1 Structure of DNA/RNA

The three parts of nucleotide are as follows:

1. 
   

   1. Nitrogen-containing bases:

   
   
   
   
   1. 
      

      ◦ purine:

      
      
      
      
      1. 
         

         — fused nitrogen-containing ring

         
         
      2. 
         

         — include adenine and guanine

      
      
      
      
   2. 
      

      ◦ pyrimidine:

      
      
      
      
      1. 
         

         — single nitrogen-containing ring

         
         
      2. 
         

         — include cytosine, thymine and uracil

      
      
      
      
   3. 
      

      ◦ DNA: adenine, guanine, cytosine and thymine

      
      
   4. 
      

      ◦ RNA: adenine, guanine, cytosine and uracil

   
   
   
   
2. 
   

   2. Pentose (5-carbon sugar)

   
   
   
   
   1. 
      

      ◦ DNA: deoxyribose

      
      
   2. 
      

      ◦ RNA: ribose

   
   
   
   
3. 
   

   3. Phosphate: linked to pentose by phosphodiester bond

2 Nucleotide

• 
  

  basic structural unit of DNA/RNA:

  
  
  
  
  1. 
     

     ◦ DNA: 100 million nucleotides; double-stranded helix

     
     
  2. 
     

     ◦ RNA: 100–1000 nucleotides, single stranded

  
  
  
  
• 
  

  triplet nucleotide = codon = produces a single amino acid:

  
  
  
  
  1. 
     

     ◦ one amino acid can have several codons

     
     
  2. 
     

     ◦ one codon can produce only one amino acid

  
  
  
  
• 
  

  two ends:

  
  
  
  
  1. 
     

     ◦ five-prime end: phosphate attaches to C5 of pentose

     
     
  2. 
     

     ◦ three-prime end: hydroxyl attaches to C3 of pentose

  
  
  
  
• 
  

  base pairing:

  
  
  
  
  1. 
     

     ◦ A-T (two weak hydrogen bonds, DNA)

     
     
  2. 
     

     ◦ C-G (three weak hydrogen bonds, DNA)

     
     
  3. 
     

     ◦ A-U (three weak hydrogen bonds, RNA)

  
  

3 Types of RNA

• 
  

  messenger RNA (mRNA): carries codon

  
  
• 
  

  transfer RNA (tRNA): carries anticodon and single amino acid

  
  
• 
  

  ribosomal RNA (rRNA): assembles amino acids

4 DNA Replication

1. 
   

   1. DNA double helix unwinds as helicase breaks down hydrogen bonds and forms replication fork

   
   
2. 
   

   2. DNA polymerase adds nucleotides to the leading strand (oriented 3-prime to 5-prime)

   
   
   
   
   1. 
      

      ◦ polymerase travels from 5-prime to 3-prime to form a new strand continuously

   
   
   
   
3. 
   

   3. DNA polymerase adds nucleotides to lagging strand (oriented 5-prime to 3-prime)

   
   
   
   
   1. 
      

      ◦ polymerase travels from 5-prime to 3-prime to form a new strand, but in segments (called Okazaki fragments)

      
      
   2. 
      

      ◦ DNA ligase joins the Okazaki fragments together

   
   
   
   
4. 
   

   4. This process is performed at many points on DNA simultaneously to save time

5 Process of DNA Synthesising Protein

1. 
   

   1. Transcription: DNA is copied into mRNA

   
   
2. 
   

   2. Gene splicing

   
   
3. 
   

   3. Translation: mRNA translates into amino acids into protein

   
   
4. 
   

   4. Protein structure formation

   
   
5. 
   

   5. Posttranslational modification

6 Transcription

1. 
   

   1. DNA strands unwind, and the DNA template strand opens up

   
   
2. 
   

   2. RNA polymerase adds nucleotides from 5-prime to 3-prime onto coding strand

   
   
   
   
   1. 
      

      ◦ uracil replaces thymine in the process

   
   
   
   
3. 
   

   3. DNA strands reanneal after mRNA is released

   
   
4. 
   

   4. This process is performed at many points on DNA simultaneously

7 Gene Splicing

• 
  

  removes introns (noncoding sequences) and joins exons (protein-coding sequences)

  
  
• 
  

  alternative splicing: single gene translated into many proteins with different functions

8 Translation

1. 
   

   1. tRNA carries anticodon to pair onto codon on mRNA

   
   
2. 
   

   2. tRNA releases amino acid as anticodon attaches to codon

   
   
3. 
   

   3. Ribosome (containing rRNA + proteins) carries the growing polypeptide and releases rRNA

   
   
4. 
   

   4. Ribosome falls off after polypeptide completed

   
   
5. 
   

   5. This process occurs at many points on mRNA simultaneously to save time

9 Protein Structure Formation

1. 
   

   1. Primary structure: initial polypeptide strand

   
   
2. 
   

   2. Secondary structure: folding into chains (alpha chain) or sheets (beta sheet) via hydrogen bonds

   
   
3. 
   

   3. Tertiary structure: folding into final structure via hydrophobic interaction + disulphide bonds

   
   
4. 
   

   4. Quaternary structure: multiple polypeptides combine into multimeric proteins

10 Posttranslational Modification

• 
  

  acetylation (add acetyl group): affects lifespan of protein

  
  
• 
  

  phosphorylation (add phosphate group): affects activity of protein

  
  
• 
  

  glycosylation (add carbohydrate chain)

11 Chromosome

• 
  

  cells and number of chromosomes:

  
  
  
  
  1. 
     

     ◦ somatic cells (most of the body): 46 chromosomes or 23 pairs (diploid); identical in body

     
     
  2. 
     

     ◦ germ cells or gametes (ovum and sperm only): 23 chromosomes (haploid); nonidentical due to recombination

     
     
  3. 
     

     ◦ autosome: first 22 pairs (numbered 1–22); numbering based on decreasing chromosome length

     
     
  4. 
     

     ◦ sex chromosomes: twenty-third pairs are X and Y: XX (female), XY (male)

  
  
  
  
• 
  

  shape of chromosomes

  
  
  
  
  1. 
     

     ◦ short arm of chromosome: p arm

     
     
  2. 
     

     ◦ long arm of chromosome: q arm

     
     
  3. 
     

     ◦ metacentric chromosome: short and long arm equal length (centromere near middle of the chromosome)

     
     
  4. 
     

     ◦ submetacentric chromosome: one arm longer than the other (centromere closer to one side of the chromosome)

     
     
  5. 
     

     ◦ acrocentric chromosome: one arm much longer than the other (centromere at top of the chromosome)

     
     
  6. 
     

     ◦ isochromosome chromosome: unbalanced, with duplication of the arms, which are mirror images resulting in two copies of either the long arm or the short arm (simultaneous duplication and deletion of genetic material)

     
     
  7. 
     

     ◦ chromatid: single strand of a chromosome (chromosomes in somatic cells come in pairs)

     
     
  8. 
     

     ◦ centromere: special DNA sequence where the two sister chromatids link

     
     
  9. 
     

     ◦ telomere: repetitive nucleotides at the end of chromosomes; protects end of chromosome (like plastic tips on shoelaces)

     
     
  10. 
      

      ◦ histone: protein that DNA wraps around to condense into chromosomes (like spools for thread)

  
  

12 Genes

• 
  

  definition: stretch of DNA sequence, which codes for a specific protein

  
  
• 
  

  human genome: 20 000 genes

13 Mitosis

• 
  

  occurs in somatic cells

  
  
• 
  

  the aim is to produce identical diploid cells for cell reproduction

  
  
• 
  

  cytokinesis: the process of division into two cells; occurs in anaphase and telophase (see Table 19.1)

14 Meiosis

• 
  

  occurs in germ cells (gametes) only

  
  
• 
  

  the aim is to produce haploid cells, so that after fertilisation there will be 46 chromosomes (instead of 92 chromosomes)

  
  
• 
  

  meiosis I and meiosis II

15 Spermatogenesis

• 
  

  produces four spermatozoa

16 Oogenesis

• 
  

  produces one ovum and three polar bodies

  
  
• 
  

  polar bodies cannot fertilise (see Table 19.2)

17 Chromosome and Gene Abnormalities

See Table 19.3.

18 Nondisjunction

See Tables 19.4, 19.5 and 19.6.

• 
  

  due to mistake in meiosis I or meiosis II

  
  
• 
  

  leads to trisomy and monosomy

  
  
• 
  

  risk increases with maternal age

Table 19.5 Age and risk of nondisjunction

Age	Risk
20	1/1500
30	1/900
34	1/500
36	1/300
40	1/100
42	1/60
45	1/30

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