Chapter 13: Meiosis
• Asexual reproduction: single individual is parent
o Produces genetically identical offspring
o Some multicellular organisms, but mainly single cellular
• Sexual reproduction: two parents
o Unites genes from both parents
o Sperm cell and egg cell combine to form zygote
o Greater genetic variation
• Somatic cells: all body cells except gametes
• Gametes: reproductive cells (sperm and egg)
• Diploid (2n): two copies of each chromosome
o Homologs: pairs of chromosomes
o Tetrads: paired homologs
o Somatic cells are diploid (46 chromosomes in humans)
• Haploid (n): only one copy of each homologous chromosome
o Gametes are haploid (23 chromosomes in humans)
• Meiosis: diploid -> haploid, fertilization: haploid -> diploid
• Sex chromosomes determine sex, autosomes are all the other chromosomes
o In humans… XX is female, XY is male
Meiosis
• Mother cell is diploid somatic cells
• 2 successive divisions: Meiosis I and Meiosis II
• produces 4 haploid daughter gamete cells
• DNA replicates in interphase
• Cohesins: proteins that hold sister chromatids together after interphase
• Meiosis I
o Produces 2 haploid cells
o Each cell has 1 chromosome, but 2 chromatids
o Prophase I
§ Chromosomes condense
§ Homologs line up side by side
§ Cross over
• Non-sister chromatids broken at chiasmata
• Synaptonemal complex: holds homologs together
• Synapsis: DNA breaks are repaired such that the DNA
from one homolog is joined to the DNA from another
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§ Nuclear envelope breakdown
§ Spindle fiber formation
o Metaphase I
§ Homologous chromosomes line up midway between poles
o Anaphase I
§ Homologs separate, sister chromatids stay together
o Telophase I
§ Chromosomes complete movement to poles
§ Nuclear envelope formation
o Cytokinesis I
§ Cell divides
Meiosis II
o Produces 4 haploid cells (2 for each of the cells produced by meiosis I)
o Each cell has 1 chromosome and 1 chromatid
o Same as mitosis, but there is no DNA synthesis between MI and MII
Mechanisms to Increase Genetic Variation
• Independent assortment of chromosomes
o Homologous pairs line up randomly during Metaphase I
o Number of possible combinations is 2n where n = haploid number
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These two arrangements of
chromosomes at metaphase I are
equally probable.
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Crossing Over
o Produces recombinant chromosomes which combine DNA from each
parent into a single chromosome
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Random Fertilization
o Any sperm can fuse with any egg to produce a zygote
o Each zygote has a unique genetic identity
Meiosis Worksheet
Stage of Cell Cycle
G1
Interphase
S
G2
What Happens
Gap or Growth: Replace organelles etc. split
during cell division
Synthesis; Make an exact copy of DNA via
replication
Gap or Growth; Prepare materials for meiosis
# of Chromosomes
46
46
46
Nuclear envelope breaks apart; Chromosomes
Prophase I
condense. Homologous chromosomes pair up 46
and crossing over occurs.
Chromosomes line up in homologous pairs.
Metaphase I
46
Independent assortment occurs.
Meiosis I
Homologous chromosomes separate.
Homologous Anaphase I
46
Chromosomes
Chromosomes move further apart; Nuclear
Telophase I
envelopes reform; and Chromosomes
46
decondense.
Physicial cell division; Cytoplasm, and
23/cell
Cytokinesis I
organelles are divided into 2 cells.
46/ total
Nuclear envelope breaks apart; Chromosomes 23/cell
Prophase II
condense
46/ total
Chromosomes line up on the metaphase
23/cell
Metaphase II
plate, specifically here in a single file straight
46/ total
line.
Meiosis II
Chromosomes separate, specifically here
46/cell
Anaphase II
Sister
sister chromatids separate.
92/ total
Chromataids
Chromosomes move further apart; Nuclear
46/cell
Telophase II
envelopes reform; and Chromosomes
92/ total
decondense.
Physicial cell division of each cell; Cytoplasm, 23/cell
Cytokinesis II
and organelles are divided into 2 cells.
92/ total
Meiosis begins with one cell and end with 4 cells. Each of the new cells have 1/2 the genetic information
cell.
Meiosis II occurs in each of the cells generated in Meiosis I.
Meiosis occurs in sex cells.
Replication occurs one time in meiosis during the S phase.
Crossing over is the exchange of genetic material between homologous chromosomes.
Independent assortment is random alignment of homologous chromosomes on the metaphase plate.
Meiosis II is the "same" events as in mitosis.
Mitosis vs. Meiosis
Mitosis
• Each daughter cell contains exact
same genetic info as mother cell
• DNA replicates in interphase
Prophase
• Chromosomes condense
• Nuclear envelope breakdown
• Spindle fiber formation
Metaphase
• Single line of chromosomes at
center of cell
Anaphase
• Sister chromatids separate
Telophase
• Chromosomes complete
movement to poles
• Nuclear envelope formation
Cytokinesis
• Cell divides
Meiosis I
• Each daughter cell contains
exactly half of the genetic
information of the mother cell
• DNA replicates in interphase
Prophase I
• Chromosomes condense
• Homologs line up side by side
• Sister chromatids cross over
• Nuclear envelope breakdown
• Spindle fiber formation
• Lasts longer than in mitosis
Metaphase I
• Line of homologous pairs of
chromosomes at center of cell
Anaphase I
• Homologs separate
• Sister chromatids stay together
Telophase II
• Chromosomes complete
movement to poles
• Nuclear envelope formation
Cytokinesis I
• Cell divides
Chapter 14: Mendel and the Gene Idea
• Gregor Mendel: “father” of genetics
o Studied in late 1850s
o Largely unnoticed until rediscovered in early 20th c after his death
• Character: feature that differs between individuals
• Trait: differences in character
• Gene: a heritable factor that causes a trait
• True-breeding: all individuals are identical and self fertilization gives rise to
offspring like parents, generation after generation (homozygous)
• Mendel’s experiments
o Studied inheritance of 7 characters that existed in 2 forms
o Tracked 3 generations
o Pea plants
• Hybridize: produce offspring between genetically different strands
• Monohybrid cross: mating following one character
• Parental generation (P)
• First filial generation (F1): offspring of P mating
• Second filial generation (F2): offspring of F1 mating
• Results of monohybrid crosses:
o F1: all offspring display one trait (dominant trait)
o F2: both traits are displayed in 3:1 ratio (dominant:recessive)
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Mendel’s explanation:
o Each individual has two copies of each gene
o Alleles: different variants of the gene
o In an individual with both alleles, only the dominant allele is expressed
o The recessive allele will only be expressed if both alleles are recessive
Phenotype: physical attributes
Genotype: genetic makeup
Homozygous: both alleles are the same
Heterozygous: two alleles are different
Punnett square: diagram of outcomes of fertilization
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Probability = (# of outcomes with a particular result) / (# of possible outcomes)
Mendel’s Law of Segregation (actually a theory not a law)
o There are different variants of some heritable factor that causes a trait
o Only one allele contributes to the character
o Each gamete only has one allele
o Each individual receives one allele from each parent
Scientific theory: explanation of a phenomenon that cannot be observed
directly
Dihybrid cross: follows two characters
Mendel’s Law of Independent Assortment
o Genes for a pair of traits are distributed independently of one another
when gametes are formed
o This only applies if the genes are on separate chromosomes
o Segregation of alleles for one gene does not affect segregation of alleles
of a second gene
Incomplete dominance: heterozygote has a phenotype intermediate between
the homozygous phenotypes
Codominance: both alleles are expressed
Multiple alleles: gene has more than 2 alleles
o An individual only has two alleles
o There are more than two alleles in the population
Epistasis: genotype at one gene affects phenotypic expression of a second gene
Polygenic inheritance: multiple genes affect the same character have additive
effects on phenotype
Pedigree analysis: diagrams to follow traits through families using pedigrees
Prenatal diagnosis: sample of fetal tissue from amniotic fluid or fetal portion of
placenta analyzed for chromosomal defects or DNA mutations
Chapter 15: Genes and Chromosomes
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Chromosome Theory of Inheritance (early 20th c): genes reside on
chromosomes
Law of Independent Assortment: alleles separate independently of each other
during metaphase I
Law of Segregation: 2 alleles separate during gamete formation such that each
gamete has only one allele
Linked genes: genes that are close together on the same chromosome
o Don’t exhibit independent assortment
Thomas Hunt Morgan
o Studied fruit flies
o Wildtype: individual with normal phenotype
o Discovered sex linked traits: involve genes on sex chromosomes
Sex chromosomes: X and Y determine sex of zygote
o Females have XX
§ Barr body: inactivated X (all females have one)
o Males have XY
Autosomes: all other chromosomes
You can measure the relative distances between genes on the same
chromosome based on the percent recombination
o 1% recombination = 1 map unit = 1 centimorgan
genes that are very far apart on the same chromosome obey the law of
independent assortment
aneuploidy: too few or too many copies of a chromosome caused by
nondisjunction
o nondisjunction: error in chromosome distribution during meiosis
deletion: portion of chromosome is missing
duplication: portion of chromosome is repeated
inversion: segment within chromosome is reversed
translocation: segment of chromosome is moved to another non homologous
chromosome
DON’T HAVE TO KNOW HOW TO USE CHI SQUARE TEST!!!
Chapter 16: the molecular basis of inheritance
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Genes are made of DNA
DNA Structure
• Polynucleotides: chains of nucleotides
• Sugar phosphate backbone with nitrogenous bases as appendages
• Nucleotides bound by covalent bonds
• Double helix: double stranded molecule that spirals around
o Watson and Crick discover, win Nobel prize 1962
• Antiparallel strands: 5’ end of one strand bonds to 3’ end of other strand
• Nitrogenous bases bound to each other by h bonds
o A—T
o G—C
o Purine bonds to pyrimidine
• Base pair: a pair of complementary nucleotides
DNA Synthesis
• Two strands separate and each strand acts as the template for synthesis of the
complementary strand
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Results in 2 identical double-stranded molecules
DNA polymerase: enzyme that adds nucleotides in DNA synthesis and
proofreads for incorrect bases
o Nucleotides only added at free 3’ end
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Nucleotide that is added is a triphosphate, it loses two Pi molec. when it’s
added making it a monophosphate
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Replication fork: bubble where new nucleotides are added, expands until entire
molecule is replicated
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Helicase: binds at the replication fork and breaks h bonds between base pairs
to make the DNA into two template strands
Topoisomerase: protein that binds ahead of the replication fork and breaks
covalent bonds in the sugar phosphate backbone to relieve over winding in the
double helix
Single strand binding proteins bind after the replication fork to prevent new h
bonds from forming between the base pairs, this keeps the two parental
strands separate so DNA can be synthesized on them
Primase adds primer
o DNA can’t act on a single strand alone; it needs a few nucleotides to
already be present (these nucleotides are called the RNA primer)
o The primer is later removed by DNA polymerase
DNA ligase fills the gap where the RNA primer used to be and connects Okazaki
fragments
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Leading strand: synthesized continuously
Lagging strand: synthesized discontinuously
o Okazaki fragments: short fragments of DNA on lagging strand
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Excision: a nuclease enzyme recognizes and removes a mispaired nucleotide
Repair: DNA polymerase fills in the gap using an undamaged strand as a
template
Circular chromosomes(bacteria) have one origin of replication
Linear chromosomes (eukaryotes) have many origins of replication
Each chromatid has one molecule of DNA
o When DNA replication occurs during s phase, there are two chromatids
and therefore two molecules of DNA
o After mitosis each cell has only one chromatid and one molecule of DNA
Chromatin: combination of DNA and nucleosomes
o DNA is wound around nucleosomes like beads on a string
o Histone: small basic proteins
o Nucleosome: complex of 8 histones
Telomere: end of a chromosome
o Every time a DNA molecule is synthesized the end will get shorter by the
amount equal to the length of the RNA primer
Overview of DNA Synthesis
Chapter 17: Gene Expression
DNA---(transcription)---> RNA ---(translation)---> protein
RNA Structure
• Usually single stranded
• Contains ribose sugar instead of deoxyribose sugar
• Contains uracil nitrogenous base instead of thymine nitrogenous base
Transcription
• Transcription: synthesis of RNA from a DNA template
o DNA strands are separated and RNA is synthesized complementary to
one of the strands
o RNA processing: RNA molecules are modified in the nucleus to produce
the three types of RNA (mRNA, tRNA, and rRNA)
• Transcription units: shorter segments that are transcribed (genes)
• Spacer dna: dna between transcription units that is never transcribed
• RNA polymerase: enzyme that carries out transcription
o Reused over and over again to create multiple mRNA’s
• Each gene has a start and stop signal
o Promoter: dna sequence that is start signal (TATA box)
o Terminator: dna sequence that is stop signal
• Transcription is a lot like dna synthesis except the rna detaches from the
template strand and the two dna strands rejoin
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RNA processing
o Capping: modified guanine nucleotide is added to 5’ end of RNA
§ Necessary for ribosome attachment and increases stability
o Polyadenylation: 50-200 adenosine nucleotides added to 3’ end
§ Increases stability
o RNA splicing: internal segments of RNA molec are removed
§ Intron: removed segments
§ Exon: retained segments (remember the exons are expressed)
§ Spliceosome: large complex of proteins and small RNAs where
splicing occurs
Translation
• Translation: synthesis of polypeptide from an mRNA template in cytoplasm
• Messenger (mRNA): produced by transcription
o Read by rRNA to make protein
o Template to synthesize polypeptide
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o Codon: 3 nucleotides that codes for 1 amino acid
Ribosome: molecule that carries out translation
o Composed of several proteins and 3 rRNA molecules
o 2 subunits join together during translation
Transfer (tRNA): adapter molecule in translation
o Folded into lowercase t shape
o Aminocyl tRNA synthase: enzyme that attaches amino acid to tRNA
o Anticodon: 3 nucleotide region that base paires with the codon on
mRNA during translation
Codon: 3 nucleotide sequence in mRNA molecule
o Indicates which amino acid should be added to a polypeptide
o Multiple codons can code for the same amino acid
o Often the third base in a codon doesn’t matter
Initiation: ribosome begins translation at start codon (methionine)
Elongation: ribosome moves along mRNA strand reading each codon, tRNA
attaches amino acid for each codon onto chain
Termination: ribosome reads a stop codon and detaches from the mRNA and
polypeptide
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Both transcription and translation require nrg
Mutations
• Substitution: one nucleotide is substituted for another
o Missense mutation: change in codon causes change in amino acid
o Nonsense mutation: codon changed to stop codon causing
premature termination
o Silent mutation: nucleotide change has no effect on amino acid
sequence because the codon was changed to another codon that
codes for the same amino acid
• Frameshift: insertion of 1 or 2 bases causes all following codons to be read
wrong
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Mutagen: chemical or environmental condition that causes mutations
Germline mutations: occur in gametes, passed on to next gen
Somatic mutations: occur in body cells, maybe cancerous