IB DP Biology: SL复习笔记2.5.1 DNA & RNA Structure

Nucleic Acids: Structure

  • The nucleic acids DNA and RNA are polymers of nucleotides
  • Both DNA and RNA are polymers that are made up of many repeating units called nucleotides
  • Each nucleotide is formed from:
    • A pentose sugar (a sugar with 5 carbon atoms)
    • A nitrogen-containing organic base (with either 1 or 2 rings of atoms)
    • A phosphate group (this is acidic and negatively charged)
  • The base and phosphate group are both covalently bonded to the sugar

Basic-structure-of-a-nucleotide

The basic structure of a nucleotide

  • Nucleotides join together in chains to form DNA or RNA strands
  • The phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next one
    • This carries on to form a large polymer
  • This forms a 'sugar-phosphate backbone' with a base linked to each sugar
  • The polymer of nucleotides is known as a strand
  • DNA is double-stranded, RNA is usually single-stranded
  • There are just 4 separate bases that can be joined in any combination/sequence
    • Because the sugar and phosphate are the same in every nucleotide
  • This sequence is the basis of the genetic code as a store of genetic information

Exam Tip

A common error is to describe DNA or RNA as polymers of bases; more correctly, they are polymers of nucleotides

DNA & RNA: Comparison

  • Like DNA, the nucleic acid RNA (ribonucleic acid) is a polynucleotide – it is made up of many nucleotides linked together in a chain
  • Like DNA, RNA nucleotides contain the nitrogenous bases adenine (A), guanine (G) and cytosine (C)
  • Unlike DNA, RNA nucleotides never contain the nitrogenous base thymine (T) – in place of this they contain the nitrogenous base uracil (U)
  • Unlike DNA, RNA nucleotides contain the pentose sugar ribose (instead of deoxyribose)

Comparison-between-RNA-nucleotide-and-DNA-nucleotide

An RNA nucleotide compared with a DNA nucleotide

  • Unlike DNA, RNA molecules are only made up of one polynucleotide strand (they are single-stranded)
  • Unlike DNA, RNA polynucleotide chains are relatively short compared to DNA
  • Like DNA, the sugar-phosphate bonds (between different nucleotides in the same strand) are strong covalent bonds
  • Like DNA, the nitrogenous bases stick out sideways from the sugar-phosphate backbone

mRNA-as-an-example-of-RNA-structure

The structure of RNA

Nucleotide Structure Summary Table

Table-25-Nucleotide-structure

Exam Tip

You need to know the difference between DNA and RNA molecules (base composition, number of strands, pentose sugar present).

Double Helix Structure

  • DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs
  • The nucleic acid DNA is a polynucleotide – it is made up of many nucleotides bonded together in a long chain

DNA-nucleotide-with-carbon-numbers

A DNA nucleotide

  • DNA molecules are made up of two polynucleotide strands lying side by side, running in opposite directions – the strands are said to be antiparallel
  • Each DNA polynucleotide strand is made up of alternating deoxyribose sugars and phosphate groups bonded together to form the sugar-phosphate backbone
  • Each DNA polynucleotide strand is said to have a 3’ end and a 5’ end (these numbers relate to which carbon atom on the pentose sugar could be bonded with another nucleotide)
  • Because the strands run in opposite directions (they are antiparallel), one is known as the 5’ to 3’ strand and the other is known as the 3’ to 5’ strand
  • The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule

A-single-DNA-polynucleotide-strand

A single DNA polynucleotide strand showing 3 nucleotides in a sequence

Hydrogen bonding

  • The two antiparallel DNA polynucleotide strands that make up the DNA molecule are held together by hydrogen bonds between the nitrogenous bases
  • These hydrogen bonds always occur between the same pairs of bases:
    • The purine adenine (A) always pairs with the pyrimidine thymine (T) – two hydrogen bonds are formed between these bases
    • The purine guanine (G) always pairs with the pyrimidine cytosine (C) – three hydrogen bonds are formed between these bases
    • This is known as complementary base pairing
    • These pairs are known as DNA base pairs

DNA-molecule-with-hydrogen-bonding

A section of DNA – two antiparallel DNA polynucleotide strands held together by hydrogen bonds

Double helix

  • DNA is not two-dimensional as shown in the diagram above
  • DNA is described as a double helix
  • This refers to the three-dimensional shape that DNA molecules form

27.-DNA-double-helix-formation

DNA molecules form a three-dimensional structure known as a DNA double helix

Exam Tip

Make sure you can name the different components of a DNA molecule (sugar-phosphate backbone, nucleotide, complementary base pairs, hydrogen bonds) and make sure you are able to locate these on a diagramRemember that covalent bonds join the nucleotides in the sugar-phosphate backbone, and hydrogen bonds join the bases of the two complementary strands togetherRemember that the bases are complementary, so the number of A = T and C = G. You could be asked to determine how many bases are present in a DNA molecule if given the number of just one of the bases.

Crick & Watson

  • Francis Crick and James Watson were two Cambridge scientists who worked together to establish the double helix structure of DNA in 1953
  • They used data from their previous experiments on the composition of DNA
  • Published findings from other research labs played a role in developing their model
    • Rosalind Franklin, Edwin Chargaff and Linus Pauling, all of whom were leading research efforts in other universities, contributed important data to Crick & Watson's discovery
    • This suggests that there was a close-knit collaboration, but in fact, there was a lot of competition between the groups to make the breakthrough discovery
  • Physical model-making played a large role in their success
  • Early versions of the model were rejected for various reasons
    • It was not compact enough
    • It would have been too unstable (DNA is a highly stable molecule)
    • It did not allow equivalent amounts of A and T, and C and G bases to be present (Chargaff's findings)
    • It fitted together much better once the second strand was flipped to become antiparallel
  • Their final model was constructed carefully with clamps, metal rods for bonds and with the correct bond angles
  • Their model was the basis of a lot of genetic research globally in the years that followed
    • Notably, Crick and Watson's model sparked work to prove the way in which DNA replicates in cells

NOS: Using models as a representation of the real world; Crick and Watson used model making to discover the structure of DNA

  • Models in science are built to represent concepts and ideas in a way that can be pictured by our brains
  • Models can be accepted or rejected based on experimental data generated by further research
  • Crick and Watson built physical, scale models of DNA to explain biological observations
    • Using simple laboratory equipment (clamps, stands, metal rods etc)
    • They adapted their models by making them more realistic eg. by building in the correct bond angles within molecules
  • They built successive models, using trial-and-error to arrive at the finalised model
  • Their first model of DNA was rejected, based on the findings of Rosalind Franklin
    • Crick and Watson received the Nobel prize for their work
    • Franklin died aged 37 so never received the recognition she deserved for her significant role in defining DNA structure
  • Today, sophisticated computer modelling is performed, that
    • Takes the place of physical model-making and provides further explanation of the functions of various biomolecules
    • Shortens the 'trial-and-error' cycles of model-making as experienced by Crick and Watson

Exam Tip

Crick and Watsons' model has been universally accepted because all further research findings have supported their model.

 

 

 

 

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