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

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nscri

pti

on

tran

sla

tion

mRNA