DNA, RNA and Protein Synthesis

TAKS Review

Spring 2012

Structure of DNA

• DNA is deoxyribonucleic acid

• DNA is a large molecule that has subunits called nucleotides.

• The nucleotides come together to make the DNA molecule in the shape of a double helix (looks like a twisted ladder).

Nucleotides

• Nucleotides are the subunits that make up DNA and they have 3 major parts:

1. A sugar molecule (deoxyribose sugar)

2. A phosphate group 3. A nitrogen base

Nitrogen bases

There are 4 kinds of nitrogen bases:

Adenine (A)

Guanine (G)

Thymine (T)

Cytosine (C)

DNA Structure

• DNA is a double helix (twisted ladder)

• The handrails of the ladder are made of phosphate and sugar

• The rungs of the ladder are made of nitrogen bases.

Bonding

• Sugar and phosphate bond using covalent bonds (strong) and nitrogen bases bond using hydrogen bonds (weak)

• Nitrogen bases are specific: Adenine (A) always attaches to Thymine (T); and Cytosine (C) always attaches to Guanine (G)

• (Think apples in trees, cars in garages)

DNA Replication

A review:

A cell will make an exact copy of itself during a process called mitosis

Before the cell can copy itself it must copy all of it’s DNA so that both daughter cells have the same number of chromosomes.

DNA replication takes place in 3 steps

• 1. DNA unwinds at the hydrogen bonds

• 2. nucleotides are added to the exposed nitrogen bases. It follows base pairing rules. (A –T and C-G)

• 3. The DNA winds back up producing 2 new strands.

What’s the complementary strand?

• If one side is CAG, what is the complementary strand?

• Answer: GTC

• If one side is AAC, what is the complementary strand?

• Answer: TTG

RNA vs DNA

Characteristic RNA DNA

Strands 1 2

Sugar molecule Ribose Deoxyribose

Bases A, U, C, G A, T, C, G

Found Nucleus and cytoplasm

Nucleus only

Building block Nucleotide Nucleotide

Kinds of RNA

• There is only one kind of DNA, but there are 3 kinds of RNA.

– Messenger RNA (mRNA)

– Transfer RNA (tRNA)

– Ribosomal RNA (rRNA)

http://onlinetc.its.brooklyn.cuny.edu/Core81/chap5.html#jump2

Gene Expression

DNA (double-stranded)

RNA (single-stranded)

Protein (amino acid chain)

Transcription Translation

Central Dogma of Molecular Biology

http://www.wappingersschools.org/RCK/staff/teacherhp/johnson/visualvocab/mRNA.gif

Transcription= taking DNA and making an RNA copy of it. Uses complementary

nucleotides. C=G, G=C, T=A, A=U

The Genetic Code

• Remember that messenger RNA – mRNA- is a copy of the DNA. It carries instructions for making a protein.

• The instructions (nucleotides) have to be translated into proteins (amino acids).

• A sequence of three nucleotides is called a codon, and it corresponds to an amino acid.

• We use a codon chart for translation

•

Genetic Mutations

Mutation• A mutation is any change in a genetic code (DNA).• It may not change the resulting amino acid chain and

is called a silent mutation• EXAMPLE: Notice how the resulting a.a. sequence is

the same in spite of the change:– Original DNA: CAA CCC AAA– Resulting mRNA: GUU GGG UUU– Resulting amino acid: Val – Gly - Phe– Mutated DNA: CAA CCC AAG– Resulting mRNA GUU GGG UUC– Resulting amino acid: Val – Gly – Phe

Point Mutation or Substitution Mutation• A point mutation is a change in one nucleotide. It

can be silent or it can cause a change in one amino acid

• Can be devestating if the resulting amino acid has a STOP codon inserted as a result– Original DNA: ATG CCC AAA– Resulting mRNA: UAC GGG UUU– Resulting amino acid: Tyr – Gly - Phe– Mutated DNA: ATG ACC AAA– Resulting mRNA UAC UGG UUU– Resulting amino acid: Tyr – Trp – Phe– Mutated DNA: ATT CCC AAA– Resulting mRNA: UAA GGG UUU– Resulting amino acid: Stop

Insertion and Deletion Mutations• Insertion – addition of one or more nucleotides• Deletion – deletion of one or more nucleotides• Insertion and deletion mutations are almost

always devastating because it will cause a frame-shift to occur.

• Imagine if a sentence of 3-letter words lost a letter?

• Original sentence: The cat and dog are fat.• A mutation occurs that deletes the c in cat.• Mutated sentence: Thc ata ndd oga ref at.• The same things happen to DNA when it mutates

Frame Shift Mutation

• Original DNA: CAT AGC TAG GAT• Resulting mRNA: GUA UCG AUC CUA • Resulting amino acid: Val–Ser-Ile-Leu • Mutated DNA: CAA GCT AGG AT• Resulting mRNA: GUU CGA UCC UA

• Resulting amino acid: Val-Arg-Ser-?

Mendel’s Theory and Studying Heredity

The Flower

Mendelian Theory of Heredity

• 2 sets of chromosomes = 2 copies of each gene

• Alleles—alternative forms of a gene– Purple vs. white flowers

• Dominant allele—trait that is expressed or seen.

• Recessive allele—trait that is not seen. To be seen, must be present in 2 copies.

Terms and Notations

• Dominant alleles = capital letters

• Recessive alleles = lower case letters

• Dominant allele is always written first

• Homozygous individual—alleles are the same (EX: PP or pp)

• Heterozygous individual—alleles are different (EX: Pp)

Terms and Notations Continued

• Genotype—set of alleles, or genes.

• Phenotype—physical appearance of a trait.

Genotype PhenotypePP purple flowers, homozygous

dominant

Pp purple flowers, heterozygous

pp white flowers, homozygous recessive

Punnett Squares

• Diagram that predicts the outcome of a genetic cross

• Top represents one parent, side the other

• Letters in the box indicate the possible genotypes of the offspring.

Example Punnett Square

Y y

Y

y

Yy—heterozygote

Yy—heterozygote

YY Yy

Yy yy

¼=YY homozygous dominant

2/4=Yy heterozygous

¼=yy homozygous recessive

Inheritance of Traits

• Pedigree—family history that shows how a trait is inherited over several generations.

• Helps identify carriers of genetic disorders

• Carriers—individuals who are heterozygous for a genetic disorder but do not show symptoms—can pass the mutant allele to their offspring

Example Pedigree

Male

Female

Male with disorder

Female with disorder