🧬DNA, GENES AND PROTEIN SYNTHESIS🧬
Genes and Genetic Code
A gene is a section of DNA on a chromosome coding for one or more polypeptides
The locus of a gene is its position on a chromosome
The gene is a base sequence that codes for the amino acid sequence of a polypeptide or a functional RNA (eg. mRNA or tRNA)
Features of Genetic Code
The code is non-overlapping; each base in the sequence is only read once
The code is universal; all living organisms share the same triplet code system, which is indirect evidence for evolution.
The code is degenerate; an amino acid may have more than one triplet code
DNA in prokaryotic cells is short, circular and not associated with any proteins
DNA in eukaryotic cells is much longer than in prokaryotic cells, linear, and associated with proteins called histones
mtDNA and cpDNA is similar to that of prokaryotic DNA, which is indirect evidence of evolution
The function of histones is to package the DNA into nucleosomes, by acting as spools around which DNA winds
The start of a DNA sequence is always the same triplet, methonine, although this triplet is an intron (removed before the final polypeptide is formed)
The triplets are stop codes, which mark the end of a polypeptide chain
Some sections are non-coding, and are made up of repeats of the base sequences.
DNA and Chromosomes
A chromosome is a thread-like structure made of DNA and histones by which hereditary information is passed from one generation to the next
Chromosomes are only visible during cell division
Each thread of a chromosome is called a chromatid, the chromatids are held together by a centromere
Homologous chromosomes are a pair of chromosomes, one maternal and one paternal, that have the same gene loci and therefore determine the same features
An allele is an alternative form of a gene (eg. for eye colour)
Homologous chromosomes are capable of pairing during mitosis
Structures of RNA
RNA is made up of a ribose pentose sugar, a base (A, G, U, C) and a phosphate group
A codon is a sequence of three adjacent nucleotides in mRNA that codes of one amino acid
The genome is the complete set of genes in a cell or organism, mtDNA and cpDNA
The proteome is the full range of proteins which can be produced my the genome
mRNA is a long strand of RNA which is arranged in a single helix. Once formed, mRNA leaves the nucleus via the nuclear pores and goes to the ribsomes where it acts as a template for protein synthesis.
tRNA is a smaller molecule than mRNA that is made up around 80 nucleotides in a single strand, which is folded into a clover type shape.
tRNA has a has a binding site for an amino acid
The opposite end of the tRNA molecule is a sequence of three organic bases called the anticodon
Each tRNA molecule is specific to one amino acid; the anticodon is complementary to the base sequence of the codon.
Polypeptide Synthesis: Transcription and Splicing
Transcription is the process of making pre-mRNA using part of DNA as a template
The DNA double helix is unwound near the start of the gene and the weak hydrogen binds between the bases are broken
RNA polymerase makes a complementary RNA copy of DNA strand, where thymine is replaced by uracil. This complementary strand is called pre-mRNA
pre-mRNA can then be spliced, removing parts of the code and re-joining them as needed. This process only happens in eukaryotic cells.
The removed parts are called introns, and the remaining parts are called exons.
This gives the ability to make a wider variety of protiens
After splicing, pre-mRNA becomes mRNA
Polypeptide Synthesis: Translation
mRNA leaves the nucleus through nuclear pores, and travels through the cytoplasm to the ribosomes (most of which are on the RER)
The purpose of tRNA is to bring specific amino acids from the cytoplasm to the ribosomes
The anticodon on the tRNA molecule forms complementary base pair with the codon and specifies the amino acids; the specific amino acid is attached to the other end of the tRNA
The ribosomes move along the mRNA, one codon at a time. As it covers two codons, there are spaces for two tRNA molecules to be attatched
The peptide bond form while both tRNA molecule area bound in the ribosome
Once the amino acid has been added to the growing polypeptide chain, it comes off the tRNA, allowing the tRNA to return with another amino acid of the same type.