DNA, GENES AND CHROMOSOMES

• EUKARYOTIC DNA

  • in nucleus
  • linear
  • long molecule
  • associated with histone proteins which help support the DNA
  • DNA is coiled around the histone proteins into a chromosome
  • coiled up tightly to fit in the nucleus

  
• DNA in chloroplasts and mitochondria is like prokaryotic DNA as its:

  • circular
  • not membrane bound in a nucleus
  • not associated with histone proteins
• PROKARYOTIC DNA

  • free in cytoplasm (with region called the nucleoid)
  • single, circular chromosomal DNA molecule - also called nucleoid
  • also have one or more plasmids. Plasmids are very small circular DNA molecules. They usually only contain a few genes. Plasmids are more accessible for proteins required for gene expression + therefore contain genes that are required often, quickly +/or in emergencies. The genes for antibiotic resistance are found in plasmids
  • not associated with histone proteins
  • coiled Up tightly

  
• GENE: a section of DNA that contains sequence of DNA bases that codes for either a polypeptide or functional RNA

  
• LOCUS: a specific position a gene occupies on a particular DNA molecule
• GENES

  • the code is in the form of a specific sequence of bases along the DNA molecule. Different polypeptides have different number and order of amino acids. It’s the order of bases in a gene that determines the order of amino acids in a particular polypeptide. Each specific amino acid is coded for by a sequence of 3 DNA bases in a gene called a triplet or codon.
  • genes that dont code for a polypeptide code for functional RNA instead. Functional RNA is RNA molecules other than mRNA, which perform special tasks during protein synthesis eg. tRNA and rRNA which forms part of ribosomes.

  
• WHAT DETERMINES THE ORDER OF AMINO ACIDS IN A POLYPEPTIDE?

  the order of bases in a gene
• WHAT ARE AMINO ACIDS CODED FOR BY?

  By a sequence of 3 DNA bases in a gene called a triplet or codon
• ALLELES: alternative form of a gene
• ALLELES: 1

  • a gene can’t exist in more than one form - alleles. One of 2 (usually one dominant and one recessive) or more DNA sequences occurring at a particular gene locus
  • each individual inherits one allele from each of its parents, they can be the same allele or different + when different, they code for a different polypeptide. The order of bases in each allele is slightly different, so they code for different amino acids + therefore a different polypeptide + different protein
• ALLELES: 2

  • any changes in the base sequence of a gene produces a new allele of that gene (mutation) and results in a different sequence of amino acids being coded for. this changes the polypeptide coded for.
  • The different alleles are on the same point on a chromosome as they are just a different form of the same gene

  
• MUTATION: a change in the base sequence of a gene - producing a new allele of that gene and results in a different sequence of amino acids being coded for
• GENOME: the complete set of DNA (genetic material) in an organism. The full genome is present within every cell of an organism, but not every gene is expressed in every cell. Which genes are expressed depends on the cell type.
• PROTEOME: the full range of proteins that a cell is able to produce / DNA is able to code for / made by an organism. The proteome is usually larger than the genome of an organism. This due to the large amount of post-translational modification of proteins (often in the Golgi apparatus) and each gene is also capable of producing multiple different proteins via alternative splicing
• FEATURES OF THE GENETIC CODE

  GENETIC CODE: the sequence of base triplets (codons) in mRNA which code for specific amino acids

  1. Non-overlapping
  2. degenerate
  3. universal
• FEATURES OF GENETIC CODE

  1, NON-OVERLAPPING: base triplets don’t share their bases. Each triplet is read in sequence and is separate from the triplet before and after.
• FEATURES OF GENETIC CODE: 1

  2. DEGENERATE: there are more possible combinations of triplets than there are amino acids (20 amino acids but 64 possible triplets 4$^3$is 64 and there are 4 bases each read as a triplet)

  • this means that some amino acids are coded for by more than one base triplet, eg. Tyrosine can be coded for by UAU or UAC.
• FEATURES OF GENETIC CODE: 2

  2. DEGENERATE

  • Not all triplets code for amino acids though:

  -some triplets are used to tell the cell to stop production of a protein - stop signals. They’re found at end of mRNA eg. UAG is a stop signal (There are also start signals at the start of the mRNA which tell the cell when to start protein production, but these code for a specific amino acid called methionine)
• FEATURES OF GENETIC CODE

  3. UNIVERSAL: the same specific base triplets code for the same amino acids in all living things (eg. UAU codes for tyrosine in all organisms)
• NON-CODING DNA: 2

  • prokaryotic DNA doesn’t have introns
  • eukaryotic DNA also contains regions of multiple repeats outside of genes. These are DNA sequences that repeat over and over.

  
• NON-CODING DNA: 1

  • between the genes in eukaryotes, there are non-coding sequences made up of multiple repeats of base sequences. A lot of the nuclear DNA doesn’t code for polypeptides. Some genes code for functional RNA.
  • even genes that code for polypeptides contain sections that don’t code for amino acids (introns). There can be several introns within a gene + their purpose isn’t known for sure. Introns in eukaryotes are removed during splicing in transcription in protein synthesis so they don’t affect the amino acid order. Introns interrupt exons.

  
• INTRONS: section of genes in DNA that don’t code for amino acids
• EXONS: triplet / base / nucleotide sequence of a gene that do code for amino acids
• CHROMOSOMES STRUCTURE

  • chromosomes are only visible when a cell is dividing. For the rest of the time they're widely dispersed throughout the cell
  • when visible they are appear as 2 threads joined at a point - a chromatid. DNA has already replicated to give two identical DNA molecules
  • the DNA is held in position by histone proteins
  • DNA is around 2m in every human cell and is tightly coiled and folded
• DNA STRUCTURE ORDER

  1. DNA double helix molecule
  2. combined with histone proteins, DNA wound around it to fix it in position
  3. DNA- protein complex is coiled to form loops
  4. loops coil and pack together, condensing into a single chromosome

  
• HOMOLOGOUS CHROMOSOMES: a pair of matching chromosomes, one maternal and one paternal
• HOMOLOGOUS CHROMOSOMES: 1

  • humans have 23 pairs of chromosomes, 46 in total - number 1s, two number 2s, two number 3s etc
  • 46 is the diploid number
  • in a homologous pair both chromosomes are the same size and have the same genes, determining the same genetic characteristic but they could have different alleles
  • determining the same genetic characteristics isn’t the same as being identical
  • one chromosomes is derived from the chromosomes provided from the mother in the egg and one from the father in the sperm
  • human sex cells are haploid cells, they have 23 chromosomes

  
• HOMOLOGOUS CHROMOSOMES: 2

  • alleles coding for the same characteristic will be found at the same fixed position (locus) on each chromosomes in a homologous pair
  • homologous chromosomes can be double or single armed
  • they aren’t joined, just the same as each other with different alleles