After a short hiatus, I am back with a post on the nature of DNA, including a summary of what it is, what genes are, and what kinds of mutations occur. Next up is meiosis and I have plans for a post on protein synthesis (transcription, translation). Happy studying!
DNA is a polynucleotide made up of the monomer units of nucleotides. Nucleotides consist of an organic base, a pentose sugar and a phosphate group. In DNA, the bases can either be adenine, cytosine, guanine or thymine, and the pentose sugar is deoxyribose. Nucleotides are held together in complementary base pairing using hydrogen bonds – the bond between the sugar and the phosphate are called phosphodiester bonds. DNA exists in a double helix, containing two strands which run antiparallel to each other. Genetic information is stored in the base sequence of DNA.
Prokaryotes have no membrane-bound nucleus and their DNA is usually free-floating in the cytoplasm. Their DNA is arranged into a singular loop (circular) but some bacterium can also have extra circles of DNA called plasmids. Plasmids usually contain additional genes for survival such as antibiotic resistance. Eukaryotes, on the other hand, have linear chromosomes tightly coiled around proteins called histones in the nucleus. The existence of circular DNA in mitochondria or chloroplasts gives evidence that these organelles were derived from prokaryotic cells that had been engulfed.
Genetic information codes for the functions of cells by instructing what proteins the cells should make. There are 20 amino acids, which are the building blocks of polypeptides and therefore proteins. Because there are 20 amino acids, we know that it must be necessary to have three bases coding for each acid. A triplet codon essentially means the three bases which make up a code for a certain amino acid.
However, having three bases coding for each amino acid offers a total of 64 coding combinations means there are 44 which code for things other than amino acids. Some of these remaining codons code for existing amino acids, e.g. UUU and UUC both code for phenylalanine. Therefore, we can say that the genetic code is degenerate, since some of the codes are redundant. Other sequences may code for the start or stop of a polypeptide production.
The genetic code is said to be universal, since most triplets code for the same amino acids in different organisms. It is also said to be non-overlapping, because each base in the sequence is only read once. This means that the code UCCGAC can only be read as UCC GAC and not UCC CCG CGA GAC.
Genes are sections of DNA which code for polypeptides and functional RNA such as rRNA or tRNA. Polypeptides make up proteins so genes essentially code for proteins in an organisms. Genes are always located on a particular position on a section of DNA called a locus. The same genes are found at the same locus on every chromosome.
Chromosomes become visible at the start of mitosis (cell division). They are shown to be two ‘threads’ joined at the centre. Each thread is called a chromatid and is held together by histones. The centre of a chromosome is a centromere. Eukaryotes have varying numbers of chromosomes – humans have 23 from each parent (haploid) making the diploid number of 46 chromosomes in every cell. Each pair of chromosomes, derived from our mothers and fathers, are called homologous pairs because each one carries the same genes but not the same alleles.
An allele is an alternative form of a gene. For example, everyone has the code for eyes but you may inherit green eyes from your mother and blue eyes from your father. If the alleles are the same, it is likely you will present that characteristic. If they are different, the alleles will have different base sequences so code for a different polypeptide.
Changes in base sequences lead to mutations, meaning a change in the protein’s function. There many kinds of mutation. Imagine we start with the code BIG RED FOX.
Substitution mutations occur when nucleotides are replaced by a different nucleotide. As with any mutation, this could cause no effect (due to the code being degenerate – the replacement may code for the same amino acid as before). However, some substitutions can change a cell entirely. The polypeptide produced will differ by a single amino acid which could be crucial in functions such as forming the tertiary structure of a protein or preventing the contraction of sickle-cell anemia.
Our code may now read correctly but does not mean the same thing:
Another kind of mutation is the deletion of bases, which could cause a frameshift. This occurs when a nucleotide is lost from the normal DNA sequence and usually means the code for the polypeptide completely changes. Since the code is non-overlapping and read in triplets, one deleted nucleotide causes the sequence to be read differently.
BIR EDF OX (and would continue with the rest of the sequence – remember, everything is read in triplets!)
A sub-type of a frameshift mutation would be the insertion of a base. As you can see, it also throws the way the code is read:
Chromosome mutations are not uncommon. These are where whole chromosomes or parts of chromosomes change in structure. They can happen in two ways: the addition of entire chromosomes (e.g. having three or more sets of homologous chromosomes), often called polyploidy, or in a process called non-disjunction.
Non-disjunction is where pairs of chromosomes fail to separate in meiosis. This usually results in gametes with one more or one less chromosome. A typical example of non-disjunction in humans is Down’s syndrome, which is an additional chromosome 21.
DNA is a polynucleotide made up of the monomer units of nucleotides. These consist of an organic base, a deoxyribose sugar and a phosphate group. The bases can either be adenine, cytosine, guanine or thymine and are held together in complementary base pairing using hydrogen bonds – the bond between the sugar and the phosphate are called phosphodiester bonds.
DNA exists in a double helix, containing two strands which run antiparallel to each other. Genetic information is stored in the base sequence of DNA.
Prokaryotes have DNA free-floating in the cytoplasm, arranged into a singular loop but some bacterium can also have extra circles of DNA called plasmids. Plasmids usually contain additional genes for survival.
Eukaryotes have linear chromosomes tightly coiled around proteins called histones in the nucleus. The existence of circular DNA in mitochondria or chloroplasts gives evidence that these organelles were derived from prokaryotic cells that had been engulfed.
There are 20 amino acids so we know that it must be necessary to have three bases coding for each acid. A triplet codon means the three bases which make up a code for a certain amino acid.
Having three bases coding for each amino acid offers a total of 64 coding combinations - therefore are 44 triplets which code for things other than amino acids. Some of these remaining codons code for existing amino acids, therefore the genetic code is degenerate, since some of the codes are redundant. Other sequences may code for the start or stop of a polypeptide production.
The genetic code is said to be universal, since most triplets code for the same amino acids in different organisms. It is also non-overlapping, because each base in the sequence is only read once.
Genes are sections of DNA which code for polypeptides and functional RNA. Genes are always located on a particular position on a section of DNA called a locus.
Chromosomes are made of to be two ‘threads’ joined at the centre. Each thread is called a chromatid, held together by histones. The centre of a chromosome is a centromere. Each pair of chromosomes, derived from our mothers and fathers, are called homologous pairs because each one carries the same genes but not the same alleles.
An allele is an alternative form of a gene.
Changes in base sequences lead to mutations, meaning a change in the protein’s function.
Substitution mutations occur when nucleotides are replaced by a different nucleotide. As with any mutation, this could cause no effect (due to the code being degenerate – the replacement may code for the same amino acid as before). However, some substitutions can change a cell entirely. The polypeptide produced will differ by a single amino acid which could be crucial in functions.
The deletion of bases can cause a frameshift. This occurs when a nucleotide is lost from the normal DNA sequence and usually means the code for the polypeptide completely changes. Since the code is non-overlapping and read in triplets, one deleted nucleotide causes the sequence to be read differently.
A sub-type of a frameshift mutation would be the insertion of a base.
Chromosome mutations are where whole chromosomes or parts of chromosomes change in structure. They can happen in two ways: the addition of entire chromosomes (often called polyploidy) or in a process called non-disjunction.
Non-disjunction is where pairs of chromosomes fail to separate in meiosis. This usually results in gametes with one more or one less chromosome. A typical example of non-disjunction in humans is Down’s syndrome, which is an additional chromosome 21.
