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4. Structure and Function of Cell Components:

All references made are to the unit 1 Scholar book. Page numbers may vary each year as different versions of the book are released.

• DNA and RNA • DNA – information store • RNA – protein synthesis • both composed of nucleotides

Refer to base structure from Scholar – Figure 4.24 p.84 •C atoms numbered 1-5. •Base attached to C1, phosphate on C5. •C2 – H for DNA, OH RNA. •DNA = deoxyribose sugar •RNA = ribose sugar

• chains of nucleic acids formed by condensation reaction (dehydration synthesis) •

• bond between phosphate (C5), and OH on C3 (C3-C5 linkage) => phosphodiester bond. (Figure 4.25 p.85)

• each strand has a 3’ and 5’ terminus. • 2 strands run anti-parallel to each other (i.e., one strand runs 5’-3’, other 3’-5’). • due to strands being complementary to each other, it allows base-pairing.

Bases There are 2 types of bases: 1.Purines (A and G), which have a double ring structure. 2. Pyrimidines (C, T, and U(RNA)), which have a single ring structure.

•Base pair rules – A+T (U in RNA) and G+C. •G+C join with 3 hydrogen bonds •A+T/U join with 2 hydrogen bonds. • Figure 4.26 p.86

•2 strands of DNA twist to form a double helix. One turn of the double helix = 10 base pairs. (Figure 4.27 p.86) •DNA can replicate itself. •DNA replication is semi-conservative – i.e., one new DNA molecule contains one new strand, and one strand from the parent molecule.

DNA Replication • DNA double helix unwinds • 2 strands separate to reveal replication fork. • free DNA nucleotides align according to base pair rules

• DNA polymerase can only join nucleotides in the 5’-3’ direction – i.e., the nucleotides aligning with the 3’-5’ strand. • 3’-5’ strand is called the lead strand • DNA polymerase continuously joins these nucleotides together.

• DNA polymerase cannot work in the 3’-5’ direction. • DNA polymerase helps to make short fragments of DNA in the 5’-3’ direction on the 5’-3’ strand. => okazaki fragments.

• Okazaki fragments are joined in the 3’-5’ direction by DNA ligase • synthesis of DNA on the 5’-3’ strand is discontinuous, => lag strand.

RNA Structure RNA differs from DNA in 3 ways: 1.Ribose instead of deoxyribose pentose sugar 2.Uracil replaces thymine 3.Single-stranded 3 types of RNA: 1. mRNA – produced during transcription 2. tRNA – transfers amino acids to growing protein during translation 3. Ribosomal RNA (rRNA) – component of ribosome

Nucleic Acids  
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