Molecular Genetics Dr' Watson and Dr' Crick

1
Molecular Genetics Dr. Watson and Dr. Crick
2
Structure of DNA

• Nucleotide- building block of DNA made of
• 1) 5-C sugar (deoxyribose)
• 2) Phosphate group
• 3) Nitrogenous base

3
Structure of DNA (discovered by Watson Crick in
1953)

• DNA has 4 nitrogen bases
• Purines (2 rings) Adenine Guanine
• Pyrimidines (1 ring) Cytosine Thymine
• Individual nucleotides are joined together to
  form a long chain
• Alternating sugar-phosphate backbone
• Nitrogenous bases pair to form the rungs
• Resemble spiral staircase or twisted ladder

4
DNA Double Helix
Base-pairing Rules A pairs with T (2 H-bonds) G
pairs with C (3 H-bonds)
5
DNA Double Helix
6
(No Transcript)
7
More DNA structure

• There are two sides of the DNA chain. One side is
  called the sense strand (template) and the other
  is the anti-sense strand (complementary).
• One side of the DNA runs from 5 to 3 and the
  other runs from 3 to 5. This is based on which
  carbon is attached to the phosphate.

8
(No Transcript)
9
(No Transcript)
10
DNA Replication
11
DNA replication

• A cell must replicate its DNA in order to have 2
  sets
• The 2 strands of the DNA double helix are
  separated by enzymes called helicases
• these break H-bonds as they move along the strand
• the points at which this occurs are called
  replication forks
• Enzymes called DNA polymerases then bind to the
  single strands assemble 2 new double helices by
  attaching a new nucleotide to the complementary
  strand

12
(No Transcript)
13
More replication

• DNA polymerases only run in a 5 to 3 direction
• the side that is made in one long piece is called
  the leading strand
• The other side runs 3 to 5
• this side is made in small pieces as helicase
  unwinds unzips the DNA and is called the
  lagging strand
• the small pieces are called Okazaki fragments
  after the man who discovered the process.
• DNA ligase attaches the fragments together
• The end result is 2 identical strands of DNA that
  are each half new and half old DNA

14
(No Transcript)
15
RNA

• There are 3 differences between RNA and DNA
• RNA is single stranded, DNA is double stranded
• RNA contains uracil, DNA has thymine (A pairs
  with U in RNA)
• RNA has ribose as its sugar, DNA has deoxyribose
• There are 3 types of RNA
• Messenger RNA (mRNA)- made by copying the DNA,
  takes the message to the ribosome to make
  proteins allowing the DNA to safely stay in the
  nucleus (blueprint)
• Transfer RNA (tRNA)-carries the correct amino
  acid to the ribosome from the cytoplasm to add it
  to the protein
• Ribosomal RNA (rRNA)-makes up the ribosome

16
RNA Nucleotide
17
Protein synthesis

• The DNA of a person contains about 75,000
  genes-each of which codes for the making of a
  protein
• whenever the body or a cell needs more protein,
  it must create it through this process
• Ex. when you grow hair, you must make more
  keratin which is the protein that makes up hair
  when you need new red blood cells, you must make
  more hemoglobin, the protein that carries O2
  when you need to digest milk, you must make more
  lactase to process the lactose

18
(No Transcript)
19
Transcription

• The 1st stage of protein synthesis is
  transcription
• takes place in the nucleus
• process of copying DNA ? mRNA (A-U, G-C)
• RNA polymerase will initiate the formation of
  mRNA at an area called the promoter in the same
  way that DNA polymerase replicates DNA.
• This binding causes the DNA to unwind
• Only the template side of the DNA is read
• The promoter region is always followed by the set
  of nucleotides T-A-C called the start codon
• A codon is a set of 3 bases all RNA is divided
  up in sets of 3
• Transcription continues until the a stop codon is
  reached, where RNA polymerase releases the DNA
  the newly synthesized RNA strand

20
(No Transcript)
21
(No Transcript)
22
(No Transcript)
23
(No Transcript)
24
(No Transcript)
25
Anti-sense strand Complement strand
Sense strand Template strand
26
Translation

• The 2n part of protein synthesis is called
  translation
• takes place at the ribosome
• mRNA that was made in transcription travels out
  of the nucleus and to a ribosome on the ER or in
  the cytoplasm
• Amino acids that are freely floating in the
  cytoplasm are transported to the ribosome by the
  tRNAs
• Part of the tRNA contains a region of three bases
  called the anticodon
• Only the anticodon that matches the mRNA codon
  will be able to donate an amino acid so that the
  DNAs message is translated into a protein
  correctly
• DNA codon TAC
• mRNA codon AUG
• tRNA anticodon UAC

27
(No Transcript)
28
(No Transcript)
29
(No Transcript)
30
Making a protein

• The mRNA attaches to a binding site on the
  ribosome
• Start codon AUG on mRNA complements UAC on tRNA ?
  1st amino acid in every protein chain starts as
  Methionine
• As the ribosome moves along the mRNA, each mRNA
  codon is paired with the correct tRNA anticodon
• The pairing of the next amino acid creates a
  covalent bond between the 2 amino acids called a
  peptide bond
• The entire mRNA molecule is read in this way,
  making an increasingly long chain of amino acids
  all bonded to one another until a stop codon is
  reached

31
(No Transcript)
32
(No Transcript)
33
(No Transcript)
34
(No Transcript)
35
(No Transcript)
36
Finishing Protein Synthesis

• Once the stop codon is reached, the mRNA molecule
  is released from the ribosome
• It may be translated by many ribosomes many times
  if the protein is needed in high amounts
• The completed amino acid chain is the primary
  structure of a protein
• Its structure will be finalized at the ER or the
  Golgi

37
Mutations

• Many types of mutations that can occur in DNA
• If the mutation involves 1 or only a few base
  pairs (only one gene), it is called a point
  mutation (See pages 225-6)
• Substitution-effects only one amino acid
• Frame shift-can effect the entire protein

38
Chromosomal mutations

• Duplication occurs when a piece of a chromosome
  is duplicated and becomes too long.
• Deletion occurs when a piece of chromosome is
  entirely lost or broken off.
• Inversion occurs when a chromosome piece breaks
  off and is reinserted in reverse orientation.
• Translocation occurs when a piece of a chromosome
  breaks off and is attached to a different
  chromosome.
• Nondisjunction occurs when an entire chromosome
  fails to separate in meiosis. One gamete will
  receive an extra chromosome and the other will
  lack the chromosome entirely.

39
(No Transcript)
40
(No Transcript)
41
DNA Packing
Nucleosome
Histone
42
(No Transcript)