KEY CONCEPT DNA structure is the same in all organisms.

8.2 Structure of DNA

KEY CONCEPT DNA structure is the same in all organisms.


• We love D N A• Made of nucleotides• Sugar, Phosphate and a

Base• Bonded down one Side

• Adenine and Thymine• Make a Lovely Pair• Cytosine without

Guanine• Would feel very bare

•O-O-O deoxy-ribo-nucleic acid•R-N-A is ribo-nucleic acid


• DNA is made up of a long chain of nucleotides.• Each nucleotide has three parts.

– a sugar – deoxyribose– phosphate group– a nitrogen-containing base

phosphate group

deoxyribose (sugar)

nitrogen-containingbase


• The nitrogen containing bases

- Adenine (A)

- Thymine (T)

- Cytosine (C)

- Guanine (G))

* Purines – have 2 rings = Adenine & Guanine

* Pyrimidines – have 1 ring = Cytosine & Thymine


Watson and Crick determined the three-dimensional structure of DNA

• 1953• Two nucleotide chains that wrap

around each other to form a double spiral (double helix)

- Temperature liable – a change in T can break apart the DNA strand


• Rosalind Franklin and Erwin Chargaff.

– Franklin’s x-ray images suggested that DNA was a double helix of even width.

– Chargaff’s - Complementary Base Pairing - A=T and C=G.


TAC

G

Nucleotides always pair in the same way.

• Because a pyrimidine (single ring) pairs with a purine (double ring), the helix has a uniform width.

• A-T• C-G


• The backbone is connected by covalent bonds.

hydrogen bond covalent bond

• The bases are connected by hydrogen bonds.


KEY CONCEPT DNA replication copies the genetic information of a cell.

8.3 DNA Replication


Replication copies the genetic information.

• The rules of base pairing directreplication.

• A-T• G-C• DNA is replicated during the

S stage of interphase.

8.3 DNA Replication


Proteins (Enzymes) carry out the process of replication.

• DNA serves as a template.

1. Helicase – enzyme that breaks the Hydrogen bonds between the bases

• Replication Fork – point at which the two chains separate (last bond broken)

nucleotide

The DNA molecule unzips in both directions.

8.3 DNA Replication


2. New complimentary nucleotide bases pair up on both sides of old DNA template

3. DNA polymerase (enzyme) forms new Hydrogen bonds between the nucleotides

DNA polymerase

new strand nucleotide

8.3 DNA Replication


original strand new strand

Two molecules of DNA

• Two new exact copies of DNA are formed, each with an original strand and a newly formed strand.

8.3 DNA Replication


There are many origins of replication in eukaryotic chromosomes.

• DNA replication starts at many points in eukaryotic chromosomes.

Replication is fast and accurate.

• Mutation – change in the nucleotide sequence• DNA polymerases can find and correct errors.• Error Rate - one error per 1 billion nucleotides

8.3 DNA Replication


• RNA differs from DNA in three major ways.

1. RNA has a ribose sugar.

2. RNA has uracil instead of thymine.U - A

3. RNA is a single-stranded structure.

8.4 Transcription


Three types of RNA.

Mesenger RNA (mRNA) – single uncoiled chain– • carries genetic information from the DNA in the nucleus to

the cytoplasm

Transfer RNA (tRNA) – single chain of about 80 • RNA nucleotides folded into a hairpin shape – • binds to specific amino acids

Ribosomal RNA (rRNA) – makes up the ribosomes where proteins are made

8.4 Transcription


• Process of copying DNA into mRNA • RNA polymerase – starts RNA transcription by binding to

specific regions of DNA – Promoters

• RNA polymerase breaks H-bonds and makes H-bonds between the DNA bases

• One chain is used as a template to build RNA (mRNA=transcript)

• Transcription continues one nucleotide at a time until the RNA polymerase reaches a DNA region

– termination signal

start site

nucleotides

transcription complex

8.4 Transcription


The transcription process is similar to replication.

• The two processes have different end results.– Replication copies

all the DNA;transcription copiesa gene.

growing RNA strands

DNA

onegene

8.4 Transcription


KEY CONCEPT Translation converts an mRNA message into a protein.

8.5 Translation


Amino acids are coded by mRNA base sequences.

• Codon – 3 nucleotides of mRNA • AUG = start• UAA, UAG, UGA = stop

codon formethionine (Met)

codon forleucine (Leu)

8.5 Translation


• The genetic code matches each codon to its amino acid

8.5 Translation


• tRNA – transports amino acids to the ribosomes• Anticodon – tRNA sequence of 3 nucleotides

– complementary to an mRNA codon.

8.5 Translation


Ribosomes that are attached to the endoplasmic reticulum build proteins for use outside cell

Ribosomes that are free floating make proteins for use inside cell

8.5 Translation


• For translation to begin - Ribosomes attaches to a start codon on mRNA (AUG)

• Start codon pairs with the anticodon on tRNA (UAC)• codes for the first amino acid – methionine – may be

removed later if not needed

8.5 Translation


• Amino acids are bonded together with peptide bonds

8.5 Translation


– Once the stop codon is reached, the ribosome releases the protein

8.5 Translation


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

8.6 Gene Expression and Regulation


• 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.”

– The lac operon was one of the first examples of gene regulation to be discovered.

– The lac operon has three genes that code for enzymes that break down lactose.



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

– Introns are nucleotides that are removed and exons nucleotides that are spliced together.



Coding DNA (genes) make proteins• Humans = 20,000

Non-coding DNA (genes) make RNA (transcribed but never translated)

• Human = 500

Human Total = ~ 20,500 genes


KEY CONCEPT DNA structure is the same in all organisms.

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