What is DNA?

Hershey and Chase—scientists that discovered DNA

Blueprint of living organisms Can produce a variety of species with

a common body plan A complex polymer Stands for deoxyribonucleic acid

DNA location

DNA is found in every cell, in every nucleus of every cell, and makes up each CHROMOSOME

Remember: Humans have 46 chromosomes, 23 pair, that code for all traits

DNA structure:

DNA structure is called a double-helix (looks like a twisted ladder) Formed by 2 strands of nucleotides

bonded together in the middle (Called the Watson and Crick model)

DNA components:

DNA is made up of NUCLEOTIDES linked together

1 nucleotide = 1 phosphate, 1 sugar and 1 nitrogen base

The SIDES of DNA are made up of phosphates and sugars alternating

(P-S-P-S-P-S)

DNA Components:

The steps/rungs of the ladder structure are made up of pairs of nitrogen bases

The four nitrogen bases are: Adenine, Thymine, Cytosine,

Guanine*The bases always pair as follows: A –T and C—G

DNA Components:

Base pairs are held together in the middle by HYDROGEN BONDS-which are weak bonds

*hydrogen bonds break and reform when DNA replication occurs

DNA Nucleotides

The sequence of the nucleotides determines the traits of the organism it composes

All organisms are made of DNA with the same four bases (A, T, C, G)

The ORDER of the bases determines the characteristics of the organism

DNA Replication

Before a cell divides, Replication must occur

Otherwise, offspring would have half the DNA of their parent cells

Replication: DNA’s ability to make a copy of itself (identical to the original)

this happens in the nucleus of the cell

Steps of Replication: 1. The DNA double strand unzips and

untwists (beginning at one end) 2. Free floating nitrogen bases (A, T, C and

G) assemble themselves along each side of the unzipped strand according to base-pairing rules

3. An enzyme glues the new base pairs in place

4. Each strand re-twists and re-coils

RNA

RNA—ribonucleic acid mRNA—messenger RNA—used in

forming proteins Single-stranded Contains ribose not deoxyribose Replaced thymine w/URACIL (there are

no T’s in RNA)

Genetic Mutations (DNA)

Mutation—any mistake or change in the DNA sequence

May be caused by errors in making proteins, or cell division or by external agents (carcinogens, etc.)

CODON: a 3-base unit of a DNA strand—each codon codes for a particular protein

Mutations-cont’d. Some mutations affect reproductive

cells or gene in an organism. If the altered sperm or egg is fertilized, the mutation would then be inherited by offspring

Some gene mutations have positive effects (plant variations)

Some mutations effect genes that control cell division (cancer)

Mutations that occur during translation

Point mutation—a change in a single base pair in DNA (would effect that one codon only, and therefore one protein)

Frameshift mutation—if a single base were lost or added (would effect all codons read from that point on)

Shifts all codons up or down a base (more harmful than a point mutation)

Chromosomal Mutations: Chromosomal mutations: structural

changes in chromosomes, more common in plants

Homologous chromosomes do not pair correctly when one chromosome has extra or missing parts, so separation of the chromosomes does not occur normally

Gametes could have extra copies or lack genes

Causes of Mutations

Some causes are unknown Some are environmental Mutagens: any agent that can cause

a change in DNA Ex: radiation, chemicals, even high

temps

Mendel and Heredity

Mendel—Austrian monk that studied pea plants to understand inheritance of traits

He carefully cross-pollinated pea plants to determine certain characteristics being passed from parent plant to offspring

He was the first person to succeed in predicting how traits are passed from one generation to the next

Heredity: the passing of characteristics from parents to offspring

Traits: characteristics that are inherited

Genetics: the branch of biology that studies heredity

Hybrid: the offspring of parents that have different forms of a trait ex: (tall and short height)

*Mendel’s first experiments were called MONOHYBRID CROSSES because they tested for a single trait’s inheritance

Mendel concluded each organism has 2 factors that control each of its traits

Genes: pieces of a chromosome (pieces of DNA) that code for each trait

Genes exist in alternative forms called ALLELES

Alleles/Genes/Traits

Alleleles: are located on different copies of a chromosome-one inherited from a female parent and one inherited from a male parent

The two forms of alleles: Dominant and Recessive

Dominant: (T) the observed allele/trait Recessive: (t) the “masked” allele/trait Dominant traits are those we actually observe,

since they “mask”/overpower the recessive trait

Homozygous: both alleles are the same (can be dominant or recessive-TT or tt)

Heterozygous: both alleles are different (one of each-Tt)

Homozygous Dominant: TT-Tall Homozygous Recessive: tt-short Heterozygous: Tt-Tall

*The only time the recessive allele is expressed, is when the alleles are homozygous recessive

Phenotype: the physical appearance of an organism-how it looks and behaves: (ex: tall, short, etc.)

Genotype: the allele combination of an organism-the actual letters (ex: Tt, TT, tt)

Punnett Squares Punnett Square: A way of showing possible

genotypes of offspring of two particular parents, for a certain trait

Monohybrid cross-tests outcome of only 1 trait

T t

T

T