more about the nucleus nucleus nuclear envelope - a double membrane that separates the nucleus from...
TRANSCRIPT
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 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
A TT AG CC
G
G C
TA
T
AG
C
C G
G C
T A
A T
Packaging DNA
Histone proteins
Histoneoctomer
DNA Helix 2 nm
A TT AG CC
G
G C
TA
T
AG
C
C G
G C
T A
A T
Packaging DNA
Histone proteins
DNA Helix
Histoneoctomer
2 nm
A TT AG CC
G
G C
TA
T
AG
C
C G
G C
T A
A T
Packaging DNA
Histone proteins
Histoneoctomer
Nucleosome
11 nm
DNA Helix 2 nm
Packaging DNA
A TT AC G
C G
G C
T A
A T
Packaging DNA
A TT AC G
C G
G C
T A
A T
Packaging DNA
A TT AC G
C G
G C
T A
A T
11 nm“Beads on a string”
30 nm
Tight helical fiber
Looped Domains200 nm
Packaging DNA
G
C
A
T
Chromosome
700 nm
11 nm
30 nm200 nm
2 nm
Looped Domains
Nucleosomes
DNA Helix
Tight helical fiber
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
met
met
arg
arg
ile
Steps of Translation1. The start codon of mRNA
attaches to the ribosome
2. tRNA brings aa to the ribosome; matching its anticodon to codon of mRNA
3. Ribosome moves along to the next mRNA codon and new tRNA brings in a second aa
4. Peptide bond forms between the 1st aa and 2nd aa
5. As the process continues, a chain of aa is formed until the ribosome reaches a stop codon
Peptide bond
A TYPICAL PROTEIN
Remember: proteins are made of amino acids
• 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
A U G C G AU UC C C A
AA
A
A
U
U
U
U
U
GG G G
G G
G
G G G
C
Cile
metpro
pro
stop
pro
his
Anticodon
codon
Codon Chart
tra
nscri
pti
on
tran
sla
tion
mRNA