Download DNA and Molecular Genetics

DNA and Molecular Genetics
Chapter 3
Introduction
• Until now we have talked
about genes simply as a
functional part of the
chromosome
• Need to consider how genes
actually work in the cell
(called expression of a
gene)
• To understand how genes
express, need to understand
both their chemical
composition and
biochemical function
DNA and RNA Structure
and Function
• Review:
– DNA found mostly in chromosomes in nucleus
– When cell is undergoing mitosis, chromosomes are
short and thick
– Rest of the time, chromosomes (and DNA) are long
and thin [>5 ft in each cell!)
– DNA exists in the form of a double-stranded helix
– Helix – spiral staircase or twisted ladder shapedstructure
– Double strand structure allows “easy” replication
(making more) of the long, complicated DNA
Fig. 03-01
DNA
structure
and
location
Fig. 03-01
DNA Structure and Replication
• DNA stands for deoxyribonucleic acid
• All nucleic acids formed by bonding together of
nucleotides (type of molecule)
• Nuleotides – formed by bonding of three smaller
molecules
1. Phosphate
2. Sugar (= deoxyribose molecule)
3. Nitrogen-containing base
• Note a base can “take up” (soak up) protons (positively
charged particles = H+)
• Acids “give off” protons (H+)
• When phosphate and N-containing base bond together 
give off (H+) protons = the acid in DNA
DNA Structure and Function (con’t)
Nucleotides
Fig. 03-02
•
Four nucleotides that
make up DNA are
1.
2.
3.
4.
•
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
Nucleotides are joined
together in a specific
way, with phosphates
forming the backbone
of the DNA strand and
bases projecting to the
side
DNA Structure and Function (con’t)
Double strands and base pairing
• DNA has two strands of
nucleotides; this makes DNA a
double helix
• Weak hydrogen bonds between
the bases hold the strands
together
• Different #’s of bonds causes
only certain bases to bond
together (called complementary
base pairing)
• Complementary base pairing
– Adenine (A) with thymine (T)
– Guanine (G) with cytosine (C)
– Could be vice-versa (T-A, C-G)
Fig. 03-03
Replication of DNA
•
•
Occurs as part of
chromosome duplication
Requires four steps
1.
2.
3.
4.
•
H-bonds between two
strands of DNA break as
enzymes unwind and “unzip”
the DNA molecule
New nucleotides (always
present in the nucleus) fit
into place beside each old
(parental) strand by
complementary base pairing
New nucleotides become
joined by enzyme called
DNA polymerase (forms
DNA polymer (molecule)
End up with two complete
DNA molecules, identical to
each other and to the
original molecule
Each new DNA is partly old
(parental strand) and partly
new (daughter strand)
Gene Expression
• Gene expression is the making of specific
proteins (in ribosomes in cytoplasm and
on rough ER) from specific nucleotide
sequences (in DNA of genes in nucleus)
• Need a way to get information from
nucleus to ribosomes  done with RNA
• Actually three types of RNA that all help to
read the DNA code and produce proteins
Structure of RNA
•
•
•
RNA (ribonucleic acid)
made up of nucleotides
containing the sugar
ribose
Four nucleotides
making up RNA have 3
of the same bases as
DNA (A,C,G) and one
different, uracil (U)
instead of thymine (T)
RNA is single-stranded
Fig. 03-05 Structure of RNA
Types of RNA
Note: all types produced in nucleus by DNA according to DNA
nucleotide sequence in specific gene
• Messenger RNA (mRNA) – carries genetic
information from DNA to ribosomes where
protein synthesis occurs
• Ribosomal RNA (rRNA) – combines with
certain proteins to form ribosomes
• Transfer RNA (tRNA) – transfers amino
acids “floating around” in cytoplasm,
brings them to ribosomes in certain order
specified by mRNA, bonds them together
to form proteins
Structure and Function of Proteins
• Made up of subunits called amino acids (20 different AA’s)
• Specific sequence of amino acids dictates specific protein
A (shortened) protein
Another (shortened) protein
• Proteins can be structural (muscles) or enzymes = catalyze (speed up)
chemical reactions
Structure and Function of Proteins
(con’t)
• Reactions in cells form metabolic (chemical) pathways
EA
EB
EC
ED
A  B  C  D  E
• Letters represent molecules, notations over arrows are enzymes.
• For example EA catalyzes reaction converting chemical A to chemical B, EB
catalyzes catalyzes B to C, etc
• By DNA producing certain enzymes, can “turn on” certain chemical
pathways in cell as needed  can form and maintain entire organism!
Back to Gene Expression
• Requires two steps
– Transcription – making mRNA from specific
portion of DNA (gene)
– Translation – mRNA goes out into cytoplasm
to ribosome, directs tRNAs to bring certain
amino acids to ribosome, rRNA joins them
together in certain sequence = a specific
protein!
Transcription
• Occurs
in nucleus
• Nucleotides in DNA are
complementarily matched
to form mRNA, substituting
U for T
• mRNA then goes out of
nucleus to ribosome for
translation
Translation
• Occurs in cytoplasm
• Synthesis of polypeptide
(many amino acids bonded
together) under direction of
mRNA
• mRNA tells rRNA which
amino acid to go get from
cytoplasm
• rRNA and protein in ribosome
binds amino acids together in
sequence directed by mRNA
Overview of Transcription and Translation
Transcription
Translation
Genetic Mutations
• Defined as a
permanent change
in the sequence of
nucleotides in DNA
• Effect on protein
activity
(construction
and/or function)
may range from no
effect to complete
inactivity
Effect of Mutations
• Some genetic disorders already talked about
in class are due to mutations
• Example: Phenylketonuria due to defect in
gene expression for EA below; Albinism due
to defect in gene expression for EB below
• Other genetic disorders due to gene defects
include hemophilia B, Cystic fibrosis, and
androgen insensitivity
Androgen Insensitivity
• Androgens are hormones needed by males
(e.g. testosterone) to show secondary sex
characteristics (broad shoulders, extra body
hair, deeper voice, etc)
• In androgen insensitivity, a mutated gene
prevents proper formation of androgen
receptors on cells
• Results in cells not responding to androgens at
puberty --> individual will instead develop some
female secondary sexual characteristics
(breasts, wider hips, etc.)
• A problem is realized when the person does not
start to menstruate and seeks medical
assistance  both X and Y chromosomes found
in cells and person found lacking in internal
sexual organs of a female.
A genetic male with
androgen insensitivity