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Science STAAR
Notebook
Biology - Category 2
Mechanisms of Genetics
Structure of DNA

DNA is a nucleic acid and is made of nucleotides.

A nucleotide has 3 parts:
 Sugar (i.e. deoxyribose in DNA; ribose in RNA)
 Nitrogen base ( A, T, G, or C)
 Phosphate group

Nitrogen bases are held together by a hydrogen bond

Components that make up DNA are common to all
organisms.

DNA is a blueprint for producing proteins needed for an
organism.

DNA is held in the nucleus of a eukaryotic cell and in
the cytoplasm of a prokaryotic cell.
STAAR Question
Which of these is a nucleotide?
A1
C3
B2
D4
DNA

The part of the DNA that is referred to as
the genetic code are the nitrogen bases.
It is a universal code!!!

The DNA strand (a twisted ladder) must
separate between the nitrogenous base
pairs in order to replicate.

Genes are short segments of DNA that
code for certain proteins.
STAAR Question
The sequence of nitrogenous bases in DNA varies
widely. The sequence of the bases in DNA is most
important for which of the following?
A Providing the instructions for the traits of an
organism
B Preventing mutations from occurring during DNA
replication
C Allowing the DNA to have the shape necessary for
replication
D Helping form the sugar-phosphate backbone of
DNA molecules
Photo by Archenzo
Photo by Infrogmation
Photo by Cy
STAAR Question
The fact that a strain of yeast with a certain
defective gene can use the human version
of the gene to repair itself is evidence that
yeast and humans –
A depend on the same food supply
B share a genetic code
C both have eukaryotic cells
D have identical genomes
DNA Replication
 Enzymes unwind and unzip
the double helix
 Each strand serves as a
template for building a new
DNA molecule
 Free nucleotides bond to
the template (A-T and C-G)
forming a complementary
strand
 The final product is two
identical DNA molecules.
STAAR Question
If the template of a strand of DNA is
5' AGATGCATC 3', the complementary
strand will be —
Testing Strategy:
A 3' TCTACGTAG 5'
B 5' CTACGTAGA 3'
C 3' AGATGCATC 5'
D 5' AGACGTCTA 3'
Ignore the 5’ and 3’
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HINT
 Read questions carefully to see if they
want the DNA or the RNA.
 Remember your complementary pairs!
 DNA:
- G=C, C=G
- T =A, A=T
 RNA:
 G=C, C=G
 U=A, A=U
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DNA Mutations
 A DNA mutation is when the
sequence of nitrogen bases
have been changed.
 The damage to the DNA of
gamete cells can be passed to
an organism’s offspring.
 Mutations can be beneficial to
an organism because they may
create an advantage over other
organisms.
 Examples: insertions, deletions,
and point mutations
(substitution).
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Chromosomal Mutations
 Remember: large amounts of DNA packaged together is a
chromosome.
 A chromosomal mutation is when thousands of
nucleotides are changed.
 Examples: insertion, deletion, inversion, duplication,
translocation, and nondisjunction
Identify the following chromosomal mutations:
STAAR Question
UV radiation can cause mutations in the DNA of
skin cells that have been overexposed to the sun.
This mutated DNA has no effect on future offspring
becauseA changes in skin cell DNA are homozygous
recessive
B mutations must occur within the RNA codons
C offspring reject parental skin cells
D only changes to gamete DNA can be inherited
Gene Expression:
Building Proteins from DNA
 Transcription
• Occurs in the nucleus
• Copies certain
segments (genes) of
DNA onto an mRNA
• mRNA leaves the
nucleus to find a
ribosome
 Translation
 Occurs in the
cytoplasm with the
help of a ribosome
• Produces a chain of
amino acids that are
linked together to
form a protein from
the mRNA segment
STAAR Question
The illustration shows the transcription process.
What is the main purpose of the structure labeled W?
A Carrying instructions for protein synthesis
B Transforming into a protein
C Replacing damaged DNA
D Passing traits to offspring
Meiosis
 Meiosis is the cell division and formation of gametes (sex
cells) or spores.
 Examples of gametes: egg and sperm
 Cells undergo two divisions in meiosis to half the number
of chromosome in the nucleus
 This ensures a complete, or diploid, number of
chromosome during fertilization
STAAR Question
 If a cat has 38 chromosomes in each of
its sex cells, how many chromosomes
will be in each daughter cell after
meiosis?
A 11
B 19
C 38
D 76
Meiosis and Genetic
Variation
 Meiosis is significant to sexual reproduction
because it helps to increase genetic diversity in
organisms:
 Crossing Over: Exchange of genetic material
between homologous chromosomes. Occurs during
prophase I of meiosis.
 Random Fertilization: There is no set determination
about which sperm will fertilize the ovum (egg).
 Independent Assortment: Random arrangement of
pairs of chromosomes
Crossing Over:
STAAR Question
Crossing-over between nonsister
chromatids during meiosis is significant in
heredity. This process most likely leads to
an increase in which of the following?
A The expression of dominant traits
B The occurrence of polyploidy
C Number of gametes
D Genetic variation
Analysis of Genomes:
Karyotype - picture of the chromosomes
 Example of an abnormal karyotype: trisomy
#21 Down’s Syndrome:
Analysis of Genome:
DNA Fingerprinting
 DNA fingerprints are made using gel
electrophoresis.
 DNA is cut up using restriction enzymes than
separated by size to give you a unique
“fingerprint”:
 Segments of non-coding DNA found between
the genes (aka junk DNA) follow patterns that
vary from one individual to another.
DISCOVERING PATERNITY
USING DNA FINGERPRINTS
 In the DNA fingerprint below, the child will have bands that
match either his mom or his dad.
 The M represents mother’s DNA; C represents child’s
DNA; and AF represents possible father’s DNA.
 Question: By analyzing the results, does the evidence
from this gel indicate that the AF (alleged father) is the
father of the child?
STAAR Question
The technique of chromosome painting is the result of scientific
research. Scientists use chromosome painting to mark the
locations of genes on human chromosomes with fluorescent
tags. Its also possible to apply this technique to the
chromosomes of many different species.
Chromosome painting allows for which of the
following?
A A comparison of the genomes of different species
B The sequence of proteins from many species
C An increase in mutations in many species
D The extraction of amino acids from different species
Analysis of Genome:
Recombinant DNA
 DNA which contains genes from more than one
source, or species, is known as recombinant
DNA.
 Process of joining together fragments of DNA
is called gene splicing.
 Why make recombinant DNA?
 To make plants resistant to disease
 To make bacteria produce certain proteins for
humans that can’t, like insulin
Analysis of Genome:
Knockouts
 Organisms have one gene (piece of DNA)
broken or “knocked out”
 Making the gene inactive, scientist can
determine what the gene’s function is inside
the animal.
Genetics
 Phenotype – observable traits
 Genotype – alleles (you get one from
mom and one from dad, 2 alleles for a
trait) that the organism carries
 Homozygous – same alleles (TT, tt)
 Heterozygous – different alleles (Tt)
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Punnett Squares
 To examine what traits will be passed on to the
next generation, we use a Punnett square.
 Every child has 2 copies of EVERY gene (one
from each parent). This combination of alleles
is your genotype.
 Dominant alleles are shown by using a capital
letter.
 Recessive alleles are shown by using a lowercase letter
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How to use a monohybrid
Punnett square…
Botanists cross a heterozygous (Pp) plant having
purple flowers with a homozygous (pp) plant
having white flowers. About what percentage of
the offspring will have purple flowers?
A 0%
B 25%
C 50%
D 75%
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How to work a Dihybrid
Punnett square…
If several pea plants with the genotype TTYy are
crossed with pea plants with the genotype Ttyy,
what percentage of the offspring will be expected
to have the TTYy allele combination?
A 25%
B 40%
C 50%
D 75%
Strategy: 1. Make 2 Punnett Squares
2. Find the percentage for each
Punnett square
3. Multiply the percentages
(practice this on the calculator!!!!!)
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Co-Dominant Genes
 Both genes are expressed (made into
proteins.)
 Blood types are a perfect example of this:
IB
i
IB IBIB IBi
i
IBi
ii
i
i
IA
IA
IA
i
IA IAi
IAi
i
IAi
IAi
IA IAIA IAi
IB IBi
IBi
i
IAi
IAi
IB IAIB IBi
Incomplete Dominance
 Occurs when a combination of genes produces
a mixture of the two traits.
 One trait is not completely dominant, so neither
trait is completely expressed.
Co-Dominant or Incomplete
Dominance
Which breed shows codominance between
feather color and which shows incomplete
dominance?
Incomplete Dominance
A mixture of traits
appears in the offspring.
Codominance
Both traits appear in the
offspring.
Sex-Linked Traits
 Humans have 23 pair of chromosomes
 The 23rd pair on a karyotype are the “sex
chromosomes”:
 XX or XY
 Sex-linked traits are carried on the X
chromosome.
 Example of X-linked disorder: color
blindness
Color Blindness Pedigree
This pedigree
shows how
color blindness
is passed down
in a family.
What patterns
do you notice?
Answer:
Females seem
to be carriers
while only
males seem to
get the disease!
GENETIC MODIFICATIONS
 The direct human manipulation of an organism’s genome
using modern DNA technology is called genetic
modification.
 It involves the introduction of foreign DNA into another
organism, in many cases from bacteria.
 Why would we want to genetically modify an organism?
1) Improve crops
1) ex: inserting genes that make the organism resistant to
insects.
2) Medicine
1) ex: human growth hormone and insulin are now grown in
bacteria
3) Research
1) ex: inserting a gene into bacteria allows scientists to store
large quantities of the gene and the gene product, since
bacteria reproduce so quickly.