DNA, genes and chromosomes

Prokaryote and eukaryote

Prokaryote → genetic material in cytoplasm, nucleotide, circular shape, no introns, shorter and no associations with proteins

Eukaryote → genetic material in nucleus, chromosomes, linear, has introns, longer, associated with histones

In eukaryote → one long linear DNA molecule in a chromosome - chromosomes only become visible as distinct structures containing two identical chromatids after DNA replication

Chromosomes

Chromatid is one half of a duplicated chromosome - each chromosome contains 2 chromatids and 4 polynucleotide chains

two sister chromatids

→ DNAs double helix is wound around histone proteins to fix it into position, the DNA histone complex is then coiled and looped and further coiled before being packed into chromosome

Homologous chromosomes are pairs that contain one maternal and one paternal set, contain the same genes in the same loci but aren’t genetically identical

egg and sperm are gametes and have 23 chromosomes (haploid) which produces 46 chromosomes and is then a diploid zygote

position of loci on length is at the same position as it determines the same characteristic

Intron - noncoding Exon → codes for proteins

DNA

Properties of genetic code →degenerate - most amino acids have more than 2 triplet

Universal -. the same triplet DNA bases code for the same amino acids in all organisms

Non -overlapping -. each triplet in a base is only read once

genetic code read in triplets

DNA triplets (CAG) are complementary to RNA (GUC) which are complementary to T RNA anti codon (CAG) - still uses U

RNA

Single stranded, messenger RNA is complementary to DNA template that it has been formed from

mRNA diffuses into cytoplasm where its in conjunction with a ribosome and acts as a scaffold to form proteins, is easily broken down so only exists while the manufacture of a particular protein is needed

genome → complete set of genes

Proteome → full range of proteins a cell is able to complete

Comparison

DNA → double polynucleotide chain, largest molecule, double helix, pentose sugar is deoxyribose, TACG, found mostly in nucleus, quantity constant for all cells of a species, chemically very stable

mRNA → single polynucleotide chain, smaller than DNA, single helix, ribose, AUCG, manufactured in nucleus but found throughout cell, quantity varies cell to cell and with lvl of metabolic activity, least stable → individual cells broken down within a few days

tRNA → single polynucleotide chain, smallest molecule, clover shaped, ribose, AUCG, manufactured in nucleus but found throughout cell, quantity varies cell to cell + with metabolic activity, more stable than mRNA but less than DNA

Transcription

Protein synthesis → 1) transcription → production of mRNA via DNA as a template 2) production of polypeptides using sequence of codons carried by mRNA and occurs in ribosomes

DNA helicase unwinds section of DNA to expose gene required for protein synthesis → done by breaking hydrogen bonds between bases in the middle of double helix

→ RNA polymerase attaches to start of newly exposed polynucleotide strands (template)

→ forms pre-RNA that is spliced into mRNA that diffuses out of nucleus through nuclear pore

→ after RNA polymerase has detached DNA rewinds and forms hydrogen bonds between complementary bases

Translation

→ open end of tRNA molecule is the amino acid attachment site and opposite end is anti -codon loop consisting of 3 unpaired bases, each of 64 different tRNAs carry one of 20 amino acids, tRNA can only collect amino acid is activated which requires energy from ATP, many ribosomes work simultaneously along strand of mRNA speeding up rate of protein synthesis and increasing protein yield from a single mRNA molecule

1) mRNA associates with a ribosome in cytoplasm → 2) 1st 2 codons are now on ribosome and collect complementary tRNA → 3) anticodons of tRNA join via complementary base paring forming hydrogen bonds with mRNA codons → 4) peptide bond forms between 2nd amino acids (catalysed by peptidyl transferase) that requires ATP energy → 5) the ribosome moves forward by 1 codon (3 bases) → 6) first tRNA leaves ribosome whilst new tRNA enters it carrying its amino acids towards 3rd codon in sequence → 7) another peptide bond formed between 2nd amino acid and new amino acid on tRNA forming polypeptide → 8) ribosome continues moving along mRNA until reaches stop-codon and will detach due to presence of release factor →9) polypeptide chain is released folding into correct shape