More about
the Nucleus
Nucleus
• Nuclear envelope - a double membrane that
separates the nucleus from the cytoplasm
• Nuclear pores - numerous openings in the nuclear
envelope, control movement of substances between
nucleus and cytoplasm
• Nucleolus - spherical body that produces ribosomes
DNA is found in
chromosomes (in cells
prepared for division) or
in chromatin (in
nondividing cells)
In addition to DNA,
chromosomes contain
proteins histones
Gene = segment of DNA
coding for a protein; info
is stored in the sequence
of nitrogenous bases
Fig. 3.20
DNA
Packaging DNA
Histone
octomer
Histone proteins
DNA Helix
2 nm
Packaging DNA
Histone
octomer
Histone proteins
DNA Helix
2 nm
Packaging DNA
11 nm
Histone
octomer
Histone proteins
Nucleosome
DNA Helix
2 nm
Packaging DNA
Packaging DNA
Packaging DNA
“Beads on
a string”
11 nm
30 nm
Tight helical
fiber
Looped
200 nm Domains
Packaging DNA
Nucleosomes
11 nm
Chromosome
30 nm
Tight helical fiber
2 nm
DNA Helix
700 nm
200 nm Looped Domains
Karyotype = collection of all organism’s chromosomes
Body & Sex Chromosomes
So why is DNA the boss?
• Carries info for synthesis of proteins
• Study fig. 3.21, 3.22, 3.23
• Protein synthesis movie
So how does the info from DNA get into the
cytoplasm?
•
With the help of RNA
(ribonucleic acid)
The big picture:
•
DNA → RNA→ protein
PROTEIN SYNTHESIS
(Gene Expression):
Part I: Transcription:
DNA → RNA (in the nucleus)
Part II: Translation:
RNA → amino acids (in the
cytoplasm on ribosomes)
Part I: How do cells make RNA?
• Transcription =
DNA → RNA
(using the DNA
sequence to make
RNA)
• Takes place in the
nucleus
Steps of
Transcription
1. Enzymes “unzip” the
DNA molecule
2. Free RNA nucleotides
pair with
complementary DNA
nucleotides
3. RNA breaks away and
leaves the nucleus,
DNA strands rejoin
Part II: How do cells make proteins?
•
Translation =
RNA → amino acids
3 types of RNA function in
translation:
1. mRNA (messenger RNA) –
carries info from DNA to
ribosomes in a three letter
genetic code = codon
2. tRNA (transfer RNA) – brings
specific amino acids to
ribosomes by matching
mRNA; 3 nitrogenous bases
that are complementary to
codon = anticodon
3. rRNA (ribosomal RNA) –
makes up ribosomes
ala
Steps of Translation
1.
2.
met
3.
Peptide bond
met
4.
arg
5.
The start codon of mRNA
attaches to the ribosome
tRNA brings aa to the
ribosome; matching its
anticodon to codon of mRNA
Ribosome moves along to the
next mRNA codon and new
tRNA brings in a second aa
Peptide bond forms between
the 1st aa and 2nd aa
As the process continues, a
chain of aa is formed until the
ribosome reaches a stop codon
Remember: proteins are made of amino
acids
A TYPICAL PROTEIN
• The 20 amino acids make up all proteins
• There is redundancy in the genetic code: more than
one codon can code for the same aa
Codon Chart
Anticodon
pro
G
G
G
G
ile
pro
met
A
C
U
U
codon
A
G
U
A
C
G
G
G
A
U
G
C
C
C
U
G
A
U
C
A
G
U
mRNA