HL 1 Biology

3.5 & 7.3 Transcription

<<Central Dogma of Genetics>>

Heredity: transmission of information from parent to offspring;  to undergo reproduction

Gene: unit of heredity that consists of a sequence of DNA bases. Coded to specify the sequence of amino acids (polypetides)

Protein: composed of one/more polypeptides. Often directly/indirectly determine observable characteristics of individual

<<2 Processes to produce specific polypeptide, using the base sequence of a gene: >>

  • Transcription:synthesis of RNA, using DNA as a template
      • RNA’s base sequence = DNA strands, with uracil instead of thymine
  • Translation: synthesis of polypeptide, with amino acid sequence that’s determined by a RNA molecule
      • Messenger RNA (mRNA) = type of RNA that carries the info needed to synthesize a polypeptide
      • Transfer RNA (tRNA) = decodes the base sequence into an amino acid sequence
      • Ribosomal RNA (rRNA) = prat of ribosome structure; synthesis polypeptide by translation of mRNA
  • In most cases, one gene-one polypeptide concept: one polypeptide synthesized using one type of mRNA, obtained by transcribing one gene. Helps understand flow of information in cells. 


  • Process in which mRNA is made
  • occurs along only one strand (because RNA is single-stranded)
  • Produced RNA has complementary sequence to DNA (which was used as template)
  • Transfer RNA (tRNA): role in translation
  • Ribosomal RNA (rRNA): structural and functional component of ribosomes
  • Outline of Transcription:
      1. Enzyme RNA polymerase binds to site, promoter on the DNA. Determine where transcription stars, promoter determines which 2 DNA strands will be transcribed.
      2. DNA separated by RNA polymerase
      3. RNA nucleotides only pair with complementary bases on one strand
      4. Covalent bonds formed by RNA polymerase between nucleotides
      5. RNA and DNA separate = double helix reforms
      6. RNA polymerase reaches termination sequence, detaches new RNA released
  • Additional Details
      • RNA polymerase splits DNA into 2 single strands during transcription
      • 2 strands have complementary base sequences
      • ones referred to: sense strand and other: antisense strand
      • mRNA (created by transcription) has same base sequence as sense strand, with uracil instead of thymine

<<Introns and Exons>>

pre-mRNA: immediate product of mRNA transcription in eukaryotes.

  • must go through stages of post-transcriptional modifications to become mature mRNA

RNA Splicing: form of post-transcriptional modification

  • Introns:  sequences that will not contribute to the formation of the polypeptide throughout the mRNA; must be removed
  • Exons: coding portions of the mRNA; spliced together to form the mature mRNA


  • their genes usually do not contain introns
  • lack molecular mechanisms needed to proceed RNA splicing
  • problem if eukaryotic gens transferred to prokaryotes by genetic engineering; introns not taken away + polypeptides synthesized with extra amino acid sequences that interrupt polypeptides’ functioning
  • Thus, in order to transfer to prokaryotes, special copies of eukaryotic genes (WITHOUT introns) needed


DNA and RNA Structure (p. 67)

1) DNA consists of a deoxyribose sugar molecule, whereas RNA has ribose. DNA has a nitrogenous base of thymine, whereas in RNA the uracil takes its spot. 

2) DNA is usually double stranded whereas RNA is single stranded.


Sugar Deoxyribose Ribose
Base Thymine Uracil
Structure Double stranded Single stranded

4) Similarities between DNA and RNA:

  • both consists bases of adenine, cytosine, guanine
  • goes back and forth with phosphate sugar phosphate, creating a backbone
  • sugar molecule structure is a pentose
  • polymers of nucleotides

Exons and Species Complexity (p.69)

1) 95%

2) 5 exons

3) “The amount of noncoding DNA does seem to scale with complexity”  refers to the simplicity of the graphs only measuring the exons and leaving out the data for the other noncoding sequences.  Also, complexity can be described in a way that the number of exons would be measured with size or amount of cells in the organism.

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