DNA replication—when? Where? Why? What else does a cell do? When? Where? Why?

DNA replication—when?Where?Why?

What else does a cell do?When? Where? Why?

RNA and Protein Synthesis

The Structure of RNA:

Similar to DNA long chain of nucleotides (made of phosphate + sugar + base).

Different than DNA because 1. single stranded 2. ribose sugar instead of deoxyribose3. uracil instead of thymine

Includes three types of RNA: tRNA, mRNA, and rRNA.

What is RNA?

DNA compared to RNA

BOTH made of nucleotide monomers

DNA RNA

Contains the sugar deoxyribose

Has the bases A, C, G, and U

Typically double-stranded

Only one type of DNAThree types of RNA: mRNA, tRNA, rRNA

8.4 Transcription

RNA carries DNA’s instructions. • The CENTRAL

DOGMA states that information flows in one direction from DNA to RNA to proteins.

8.4 Transcription

• The CENTRAL DOGMA includes three processes.

• RNA is a link between DNA and proteins.

replication

transcription

translation

– Replication– Transcription– Translation

BIG PICTURE: Protein Synthesis

DNA

RNA

Protein

8.4 Transcription

• Transcription copies DNA to make a strand of RNA.

(Transcription converts a gene into a single-stranded RNA molecule.)

Transcribe means “to write”

8.4 Transcription

• Transcription makes three types of RNA.– Messenger RNA (mRNA) serves as messengers

from DNA in nucleus to ribosomes in cytoplasm. The message will be translated to form a protein.

– Ribosomal RNA (rRNA) forms structural component of ribosomes (remember, ribosomes make proteins in the cytoplasm).

– Transfer RNA (tRNA) transfers each amino acid to the ribosome as specified in a mRNA code.

8.4 Transcription

• Transcription is catalyzed by RNA polymerase.

1. RNA polymerase finds an area known as a promoter on the DNA strand. RNA polymerase and other proteins form a transcription complex.

2. The transcription complex recognizes the start of a gene (DNA) and unwinds a segment of it.

start site

nucleotides

RNA polymerase + proteins =transcription complex

8.4 Transcription

4. RNA polymerase bonds the nucleotides together.

The DNA helix winds again as the gene is transcribed.

3. The DNA is used as a template for RNA polymerase to add mRNA nucleotides. Nucleotides pair with one strand of the DNA: C-G and A-U

DNA

RNA polymerase moves along the DNA

8.4 Transcription

5. The mRNA strand detaches from the DNA once the gene is transcribed. The mRNA strand is “processed” (cleaned up) and then exits the nucleus.

RNA

Practicing transcription

Use the following DNA sequence to complete an RNA transcript. Remember. Thymine is not a base for RNA; use Uracil as a replacement.

DNA: T A C C G C T T A A C C G T A A C T

mRNA:

8.4 Transcription

The transcription process is similar to replication.

growing RNA strands

DNA

onegene

• Transcription and replication both involve complex enzymes and complementary base pairing.

• The two processes have different end results.– Replication copies all the DNA;

transcription copies a gene.– Replication makes one copy;

transcription can make many copies.

8.4 Transcription



replication

transcription

translation


8.5 Translation

KEY CONCEPT TRANSLATION converts an mRNA message into a polypeptide, or PROTEIN.

8.4 Transcription



replication

transcription

translation


8.5 Translation

The Genetic Code• There are only 4 RNA bases (A U C G)• How are there 20 different amino acids if only 4 letters make

up the alphabet of RNA?– RNA bases are taken 3 at a time– A CODON is a sequence of three consecutive

nucleotides on the mRNA strand that specify/code for a single amino acid.

• Anticodons are the “complements” of the mRNA codons Anticodons are on tRNA molecules that travel throughout the cytoplasm picking up the correct amino acids.

8.5 Translation

Amino acids are coded by mRNA base sequences.

• Translation converts mRNA messages into polypeptides.• A codon is a sequence of three nucleotides that codes for

an amino acid.

codon formethionine (Met)

codon forleucine (Leu)

8.5 Translation

Amino acids are linked to become a protein.

• An anticodon is a set of three nucleotides that is complementary to an mRNA codon.

• An anticodon is carried by a tRNA.

8.5 Translation

• Ribosomes consist of two subunits.– The large subunit has three binding sites for tRNA.– The small subunit binds to mRNA.

8.5 Translation

1. For translation to begin, tRNA binds to a start codon and signals the ribosome to assemble.

2. A complementary tRNA molecule binds to the exposed codon, bringing its amino acid close to the first amino acid.

8.5 Translation

3. The ribosome helps form a polypeptide bond between the amino acids and tRNA lets go of amino acid.

4. The ribosome pulls the mRNA strand the length of one codon.

8.5 Translation

5. The now empty tRNA molecule exits the ribosome.

6. A complementary tRNA molecule binds to the next exposed codon.

7. Once the stop codon is reached, the ribosome releases the protein and disassembles.

8.5 Translation

Some finer points…

• DNA ---- mRNA -- tRNA --- proteins

codons anticodons amino acids

• AUG is an mRNA START codon• There are three STOP codons.

– UAA, UAG, and UGA

8.5 Translation

• The genetic code matches each codon to its amino acid or function.– three stop

codons– one start

codon, codes for methionine

The genetic code matches each RNA codon with its amino acid or function.

Practicing transcription

Use the following DNA sequence to complete an RNA transcript. Remember. Thymine is not a base for RNA; use Uracil as a replacement.

DNA: T A C C G C T T A A C C G T A A C T

mRNA: A U G G C G A A U U G G C A U U G A

tRNA: U A C C G C U U A A C C G U A A C U

Amino Met Ala Asn Try His STOP

acid: (START)

8.5 Translation

• A change in the order in which codons are read changes the resulting protein.

• Regardless of the organism, codons code for the same amino acid.

8.5 Translation

8.6 Gene Expression and Regulation

Gene Expression and Regulation

Are ALL genes “on” all the time in the life of a cell?

Do skin cells need to make digestive enzymes?

Do kidney cells need to make hemoglobin?

Do muscle cells need to make amylase?

How do cells control what they make/do?

The EXPRESSION of genes is carefully REGULATED in cells.

Just remember: a cookbook is full of recipes, but you don’t make all of them every time you make a meal. You just pick and choose what you want to cook and make those items from those select recipes.


KEY CONCEPT Gene expression is carefully regulated in both prokaryotic and eukaryotic cells.


Prokaryotic cells turn genes on and off by controlling transcription.

• A promotor is a DNA segment that allows a gene to be transcribed.

• An operator is a part of DNA that turns a gene “on” or ”off.”• An operon includes a promoter, an operator, and one or

more structural genes that code for all the proteins needed to do a job.– Operons are most common in prokaryotes.


Eukaryotes regulate gene expression at many points.

• Different sets of genes are expressed in different types of cells.

• Transcription is controlled by regulatory DNA sequences and protein transcription factors.


• Transcription is controlled by regulatory DNA sequences and protein transcription factors.

– Most eukaryotes have a TATA box promoter.– Enhancers and silencers speed up or slow down the rate

of transcription.– Each gene has a unique combination of regulatory

sequences.


• RNA processing is also an important part of gene regulation in eukaryotes.

• mRNA processing includes three major steps.


• mRNA processing includes three major steps.

– Introns are removed and exons are spliced together.– A cap is added.– A tail is added.

8.7 Mutations

KEY CONCEPT Mutations are changes in DNA that may or may not affect phenotype.

8.7 Mutations

Some mutations affect a single gene, while others affect an entire chromosome.

• A mutation is a change in an organism’s DNA.• Many kinds of mutations can occur, especially during

replication.• A point mutation substitutes one nucleotide for another.

mutatedbase

8.7 Mutations

• Many kinds of mutations can occur, especially during replication. – A frameshift mutation inserts or deletes a nucleotide in

the DNA sequence.

8.7 Mutations

• Chromosomal mutations affect many genes.

– Chromosomal mutations affect many genes.– Gene duplication results from unequal crossing over.

• Chromosomal mutations may occur during crossing over

8.7 Mutations

• Translocation results from the exchange of DNA segments between nonhomologous chromosomes.

8.7 Mutations

Mutations may or may not affect phenotype.

• Chromosomal mutations tend to have a big effect. • Some gene mutations change phenotype.

– A mutation may cause a premature stop codon.– A mutation may change protein shape or the active site.– A mutation may change gene regulation.

blockage

no blockage

8.7 Mutations

• Some gene mutations do not affect phenotype.

– A mutation may be silent.– A mutation may occur in a noncoding region.– A mutation may not affect protein folding or the active

site.

8.7 Mutations

• Mutations in body cells do not affect offspring. • Mutations in sex cells can be harmful or beneficial to

offspring.• Natural selection often removes mutant alleles from a

population when they are less adaptive.

8.7 Mutations

Mutations can be caused by several factors.

• Replication errors can cause mutations.

• Mutagens, such as UV ray and chemicals, can cause mutations.

• Some cancer drugs use mutagenic properties to kill cancer cells.

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DNA replication—when? Where? Why? What else does a cell do? When? Where? Why?