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Worksheet 7.1
Chapter 7: Nucleic acids and proteins – fifteen summary
facts
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DNA has a rather simple structure with great capacity for variation. It is a double helix, made up
of nucleotides. Its nucleotides are attached at the 5' and the 3' carbons of the pentose deoxyribose.
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DNA includes covalent and hydrogen bonds. The hydrogen bonds allow linkage of the two chains
of DNA between a purine and a pyrimidine nitrogenous base. Covalent bonds occur everywhere
else within the DNA molecule. The covalent bonds are much stronger than the hydrogen bonds.
Because of the weak hydrogen bonds between the bases of the two DNA chains, the DNA can be
opened down the middle thus exposing the bases on both chains.
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In eukaryotic cells, the DNA is wrapped around molecules of protein called histone. This allows
effective packaging of DNA so that its relatively large amount may fit in the nucleus of a cell.
DNA and histone together form nucleosomes.
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The centromere of chromosomes is largely made up of highly repetitive sequences of DNA.
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DNA replication occurs in a 5' to 3' direction and involves semiconservative replication. The two
strands of DNA are said to be antiparallel. Therefore, replication of each strand occurs in opposite
directions. Normally, DNA replication results in two exact copies of the parent DNA being
produced. Changes in DNA are usually harmful and are called mutations.
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The enzymes necessary for DNA replication are helicase, primase, DNA polymerase III, DNA
polymerase I, and DNA ligase.
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There are three types of RNA involved in protein synthesis within the cell. They are messenger
RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA). The central dogma of biology
is summarized as:
DNA  RNA  protein
The first arrow represents transcription and the second arrow represents translation.
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The antisense strand of DNA is the strand that is copied during transcription. The other strand is
referred to as the sense strand; it has the same base sequence as the newly transcribed RNA except
that thymine is present in the place of uracil.
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The promoter region for a particular gene determines which DNA strand is the antisense strand.
The promoter region is a short sequence which is not transcribed. The terminator region causes the
RNA polymerase to detach from the DNA, thus stopping transcription. Nucleoside triphosphates
provide the energy for polymerization of the mRNA strand to occur.
10 Translation occurs at the ribosomes and involves mRNA, rRNA, and tRNA. The end result of
translation is a protein produced to the specifications of the DNA in the nucleus. Each ribosome is
made up of a large and a small subunit.
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11 There are three binding sites for tRNA in the ribosome between the two subunits. These are the A,
P, and E sites. The translation process involves initiation, elongation, translocation, and
termination.
12 Proteins have four levels of organization:




primary organization is the unique sequence of amino acids
secondary structure occurs as a result of hydrogen bonds between components of the carboxyl
and the amino groups
tertiary structure involves the polypeptide chain bending and folding because of interactions
among R-groups and the peptide backbone
quaternary structure occurs when multiple polypeptide chains combine to form a single
structure.
13 Proteins may be fibrous or globular, polar or non-polar.
14 Enzymes are quite specific and work in a way explained by the induced-fit model. This model
states the need for a close fit between an enzyme’s active site and the substrate. Once the active
site and substrate are together, a conformational change occurs allowing the formation of an
enzyme–substrate complex. This lowers the activation energy of a reaction thus allowing the
reaction to proceed more rapidly.
15 Inhibition of an enzyme may occur in several ways: competitive inhibition, non-competitive
inhibition, and end-product inhibition. Active sites and allosteric sites of an enzyme are involved
in different types of inhibition.
© Pearson Education Ltd 2010. For more information about the Pearson Baccalaureate series please visit
www.pearsonbacc.com
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