DNA, genes and protein synthesis

• What is the definition of a chromosome?

  A DNA molecule and its associated proteins(histones) combined together.
• What is the definition of a gene?

  A section of DNA that codes for a single protein.
• What is the definition of a histone?

  Proteins which DNA is wrapped around, package it into structures called chromosomes
• What is the definition of an intron?

  Sequences of DNA that do not code for amino acid sequences, these are located within a gene.
• What is the definition of an exon?

  Sequences of DNA that code for polypeptides/ amino acid sequences.
• What is the definition of a genome?

  The complete set of genetic information, specifically the complete set of genes, within a cell of an organism.
• What is the definition of a proteome?

  The complete set of proteins that a cell, tissue or organism can produce.
• What is the definition of an allele?

  Alternative forms of a particular gene with different base sequences.
• What can genes code for?

  An amino acid sequence of a polypeptide or a functional RNA.
• What is the definition of a locus?

  The fixed position that a specific gene occupies on a particular strand of DNA.
• What is the structure of a chromosome?

  DNA forms a double helix structure, that is linear and combines with histones. A DNA-histone complex is formed which coils, the coils then fold to form loops. The loops coil and pack together to form a chromosome.
• How is prokaryotic DNA different to Eukaryotic?

  The DNA strand is shorter and circular, there is usually just one circular strand and it is not associated with proteins. There are also no introns present. This type of DNA is also found in mitochondria and chloroplasts.
• What are 4 features of the genetic code?

  It is a triplet code, it is degenerate, it is non-overlapping and it is universal.
• Describe how the genetic code is a triplet code.

  There are 20 naturally occurring amino acids, each amino acids has its own code of bases. There are 4 bases present in DNA, 3 bases produce 64 different codes this is more than enough to satisfy 20 amino acids. Therefore 3 bases code for one amino acid.
• How is the genetic code degenerate?

  Most amino acids are coded for by more than one triplet, as 64 codes but 20 amino acids.
• Why is it important that the genetic code is degenerate?

  It means the amino acid sequence is protected against mutations
• How is the genetic code non-overlapping?

  Each base in the sequence is only read once, as the DNA sequence is read in one direction. The DNS sequence always starts with the same triplet(methionine). This protects against mutation that only affects 3 codes.
• How is the genetic code universal?

  Each triplet codes for the same amino acids in all organisms, this is indirect evidence of evolution.
• How is eukaryotic DNA and prokaryotic DNA similar?

  DNA in mitochondria and chloroplasts is similar to prokaryotes.

  They are both double stranded and both contain same 4 bases.

  They both code for proteins and nucleotides are joined by phosphodiester bonds.
• Why doesn’t DNA directly encode for a sequence of amino acids?

  DNA molecule is too large to leave the nucleus so a gene is transferred onto mRNA, that can leave the nucleus and encode for a protein. DNA determine sequence but doesn’t encode it.
• What are the 3 steps of protein synthesis?

  Transcription, pre-mRNA splicing(in eukaryotes) and translation.
• Describe the structure of mRNA?

  Single stranded, shorter then DNA, no hydrogen bonds, uracil instead of thymine, possesses codons which are 3 bases that are complementary to a triplet in DNA. The sequence of codons determines amino acid sequence.
• Describe the process of transcription?

  1.Takes place in the nucleus, DNA double helix unwinds by DNA helix add and hydrogen bonds break.

  2. Strands are separated and gene on template strand used to produce mRNA.

  3. free RNA nucleotides in nucleoplasm line up in correct order by complementary base pairing.

  4. RNA polymerase aids in the formation of phosphodiester bonds between free nucleotides.

  5. mRNA exits nucleus are splicing by pores.
• What is splicing?

  Pre-mRNA is produced by transcription in eukaryotes, therefore introns are removed by enzymes producing mRNA before it leaves the nucleus.
• Why is mRNA suited to its function?

  Its function is to act as a template for protein synthesis in the cytoplasm. It possesses genetic information in the form of codons, it is chemically unstable so polypeptide synthesis will not continue indefinitely. It is single stranded can leave the nucleus.
• Describe the structure of tRNA?

  Cloverleaf-shaped molecule with anticodon and amino acid binding sites, held together by hydrogen bonds in complementary base pairing. Each tRNA molecule is specific to one amino acid as its anticodon is specific.
• How many different tRNA molecules are there?

  There is a tRNA molecule for each triplet code.
• What are the similarities between mRNA and tRNA?

  Both single stranded, both contain the pentose sugar ribose, both have the same organic bases, both manufactured in the nucleus but found throughout the cell.
• What are the differences between mRNA and tRNA?

  tRNA is smaller and contains hydrogen bonds. mRNS is more chemically unstable. tRNA is clover shaped whereas mRNA is single helix molecule.
• What is a ribosome?

  A ribosome consists of 2 subunits with a groove in between where mRNA can fit, the ribosome moves along mRNA reading the code and assembling amino acids in correct order. Ribosomes can be free or attached to the rough endoplasmic reticulum.
• Describe the process of translation.

  1.mRNA leaves the nucleus via pores, ribosome attaches to the start codon.

  2. tRNA molecules attach to mRNA by anticodons by complementary codons.

  3. distance between anticodon and codon is always the same

  4. tRNA brings a specific amino acid in a specific order

  5. peptide bond forms between amino acids

  6. ribosome moves along mRNA process is repeated

  7. forming a polypeptide chain, the protein goes on to be folded forming more bonds and 2nd and 3rd structure until ribosome reaches stop codon.
• What requires energy (ATP) in translation?

  To join an amino acid to the binding sit on tRNA

  Form the peptide bond between amino acids.