Foundations of Biology Exam 2

How do cells divide

Four events must Occur:

Reproductive Signal

Replication

Segregation

Cytokinesis

Reproductive Signal

Step 1

To initiate cell division

Replication

Step 2

Of the Dna

Segregation

Step 3

Distribution of the DNA into two new cells

Cytokinesis

Step 4

Separation of the 2 new cells

Eukaryotic Cell Cycle

Distinct phases of the cell cycle, divided into Interphase and M phase (mitosis/cytokinesis)

Specific molecular signals trigger the transition from one phase to another

Interphase

DNA exists as long, threadlike chromatin

Begins after cytokinesis

Ends when mitosis starts

Divided into subphases: G1, S, and G2

M (mitosis) phase

Nuclear membrane dissolves

DNA condenses and divides

Cytoplasm divides

G1

each chromosome consists of one dsDNA molecule

S

DNA replication produces 2 identical dsDNA molecules (sister chromatids) for each chromosome

G2

Each chromosome consists of two associated dsDNA molecules (sister chromatids)

Mitosis Phases

Prophase/Prometaphase

Metaphase

Anaphase

Telophase

Cytokinesis

Prophase/ Prometaphase

compaction of replicated DNA into visible chromosomes: breakdown of nuclear envelope

Metaphase

Duplicated chromosomes line up in the middle of cell

Anaphase

sister chromatids separate and move to opposite sides of cell (now are daughter chromosomes)

Telophase

Decompaction and formation of new nuclear envelope around the two separated sets of daughter chromosomes

Cytokinesis

Division of cytoplasm (forms two cells)

In animal cells- a contractile ring of actin and myosin microfilaments pinches in the cell membrane

In plant cells- vesicles from the golgi apparatus appear along the plane of cell division and fuse to form a new cell membrane. contents of vesicles contribute to forming the new cell wall

Mitotic Spindle

Consists of Microtubles

Microtubules

function as the spindle fibers, which orient and move chromosomes in the dividing cell.

Microtubule Organizing Center (MTOC)

surrounded by high concentration of tubulin dimers

forms/ orients the mitotic spindle that will attach to and move the duplicated chromosomes during M-phase

Centrosome

MTOC of animal cells

consist of 2 centrioles (hollow tubes) formed by microtubules

doubling during S phase too: each will move to opposite ends of the nuclear envelope during G2-M Transition

positions determine the spindle orientation and plane of cell division

Plants

Have no centrosomes but have MTOCs

How is eukaryotic cell division controlled?

The eukaryotic cell cycle

transitions depend on activity of enzymes called Cdks= cyclin-dependent kinases

Cdk is active only when bound to its partner protein, cyclin

Cyclin

synthesized only at certain times in the cell cycle

Cdk activation occurs in the presence of cyclin

are unstable

Cyclin and Cdks

control the progression through the cell cycle

ensure that the cell is ready to move past crucial checkpoints

G1-S Cdk phosphorylates RB protein

Unphosphorylated (active) RB inhibits the cell cyle at restriction point: cell does not enter S phase

When RB is phosphorylated by G1-S cylicn-Cdk, RB is inactivated and no longer blocks the cell cucle

p16 inhibits RB phosphorylation

Types of Cell Division

Binary Fission & Mitosis

Meiosis

Binary Fission & Mitosis

DNA copied and a complete copy segregated to each daughter cell

products identical to the mother cell

division for somatic cells

Meiosis

DNA copied followed by 2 rounds of division and nuclear segregation

DNA content reduced by ½

Each Product is unique

division for gametes (sexual reproduction)

Sexual Reproduction

joining of gametes to produce a diploid phase of the life cycle, coupled with meiosis that reduces the chromosome number in the haploid phase

Meiosis specialized cell division

diploid mother cell (pairs of chromosomes) → haploid daughter cells (each with one of each kind of chromosome)

Diploid cell

has 2 complete sets of chromosomes

Pairs of chromosomes

2n

46 chromosomes

Sister chromatids

Haploid Cell

has a single set of chromosomes

has half the amount of a diploid cell

n

23 chromosomes

chromatids

Homologs

One member of each pair of chromosomes inherited from each parent

Homologous chromosomes

appear the same and contain the same genes, except for sex chromosomes

Functions of Meiosis

Reduce chromosome number from diploid to haploid

ensure each haploid has a complete set of chromosomes

Generate diversity among the daughter cells

Key Features of Meiosis

2 nuclear divisions but DNA is replicated only once

Homologous chromosomes pair and exchange genetic information then segregate from each other in meiosis I

sister chromatids separated from each other in Meiosis II

Unique Events of Meiosis I

Duplicated homologous pairs come together and pair along their entire lengths

pairing occurs during prophase 1

the four chromatids of each homologous pair form a tetrad or bivalent

Synapsis

Pairing of homologous pairs of chromosomes during prophase 1

can lead to crossing over between non-sister chromatids

After Meiosis I

Homologous pairs separate

maternal and paternal centromeres of each pair segregate to opposite poles

cells at the end of Meiosis I are haploid, but each chromosome still contains 2 chromatids

Crossing over

can produce exchange between DNA molecules of each chromatid, contributing to genetic variation among gametes

Chiasmata

sites of exchange between non-sister chromatids

Events of Meiosis II

Duplicated cells at end of meiosis I are haploid, but each chromosome still consists of 2 chromatids

critical event: separation of the sister chromatids

sister chromatids segregate to opposite poles

Nondisjunction

Homologous pairs fail to separate at anaphase I or II

results in aneuploidy-chromosomes missing or present in access

Causes of Nondisjunction

aneuploidy is sometimes caused by lack of cohesions that hold the homologous pairs together. without cohesions the pairs separate randomly

failure to undergo crossing over

frequency of nondisjunction goes up as a female ages

Consequences of errors in meiosis

• If both homologs go to the same pole and the resulting egg is fertilized

  • Trisomy 21→ down syndrome

  • Trisomy 18→ edwards syndrome

• Fertilized egg that does not receive a copy of a particular chromosome will be monosomic

  • lethal in all situations except turner syndrome (having a single X chromosome)

Independent Assortment

haploid sets of chromosomes inherited from parents mixed by segregation of homologs during Meiosis I

Trisomic

3 chromosomes on 1 side

Monosomic

1 chromosome only and on 1 side

Mendel's laws

Law of Segregation

Law of Independent Assortment

Law of Segregation

Two alleles of a gene separate and are transmitted individually and equally to gametes

Gene

sequence within a DNA molecule and resides at a particular site on a chromosome (locus)

Law of Independent assortment

Alleles of different genes assort independently during gamete formation

Thomas Hunt Morgan

Established the drosophila melanogaster as a model for genetic studies

Discovered linkage

alleles of separate loci were transmitted together to offspring,

Linkage

alleles of separate loci are transmitted together to offspring

Frequency of crossing over between 2 linked genes

Proportional to the distance between them

Frequencies of recombinant gametes, and resulting offspring are greater for loci that are farther apart

recombinant frequency= # of recombinants/ # of total offspring

Max recom. frequency is 0.5

Heterozygotes

2 different alleles of a particular gene

Homozygotes

2 identical alleles of a particular gene

Absolute linkage

is rare

Linkage group

All of the loci on a chromosome form this

Genetic Maps

Recombinant frequencies can be used to make these

shows the arrangement of genes along a chromosome

Sex chromosome

the gene(s) with primary control of sexual development

Autosomes

Other chromosomes that are not sex chromosomes

Y chromosome

carries few genes

X chromosome

carries many genes involved in a variety of functions

Hemizygous

has 1 copy of a gene

Complete dominance

The allele defined as dominant completely masks the effect of the recessive allele

used to define dominant allele and recessive allele

Incomplete dominance

both alleles of a gene at a locus are partially expressed

results in an intermediate phenotype

Co-dominance

phenotypes of both alleles appear in the heterozygote

Epistasis

Phenotypic expression of one gene is influenced by genotype of another gene

Probability rules

two events occur together use multiplication

two events occur separately use addition

Single gene disorders

caused by a mutant allele of a single gene

results in a change in phenotype

rare in the general population

Examples of single gene disorders

recessive disorders

dominant disorders

recessive disorders

both alleles have to be mutant

examples of recessive disorders

albinism, cystic fibrosis, sickle cell anemia, muscular dystrophy

dominant disorders

one mutant allele is enough

examples of dominant disorders

huntington disease, achondroplasia

Mendelian traits

single gene, affecting discrete phenotypic differences

Quantitative phenotypic variations

height, weight, skin pigmentation, hypertension, type II diabetes, asthma

Frederick Griffith

physician trying to find a vaccine for pneumonia

Griffith’s study

found 2 strains of bacterium streptococcus pneumoniae

Characterized them as smooth (S) and rough (R)

Griffith’s findings

S strain was dangerous and the R strain was not

S strain had polysaccharide capsule around them while R strain did not

Avery et al

Did experiments to identify the transforming principle

Avery et al’s study

Treated S strain bacteria to selectively destroy different types of macromolecules

Avery et al’s findings

R strain was still transformed when S-RNA or S-Protein was destroyed but was not transformed if S-DNA was destroyed

Hershey and Chase (1952)

Used bacteriophage to explore what controlled inheritance, DNA or proteins

Hershey and Chase’s study

Grew cultures of virally infected bacteria with radioactive Phosphate or Sulfate

32PO4 or 35SO4

Hershey and Chase’s experiment

DNA contains lots of PO4 but no SO4, so if phage transferred PO4 to the bacteria, then DNA was hereditary particle

Opposite for protein

Hershey and Chase’s findings

DNA contained the information needed to make the next generation of phage

Chargaff’s rule

A+G=C+T

Rosalind Franklin

Suggested that DNA is spiral or helical molecule

Watson and Crick

combined all the knowledge of DNA to determine its structure

The double helical structure of DNA

antiparallel strands

polarity of the strands is determined by the sugar-phosphate bonds

Phosphate groups

connect to the 3’ C of one sugar and the 5’C of the next sugar

2 DNA chain ends differ

One is a free 5’ phosphate group (5’ end) and the other is a 3’ hydroxyl group (3’ end)

Base pairing

5’→3’ strand paired with the 3’←5’ strand

5’ paired with 3’ of other strand

Major and minor grooves

result of base pairing, phosphate backbones are closer together on one side of the double helix than the other

Major grooves

atoms available for hydrogen bonding with other molecule are more accessible here

A and T

Minor grooves

Have C and G