Download DNA - Palmer ISD

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 BONDSwhich 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 basepairing 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