases on mRNA that code for a single amino acid.
• The genome is the complete set of genes in a cell.
• The proteome is the full range of proteins that are coded for by the genome.
• Ribonucleic acid (RNA) is a polymer made up of repeating mononucleotide sub-units. RNA is a single strand where each nucleotide is made up of a pentose sugar ribose, one of the organic bases, and a phosphate group.
• The organic bases in RNA include adenine (a), cytosine (c), guanine (g) and uracil (u).
• Two types of RNA are used in protein synthesis : messenger RNA (mRNA) and transfer RNA (tRNA).
• mRNA is a long strand arranged in a single helix. The base sequence of mRNA is determined by the sequence of bases on a length of DNA in a process called transcription.
• When mRNA reaches the ribosomes, it acts as a template for protein synthesis. mRNA possesses information in the form of codons. The sequence of codons on the mRNA determines the amino acid sequence of a specific polypeptide that will be made.
• tRNA is a fairly short molecule (made up of around 80 nucleotides). It is a single-stranded chain that folds into a clover-leaf shape, with one end extending beyond the other.
• The end of a tRNA chain allows it to bind to amino acids. At the opposite end of the tRNA molecule is a sequence of three other organic bases (an anticodon), making each tRNA specific to one amino acid.
• In RNA, adenine binds to uracil, and guanine binds to cytosine. During protein synthesis, an anticodon (on tRNA) pairs with the three complementary organic bases in the codon of the mRNA.
• Producing a protein is outlined by base sequence of DNA. Transcription is the process by which a complementary section of DNA is converted into pre-mRNA. pre-mRNA is spliced to form mRNA and then used in translation as a template for complementary tRNA molecules to attach to (with the corresponding amino acids), producing a polypeptide.
• The first stage of transcription involves DNA helicase acting on a specific region of DNA to cause the strands to separate. This exposes the nucleotide bases. One strand is known as the template strand.
• The second stage of transcription involves the pairing of complementary nucleotide bases on the template strand. RNA polymerase then moves along the strand and joins the nucleotides together to form the pre-mRNA.
• The third stage of transcription involves the rejoining of the DNA strands. As RNA polymerase adds the complementary nucleotides to the template strand, the DNA strands rejoin behind this section. This way, only about 12 base pairs are exposed at any time.
• In prokaryotic cells, transcription results in direct production of mRNA from DNA. In eukaryotes, this produces pre-mRNA, which must be spliced to form mRNA.
• Splicing involves removing introns in the pre-mRNA sequence, since these can prevent the synthesis of a polypeptide. Then, the functional exons are joined together as mRNA.