BSC2010 Exam 2

gene

unit of heritable information

why do individuals resemble each other

1. they share genes

2. they share environments

function of reproduction

1. transfer of information between parent and daughter cells or between parent and offspring in multicellular organisms

2. Apportionment of organelles

asexual (clonal) reproduction

1. daughters genetically identical to parent

2. ALL prokaryotes, many eukaryotes

3. binary fission in prokaryotes and mitosis in eukaryotes

sexual reproduction

1. parents combine genes

2. leads to recombination = daughters genetically different from parents

3. ONLY in eukaryotes

binary fission

centromere

region of chromosome where chromatids are held together

kinetochore

protein complex that forms on centromere where spindle microtubules attach

centrosome

microtubule organizing center of the cell

centriole

one of two barrel shaped structures within centrosome

stages of cell cycle

G1 (or G0), S, G2, M

interphase consists of

G1, S, G2

Interphase (G1)

• Each chromosome is a single, unreplicated, double strand of DNA

• One chromosome from each parent forms a homologous pair

• Chromosomes in nucleus surrounded by membrane

• Single centrosome

Interphase (S)

• After DNA replication, two “sister chromatids” are present for each homolog

• Each sister chromatid is the same double stranded DNA molecule

• The two sister chromatids are attached by proteins

• Centrosomes duplicate

Interphase (G2)

• Organelles duplicate

• Prepares for mitosis

Mitosis (M) Phase

Prophase, Prometaphase, Metaphase, Anaphase, Telophase

Early Prophase

• Chromosomes begin to condense

• Mitotic spindle forms from centrosome

• Centrosomes begin to migrate to poles of cell

• Nucleoli disappear

Mid Prophase

• Chromosomes fully condensed

• Centrosomes complete migration to the poles

• Nuclear envelope begins to degrade

• Spindle fibers enter nuclear area from poles

• Kinetochores form at centromeres

Prometaphase

• Nuclear envelope completely degraded

• Spindle fibers begin to attach at kinetochores

• Sister chromatids attached to opposite poles

Metaphase

• Spindle fibers attached to centromere at kinetochore

• All sister chromatids attached to opposite poles

• Chromosomes migrate to center plane of cell, “metaphase plate”

Anaphase

• Protein bond between sister chromatids degrades

• Sister chromatids separate, migrate toward opposite poles

• Poles move farther apart as non kinetochore spindle fibers (polar microtubules) lengthen

Telophase

• Non-kinetochore (polar) spindle fibers continue to elongate cell

• Nuclear envelope begins to form at poles

• Chromosomes decondense back into chromatin

• Nucleoli re-form, cytokinesis begins

Cytokinesis in Animal Cells

Cytokinesis in Plant Cells

Short-term function of meiosis

Create haploid cells from diploid cells

Ultimate function of meiosis

Generate genetic variation through recombination by which natural selection can act

Diplontic Life Cycle

Haplontic Life Cycle

Alteration of Generations (Diplo-Haplontic) Life Cycle

Interphase 1 (G1) Phase

• Begin with two homologs chromosomes (n pairs)

• DNA content = 2c

• Ploidy = 2n (diploid)

Interphase 1 (S) Phase

• Chromosomes replicate

• DNA content = 4c

• Ploidy = 2n

Early Prophase 1

• Synapsis occurs as homologs form tetrad

Late Prophase 1

• Chromosomes cross over, form chiasmata

• Exchange of DNA between homologs occurs at chiasma

• Spindles form and attach to kinetochores as in mitosis

Metaphase 1

• Chromosomes lined up on metaphase plate in homologous pairs

• Spindles from one pole attach to one chromosome of each pair

• Spindles from the other pole attach to the other chromosome of the pair

Anaphase 1

• Homologous chromosomes separate and move along spindle fibers toward pole

• Sister chromatids remain attached at centromeres

• RECOMBINATION HAS OCCURRED

Meiosis 1 - Telophase and Cytokinesis

• Homologous chromosomes reach opposite poles

• Each pole has complete HAPLOID complement of chromosomes

• Each chromosome consists of two sister chromatids

Prophase 2

• Spindle forms

• Chromosomes move toward metaphase plate

Metaphase 2

• Chromosomes reach metaphase plate, as in mitosis

• Kinetochores of sister chromatids attach to spindle fibers from opposite poles

Anaphase 2

• Centromeres of sister chromatids separate

• Sister chromatids move toward opposite poles

Meiosis 2 - Telophase and Cytokinesis

• Same mechanism as Meiosis 1

• Four haploid daughter cells from each parent cell

• Some chromosomes are recombinant and others are not

Cell Checkpoints

G1-S, S, G2-M, M

How do cyclins and CDKs regulate the cell cycle?

• CDKs are ALWAYS expressed

• Different cyclins are expressed at certain points in the cell cycle

• Binding of cyclin to CDK allows CDK to phosphorylate something by exposing its active site

• This phosphorylation can repress things that work to slow the cell cycle or halt it

Mendel’s Law of Segregation

If an individual has identical alleles at a locus, that allele is present in all its gametes. If an individual has two different alleles at a locus, half its gametes receive one allele and half receive the other allele.

Mendel’s Law of Independent Assortment

Alleles at each locus segregate independently of alleles at other loci, except for when genes are very close together on the same chromosome

What is a test cross?

When an individual with unknown genotype is crossed with an individual with a homozygous recessive genotype.

Incomplete Dominance

When heterozygous individuals have a trait that is an intermediate of the dominant and recessive traits. EX: If HH = Red and hh = Blue, Hh = Purple.

Co-Dominance

Heterozygotes have a completely different characteristic compared to homozygous dominant and recessive. Heterozygotes are not an intermediate. Ex: ABO blood types.

Pleiotropy

One gene affects multiple characters

Epistasis

Expression of one gene modifies the expression of another gene. Ex: Coat color in mice.

Polygenic Inheritance

Multiple traits control one phenotype

Transformation

Bacterium takes up free floating extracellular DNA (often from lysed cell) while in a state of competence

Transduction

When bacteriophage interacts with bacteria it can pass DNA in that is incorporated into host genome

Conjugation

Sex pilus between bacteria is used to transfer genetic information

Chargaff’s Rule

A=T and G=C in all DNA

Hershey-Chase Experiment

Put radioactive isotopes of sulfur and phosphorus in bacteriophages then blended and centrifuged them to see where the radioactivity is. Found that DNA is put into the cell, not proteins, and that DNA carries genetic info.

Meselson-Stahl Experiment

Synthesized one strand of DNA with heavier isotope of nitrogen and let bacteria replicate twice with lighter nitrogen and proved that DNA replication is semi-conservative.

Direction of DNA synthesis

5’ to 3’

Topoisomerase Function

Relives strain from super-coiling ahead of DNA replication fork

DNA Helicase Function

Unwinds/ Unzips DNA helix

Single Stranding Binding Proteins (SSBs) Function

Prevents DNA strands from rewinding as DNA is being replicated

DNA (RNA) Primase

Makes short RNA primer for replication to start on

DNA Ligase

Joins Okazaki fragments

DNA Polymerase III

Main replicative enzyme that synthesizes new strands on both leading and lagging strands

DNA Polymerase I

Removes RNA primers and fills the gaps in Okazaki fragments

Telomerase

Catalyzes lengthening of telomeres in germ line and stem cells

Stages of transcription

Initiation, Elongation, Termination

Initiation (Transcription)

• RNA Polymerase binds to promoter sequence on template strand

• RNA Polymerase unwinds DNA at promoter

Elongation (Transcription)

• RNA Polymerase generates strand as bases base-pair with template strand

• Creates mRNA from 5’ to 3’ (therefore reading template strand 3’ to 5’)

Termination (Transcription)

• Transcription stops when RNA Polymerase reaches terminator sequence

• RNA Polymerase disengages with mRNA and DNA

Transcription Factors

Help RNA Polymerase bind to promoter sequence of DNA

How is the primary transcript processed in Eukaryotes

Introns taken out, 5’ GTP cap added, 3’ Poly-A Tail added

Spliceosome Function

Formed from small nuclear ribonucleoproteins (snRNPs) which recognize splice site sequence and excise it. Cuts out introns, joins exons

Ribozymes

RNA molecules with catalytic activity

Enzyme that attaches amino acids to tRNA molecules

Aminoacyl-tRNA synthetase

Initiation (Translation)

Elongation (Translation)

• Codon recognition

• rRNA ribozyme catalyzes peptide bond formation

Termination (Translation)

• Release factor protein binds stop codon and releases mRNA

  • Release factor cleaves bond between tRNA and amino acid

What is a signal peptide

• Sequence of 20 amino acids near amino acid of peptide

• Binds to receptor protein in rough ER membrane

• Growing polypeptide translocated into the ER space through membrane pore

Point Mutation

single base substitution

Silent (Synonymous) Substitution

No effect on polypeptide, mutation results in same amino acid being made

Replacement (Non-Synonymous) Substitution or Missense Mutation

Mutation results in different amino acid being put in

Nonsense mutation

Mutation results in stop codon and premature termination

Frameshift Mutation

Insertions/Deletions which change the reading frame

Tautomerization

The spontaneous, reversible shift of a proton within a DNA nuclease, changing a single bond and an adjacent double bond which changes the base’s hydrogen bonding properties and causes replication errors and point mutations

Spontaneous Deamination

Spontaneous loss of amino group which causes point mutation or replication error

Mismatch Repair vs. Excision Repair

Mismatch repair fixes normal undamaged nucleotides that were mispaired

Excision repair removes damaged or chemically altered bases

Non-Disjunction

Improper number of chromosomes being put into the gametes

Chromosomal Inversion

Chromosomal Translocation

Transposable Elements (“Jumping Genes”)

• Genetic elements insert into new locations in the genome

• Conservative transposition: “cut and paste”

• Replicative transposition: “copy and paste”

• Counts as non-homologous recombination

Transposase

Catalyzes movement of transposable element to new location in the genome

Reverse Transcriptase

Enzyme that uses an RNA template to create DNA (cDNA)

Retrotransposon Life Cycle

Composite Transposable Elements

Contain additional genetic information besides transposase like antibiotic resistance genes

Parts of an Operon

Promoter, Operator, Structural Genes, Regulatory Gene (Outside Operon)

Trp Operon

• Repressible operon (usually function in anabolic reactions)

Lac Operon

• Inducible operon (usually function in catabolic reactions)

Constitutive (Housekeeping) Genes

Genes that are always expressed

Negative Regulation

Transcription switched off by active repressor as in trp and lac operons

Positive Regulation

Transcription switched on by an activator molecule as in the lac operon