genetic information

• describe a DNA molecule

  wound around proteins called histones to form a DNA-histone complex.
• state the differences in DNA between eukaryotic and prokaryotic cells.

  1. DNA is contained in nucleus in eukaryotic cells, and in prokaryotic cells it is contained in the cytoplasm.
  2. In eukaryotic, DNA is long and linear as opposed to prokaryotic, it is shorter and circular.
  3. DNA is associated with histones in eukaryotic cells whereas it isn't in prokaryotic.
  4. DNA in eukaryotic cells contains introns but prokaryotic doesn't contain introns.
• define 'genes'

  a section of DNA that codes for a protein.
• what are exons?

  sections of DNA that code for amino acids
• what are introns?

  sections of DNA that dont code for amino acids.
• each gene is located at a specific position along a chromosome known as a locus.
• a genome is a complete set of genes within a cell.
• define 'triplet code'


  each amino acid is coded for by a sequence of 3 DNA bases.
• what are the features of the genetic code?


  1. universal - each DNA triplet codes for the same amino acid in all organisms.
  2. non-overlapping - each base in a DNA sequence is only read once.
  3. degenerate - most amino acids are coded for by more than one triplet.
• messenger RNA (mRNA) is a type of RNA synthesised during the process of transcription.
• what is the role of mRNA?

  to carry genetic information from the DNA to the ribosomes.
• features of mRNA:

  • single-stranded, linear molecule
  • contains a base sequence complementary to a DNA sequence
  • contains codons, which are sets of three bases that code for an amino acid.
  • small enough to leave the nucleus
• transfer RNA (tRNA) is a type of RNA used in the process of translation.
• what is the role of tRNA?

  to transport amino acids to ribosomes to build up a polypeptide chain.
• features of tRNA:

  • single-stranded molecule folded into a clover-leaf shape.
  • uses hydrogen bonds between complementary base pairs to hold it in shape.
  • contains a specific sequence of three bases at one end, known as the anticodon.
  • contains an amino acid binding site at the opposite end.
• compare the functions of mRNA and tRNA.

  mRNA carries genetic information from the DNA to the ribosomes whereas tRNA transports amino acids to the ribosomes to build proteins.
• compare the shapes of mRNA and tRNA.

  mRNA is linear whereas tRNA is clover-leaf shaped.
• this diagram represents mRNA

  
• this diagram represents tRNA

  
• transcription is the initial step in the synthesis of proteins. It involves creating an mRNA copy of a gene from the DNA template.
• in eukaryotic cells, transcription takes place within the nucleus. this mRNA then exits the nucleus and enters the cytoplasm, where translation occurs.
• describe the stages of transcription:

  1. RNA polymerase binds to DNA, hydrogen bonds between the DNA bases break and the double helix unwinds.
  2. antisense strand acts as the template for mRNA synthesis and free RNA nucleotides align with the template through complementary base pairing.
  3. a complementary mRNA strand is formed, carrying the same base sequence as the DNA sense strand.
  4. RNA polymerase reaches a stop codon, detaching from DNA, terminating transcription.
  5. mRNA is released, detaches from DNA, and DNA rewinds into its double helix structure.
• what is the difference between transcription in prokaryotes and eukaryotes?

  in prokaryotes, mRNA is directly produced from transcription without any splicing, whereas in eukaryotes, pre-mRNA is spliced after transcription to remove introns before it is transported to the cytoplasm.
• splicing in eukaryotes:

  1. in eukaryotic cells, both introns and exons of DNA are transcribed into the initial mRNA transcript (pre-mRNA).
  2. the pre-mRNA undergoes RNA splicing, where introns are removed and exons are joined together.
  3. mature mRNA is produced, only consisting of exon sequences.
  4. the mRNA then travels out of the nucleus for translation.
• translation occurs in the cytoplasm, specifically on the ribosome, in both prokaryotic and eukaryotic cells.
• translation is the process of decoding the information in mRNA to synthesise a polypeptide chain, with the help of tRNA. this chain then folds into a functional protein.
• Stages of translation


  1. Ribosome attaches to mRNA strand at start codon

  2. tRNA molecule, carrying specific amino acid and with complementary anticodon, binds to mRNA

  3. Second tRNA molecule attaches to mRNA

  4. Amino acids carried by first two tRNA molecules linked via peptide bond using ATP

  5. First tRNA molecule detaches from mRNA and is free to collect another amino acid

  6. Ribosome moves along mRNA, allowing another tRNA molecule to bind to next codon

  7. Two tRNA molecules can be attached to mRNA at any point

  8. Sequence continues until stop codon on mRNA is reached

  9. Completed polypeptide chain detaches from ribosome
• a gene mutation is a change in the DNA base sequence.
• mutations occur spontaneously if DNA is misread during DNA replication. ultraviolet or ionising radiation, chemicals and viruses can increase the rate of mutations - these are known as mutagens.
• what are the three types of gene mutations?

  1. substitution - one base is replaced by another
  2. deletion - one base is removed from the sequence
  3. insertion - one base is added into the sequence.
• mutations may change the amino acid sequence coded for by the gene, which can lead to a different polypeptide and potentially a different tertiary structure.
• what happens if the mutation occurs on an enzyme?

  the active site of the enzyme could change, which would stop it from forming an enzyme-substrate complex.
• substitutions - not all substitution mutations will result in a change to the amino acid sequence of a proteins. this is due to the degenerate nature of the genetic code. many amino acids are coded for by more than one triplet.
• deletions are much more likely to change the amino acid sequence because they cause a frameshift - this is where the entire sequence after the deletion shifts to the left by one base. this changes all the triplets after the deletion and so also changes the amino acid sequence.
• insertions are likely to change the amino acid sequence because they cause a frameshift - the entire sequence after the insertion shifts to the right by one base. this changes all the triplets after the insertion and so also changes the amino acid sequence.
• chromosome mutations involve changes to the structure or number of whole chromosomes. mutations to chromosome number can arise spontaneously through errors in cell division.
• what are the types of mutations to the number of chromosomes?

  1. polyploidy - when organisms have more than two sets of chromosomes; mostly seen in plants.
  2. non-disjunction - occurs when homologous chromosomes fail to separate in meiosis, resulting in gametes with extra or missing chromosomes.
• meiosis is a type of cell division in which a parent cell divides to form four haploid cells, each genetically distinct from one another.
• before meiosis starts, DNA is replicated during interphase, so that each chromosome contains two chromatids.
• meiosis I - homologous chromosomes are separated.