BIOL 2320: Genetics
First Year Biology Review
Kelsie Doering
kelsie.doering@kpu.ca
Genetics Vocabulary
• DNA = is packaged into chromosomes
• Genes = are specific sequences of DNA that
encode proteins
• Alleles = different versions of the same gene
• Locus = the specific location of a gene on
the chromosome (plural = loci)
• Chromatin = is DNA wrapped around histone
proteins
• Chromosomes = consist of chromatin
• Genome = all the genes + non-coding DNA
stored in the nucleus
• Maternal = inherited from mom
• Paternal = inherited from dad
Allele for purple flowers
Homologous
Chromosomes
Locus for flower-colour gene
2
Allele for white flowers
DNA and DNA Replication
Learning Objectives:
1. Describe the Central Dogma of Biology
2. Define nucleotide and describe each of its molecular components.
3. Describe the cell cycle and specify the events that occur at interphase (G1, S,
G2).
4. Describe the semi-conservative method of DNA replication including: (a) the
role of helicase and DNA polymerase (b) leading and lagging strands. Explain
the significance of this process.
Central Dogma of Biology
• ​DNA provides directions for its own
replication in the nucleus​
• DNA directs synthesis of messenger RNA
(mRNA) and, through mRNA, controls
protein synthesis​
• Protein synthesis occurs on ribosomes
in the cytoplasm
• This is the Central Dogma of Biology =
genetic information flows from DNA
to RNA to protein​
4
Nucleic Acids
• DNA and RNA are composed of monomers
called nucleotides​
• Nucleotide = pentose sugar + nitrogenous base
attached to 1’-carbon of sugar + phosphate
group attached to 5’-carbon of the sugar
• Nucleotide monomers are joined by phosphodiester
linkages into polymers via dehydration reactions
• Joins the sugar of one nucleotide (OH on 3’C) to
the phosphate group of the next
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C
DNA
C
G
G
• DNA = has deoxyribose sugar and the nitrogenous bases A, G, C, T
• DNA acts as a code for RNA
• Double stranded nucleic acid polymer
• Two polynucleotide strands are held together by hydrogen
bonds between nitrogenous bases​ (more stable than RNA)
• These are called base pairs
• A pairs with T (2 H-bonds)
• G pairs with C (3 H-bonds)
G
G
C
C
Base pair
T
A
A
T
C
G
A
T
DNA
T
double helix
C
G
C
A
G
C
G
A
A
T
Polynucleotide
T
T
A
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The Cell Cycle
• Two major phases of the cell cycle:
1. Interphase = cell growth and DNA
replication
• Split into 3 parts; Gap 1, Synthesis,
Gap 2
• Cell growth occurs in all 3, but DNA
replication only in S phase
2. Mitotic Phase = karyokinesis and
cytokinesis (cell division)
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Synthesis Phase
• Homologous Chromosomes = the two copies of
each chromosome in diploid cells; one from egg
and other from sperm
• Each has the same genes, just may be different
alleles
• Sister Chromatids = the identical copies of a
chromosome formed following DNA replication
during S phase
• BEFORE S phase = each chromosome has 1
chromatid
• AFTER S phase = each chromosome has 2
chromatids
One
Chromatid
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DNA Replication: Key Terms to Know
• Origin of replication – initial point of DNA unwinding
• Helicase – unwinds DNA​
• RNA primer – short complimentary RNA strand required for binding of DNA
polymerase​
• DNA polymerase III – adds DNA nucleotides to growing daughter strands​
• DNA polymerase I – replaces RNA nucleotides from primer​
• Leading strand – Built continuously in 5’-3’ direction growing toward replication
fork​
• Lagging strand – Built in fragments in 5’-3’ directions growing
away form replication fork​
• Okazaki fragment – small fragments of DNA produced in lagging strand​
• DNA Ligase – binds together nucleotides of completed Okazaki fragments
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DNA Replication
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Mitosis and Meiosis
Learning Objectives
1. Compare and contrast mitosis and meiosis with respect to: (a) the pairing
and behaviour of chromosomes (b) the outcomes of each process (c) their
functional roles in living organisms.
2. Describe the behaviour of chromosomes during 4 stages of mitosis.
3. Describe the behaviour of chromosomes during 8 stages of meiosis.
4. Explain the significance of the first meiotic division including the role of
crossing over in producing genetic variation.
Mitosis
vs.
Meiosis
Both types of nuclear division, that involve one replication of chromosomes
• Happens in somatic cells
• Happens in germ (sex) cells
• For proliferation
• For sexual reproduction
• 1 division (4 stages)
• 2 divisions (8 stages)
• Makes 2 identical daughter cells
• Makes 4 unique daughter cells
• Chromosome number same (diploid)
• Chromosome number halved (haploid)
• No genetic variation (hopefully!)
• Lots of genetic variation
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Mitosis
• Mitosis = karyokinesis (division of DNA in the cell) and cytokinesis (division of
cytoplasm and its contents) to produce two genetically identical daughter cells
from one mother cell
• 4 stages:
1. Prophase (including prometaphase)
2. Metaphase
3. Anaphase
4. Telophase (& Cytokinesis)
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Meiosis
• Meiosis = the replication (once) and division (twice)
of DNA in the cell, to produce four genetically
unique daughter cells from one mother cell
• 8 stages:
• Prophase I = where crossing over occurs
• Metaphase I
• Anaphase I
• Telophase I (& Cytokinesis)
• Prophase II
• Metaphase II
• Anaphase II
• Telophase II (& Cytokinesis)
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Meiosis – Crossing Over in Prophase I
• Crossing over = non-sister chromatids of homologous chromosomes exchange DNA segments
• This forms new allele combinations (adds to why each gamete is unique)
• Crossing over can occur in multiple places for each chromosome pair (species
dependent), and the spots it occurs in are (mostly) random
•
•
“parental chromosome”
A, B, and C represent genes found at particular spots on the chromosome, with capital and lowercase
letters for different alleles of each gene
DNA is broken at the same spot on each homologue and reconnected in a criss-cross pattern so that the
homologues exchange part of their DNA
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Meiosis – Crossing Over in Prophase I
(before DNA
replication)
(after DNA
replication)
(resulting gametes)
• A, B, C, D, E are different genes
• Above is the resulting 4 gametes of one cross over event during prophase I between
genes B and C
• 2 of the 4 gametes will be parental – were not involved in the cross over
• 2 of the 4 gametes will be recombinants – were involved in the cross over
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Transcription and Translation
Learning Objectives
1. Describe the process of transcription and specify the molecules involved
including RNA polymerase.
2. Describe the process of translation and specify the roles of: (a) messenger
RNA (b) ribosomes (c) transfer RNA.
3. Describe each of the following in relation to the genetic code: (a) genetic
code table (b) size of the code words (c) chain initiation and chain
termination.
Transcription and Translation
• Transcription = synthesis of RNA using
information in DNA (in nucleus)
• Produces single stranded pre-mRNA,
which is processed to mature
messenger RNA (mRNA)
• Translation = synthesis of a polypeptide,
using information in the mRNA
• Ribosomes are the sites of
translation (in cytoplasm)
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Transcription: 1. Initiation
• 1. Transcription initiation = RNA polymerase attaches to the promoter
• Promoter = a short nucleotide sequence that forms the transcription start
point
• DNA strands unwind (via helicase) and RNA synthesis begins
RNA polymerase
Terminator
DNA
DNA of gene
Promoter
DNA
1
Initiation
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Transcription: 2. Elongation
• 2. RNA elongation = RNA polymerase adds RNA nucleotides in 5’ to 3’ direction
• Nucleotides can only be added to the 3’ end of the growing mRNA
• The mRNA strand peels away from the DNA template
2
Elongation
RNA Polymerase
Growing
RNA
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Transcription: 3. Termination
• 3. Termination = RNA polymerase detaches when it reaches a short sequence of
nucleotides called a terminator
• The mRNA molecule is released and the DNA strands rewind
Growing
RNA
3
Completed
RNA
Termination
Terminator
RNA
polymerase
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Translation
• Ribosomes translate the RNA “language” to protein “language”
• For translation, the ribosomes need:
• Messenger RNA (mRNA)
• Transfer RNA (tRNA)
• Amino acids
• Ribosomes have three binding sites for tRNA:
• A site = holds the tRNA carrying the next amino acid
• P site = holds the tRNA attached to the growing polypeptide
• E site = exit site where tRNA is discharged
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Translation: 1. Initiation
• 1. Translation initiation =
i. Small ribosome subunit binds with mRNA at the start codon (AUG = Start)
ii. A special initiator tRNA carrying the amino acid methionine binds to the
start codon (AUG codes for methionine)
iii. The large ribosomal subunit binds to the small subunit
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Translation: 2. Elongation
tRNA from P-site shifts to E-site and exits
tRNA pairs anti-codon
with mRNA codon at A
site
tRNA from A-site shifts to P-site
A-site is now ready for next tRNA
Ribosome moves one
codon along the mRNA
strand
Growing polypeptide
bonds to new amino
acid
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Translation: 3. Termination
• 3. Termination = occurs when a stop codon on the mRNA reaches the A site of the
ribosome
• (UAA, UAG, UGA = STOP)
• Release factor = a protein that recognizes the stop codon and release new
peptides from the ribosome
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The Genetic Code
• Codons (mRNA base triplets) are read in 5’ to 3’
direction during translation
• Each codon specifies an amino acid placed at a
corresponding position along the polypeptide
• 64 triplets
• 61 code for amino acids (including one “start”)
• 3 code for “stop” signals to end translation
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Mutations
• Learning Objectives
1. Define point mutation and describe each of the following types: (a)
substitution (b) insertion (addition) (c) deletion.
2. Explain each of the following effects of point mutations: (a) silent (b)
missense (c) nonsense (d) frameshift.
3. Define chromosome mutation and describe each of the following types: (a)
duplication (b) deletion (c) inversion (d) translocation.
4. Define genome mutation and describe each of the following types: (a)
aneuploidy by non-disjunction (b) polyploidy.
Point Mutations
• Point mutations = changes in just one base pair of DNA
• Divided into two categories:
a. Nucleotide substitutions = replaces one nucleotide and its partner with
another pair of nucleotides
b. Nucleotide insertions or deletions = additions or losses of nucleotides in
a gene
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Point Mutations: a. Substitutions
• Nucleotide Substitution = replaces one nucleotide and its
partner with another pair of nucleotides
• Three effects that a substitution can have on the protein
product:
i. Synonymous mutations = have no effect on amino
acid produced by codon because of genetic code
redundancy
ii. Missense mutations = code for an amino acid, but
not the correct amino acid
iii. Nonsense mutations = change amino acid codon
into a stop codon, nearly always leading to
nonfunctional protein
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Point Mutations: b. Insertions/Deletions
• Insertions and Deletions = additions or
losses of nucleotides in a gene
• Insertion or deletion of nucleotides
may alter the reading frame, thereby
producing a frameshift mutation
• These mutations have disastrous
effect on the resulting protein
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Chromosome Mutations
• Chromosome mutation = alteration in chromosome structure often resulting from
double-stranded breaks in the DNA molecule
• Typically involves a region of the chromosome that includes multiple genes
• Can be spontaneous; deletions and duplications often occur due to unequal crossing
over during meiosis
• Can be induced by exposure to high-energy radiation (eg. X-rays or gamma rays)
• Breakage of a chromosome leads to four possible types of alterations to
chromosome structure:
a. Deletion
b. Duplication
c. Inversion
d. Translocation
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Genome Mutations
• Genome mutation = duplication or deletion of a single chromosome or an entire set of
chromosomes
• Typically occur spontaneously due to nondisjunction during mitosis or meiosis
• Two types of genome mutations:
a. Aneuploidy – organism has one or more additional or missing chromosomes
b. Polyploidy – organism has one or more additional copies of ALL chromosomes
(3n, 4n, 5n, 6n etc…)
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Mendelian Inheritance
• Learning Objectives
1. Distinguish between the following sets of terms: (a) phenotype and
genotype (b) homozygous and heterozygous (c) gene and allele (d)
dominant allele and recessive allele.
2. Describe Mendel’s 3 laws of inheritance.
3. Describe the phenotypic and genotypic outcomes of Mendel’s monohybrid
crosses for: (a) the first filial generation (F1) (b) the second filial generation
(F2).
4. Describe the use of Punnett squares in predicting phenotypic and genotypic
outcomes.
Genetics Vocabulary
• Crossing = refers to controlled mating between two parents (or self-pollination of one
parent)
• P generation = parent generation
• F1 generation = first filial (offspring) generation
• Parents are from P generation
• F2 generation = second filial (offspring) generation
• Parents are from F1 generation
• True-Breeding = refers to individuals that always produce offspring with the same
phenotype as themselves when self-fertilized
• In Mendelian genetics, an organism must be homozygous for every trait for which it
is considered true breeding
• eg. Self-fertilized plant with purple flowers always produces only purple flowered
offspring
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Genotype/Phenotype
• Genotype = the genetic makeup that codes for a trait
• i.e., the allele combination that results in white or
purple coloured flowers
• Diploid organisms have pairs of alleles located on
homologous chromosomes
• Heterozygous Genotype = homologous chromosomes
have 2 different alleles for a given gene
• Homozygous Genotype = homologous chromosomes
have the 2 same alleles for a given gene
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Genotype/Phenotype
• Phenotype = the expression of the genotype as an
observable characteristic
• Characters may be physiological, anatomical,
morphological, or behavioral
• i.e., the actual colour of the flower; white or purple
• Note: phenotype can be determined by both genotype
AND environmental factors
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Mendel’s Laws
• Law of Dominance
• Some alleles are dominant while others are recessive
• An individual with at least one dominant allele will
have the dominant phenotype
• *this is for complete dominance/Mendelian
inheritance
• Law of Segregation
• Two alleles for a heritable character separate from
each other during gamete formation and end up in
different gametes
• Law of Independent Assortment
• Each pair of alleles segregates independently of any
other pair of alleles during gamete formation
Remember: when writing out
dominant vs. recessive alleles, best to
denote the lowercase recessive allele
with a line on top of the letter eg. s
Punnett Squares
• Punnett Squares = determine all possible genotypes and phenotypes of offspring
resulting from parents with known genotypes and phenotypes
eg. P generation: Aa X
A
If Mom = Aa
½ eggs = A
½ eggs = a
A
a
If Dad = Aa
½ sperm = A
½ sperm = a
Aa
AA
Aa
a
Aa
aa
Genotypic Ratio:
1AA : 2Aa : 1aa
Phenotypic Ratio:
3 Dominant : 1 Recessive
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Hybrid Crosses
• Monohybrid Cross = offspring possibilities resulting from cross for which both parents
are heterozygous for ONE gene (a “hybrid”)
• eg. Pp x Pp
• F1 Genotypic Ratio = 1:2:1
• F1 Phenotypic Ratio = 3:1
• Note: PP x pp is NOT a monohybrid cross! This will give an F1
genotypic ratio of 100% Pp and F1 phenotypic ratio of 100 dominant
• Dihybrid Crosses = offspring possibilities resulting from cross of TWO genes for which
both parents are heterozygous at both genes
• F1 Genotypic Ratio = 1:1:2:2:4:2:2:1:1
• eg. PpYy x PpYy
• F1 Phenotypic Ratio = 9:3:3:1
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X-Linked Inheritance
• Males only have one X-chromosome so only one allele copy; they always express the
phenotype of that one allele
• Females must inherit the recessive allele from both parents to have the recessive
phenotype
• Females can be carriers (Ie. Heterozygous); males cannot
Sex
Genotype
Alleles
Phenotype
Male
Dominant
X NY
Dominant
Male
Recessive
XnY
Recessive
Female
Homozygous
Dominant
XNXN
Dominant
Female
Heterozygous
(Carrier)
XNXn
Dominant
Female
Homozygous
Recessive
XnXn
Recessive
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