Chapter 2 Chromosome transmission during cell division and sexual reproduction

General features of chromosome

Chromosomes are structures within living cells that contain the genetic material

contain genes

What is chromosomes composed of

DNA and Proteins

DNA

genetic material

Proteins

provide an organized structure

eukaryotes DNA-protein complex

chromatin

Two types of cells

Prokaryotes and Eukaryotes

general features of Prokaryotes

No Nucleus

singular type of circular chromosome

Where is the chromosome found in the prokaryotes

region of the cytoplasm called nucleoid

What is the cytosol enclosed by in prokaryotes

plasma membrane

regulates nutrient uptake and waste excretion

What do prokayotes might have

outer membrane and flgellum

Outer membrane for prokayotes

rigid cell wall

protection from breakage

General features of Eukaryotes

membrane bound nucleus

linear chromosomes

contain specific membrane bound organelles 

What specific membrane bound organelles do Eukaryotes contain?

Mitochondria

Lysosomes

Golgi apparatus 

Mitochondria

ATP synthesis

Lysosomes

Plays a role in degradation of macromolecules

Golgi apparatus

Play a role in protein modification and trafficking

Two types of animal cells

Somatic and Germ

Somatic cells

Body cells other than Gametes

Germ cells 

Gametes

Sperm and egg cells

Cytogenetics

involves study of chromosomes

looks at karyotypes

Diploid species

have two sets of chromosomes

human diploid count 

46 chromosomes(23 per set)

Homologs

members of a pair of chromosomes

homologous pair

two homologs

two chromosomes in a homologous pair

nearly identical in size

same banding pattern and centromere location 

same genes but not same alleles 

DNA sequence on homologous chromosomes 

less than 1% difference between homologs

Genotype AA

homozygous for dominant allele

Genotype for BB

Heterozygous

Genotype for CC

Homozygous for the recessive allele

sex chromosomes X and Y

not homologous

differ in size and genetic composition

short regions of homology 

Two homologous chromosomes labeled with 3 different genes

Physical location of a gene on a chromosome is called locus

Asexual reproduction

unicellular organisms produce new individuals

such as binary fission

both purposes of cell division

asexual reproduction

achieving multicellularity

Binary fission steps overview

1. Mother cell with bacterial chromosome

2. replication of bacterial chromosome

3. z-ring formed from FtsZ filaments

4. Septum

5. two daughter cells 

How fast does E.coli divide

20 minutes

What happens prior to cell division in binary fission

the bacterial cell replicates its chromosome

Binary fission

cell divides into two daughter cells

does not involve genetic contributions from two different gametes

Eukaryotic cells progress through a cell cycle

divide progress through a series of stages known as the cell cycle

interphase

In actively dividing cells, G1 (Gap 1), S and G2

G₀ phase

cell may remain for long periods of time

what happens in G₀ phase

Either postponed progression through the cell cycle

Or made the decision to never divide again

Why would a cell in the cell cycle not divide again

Terminally differentiated cells( like adult nerve cells)

G1 phase

cell prepares to divide

reaches a restriction point and committed on a pathway to cell division 

S phase

chromosomes are replicated

chromatids happen

joined at centromere to form dyad 

chromatid 

two copies of a replicated chromosome

dyad 

pair of sister chromatids

End of S phase

cell has twice as many chromatids as there are chromosomes in the G1 phase

human cell example for G1 and S phase

46 distinct chromosomes in G1 phase

46 pairs of sister chromatids after S phase

chromosome in G1 and late M phase 

refers to the equivalent of one chromatid

Chromosome in G2 and early M phase

refers to a pair of sister chromatids joined at the centromere

During the G2 phase

e cell accumulates the materials that are necessary for nuclear and cell division

What occurs in M phase 

mitosis

What is the primary purpose of mitosis

to distribute the replicated chromosomes to the nuclei of two daughter cells

What is the purpose of cytokinesis

to divide the mother cell into two daughter cells in humans

Mitosis and Cytokinesis

The 46 pairs of sister chromatids are separted and sorted

Each daughter cell receives the same complement of chromosomes 

each daughter cell receives 46 chromosomes (or 46 chromatids)

Mitosis is subdivided into 5 phases

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

example of mitosis

original mother cell that is diploid (2n)

contains total of six chromosomes

3 chromosomes per set n=3

Interphase

Chromosomes decondense

end of interphase, chromosomes already replicated (6 pairs of sister chromatids are not seen until prophase)

centrosome, the attachment point of the mitotic spindle divides 

Spindle apparatus 

needed for M phase

microtubules are formed by rapid polymerization of tubulin proteins 

3 types of spindle microtubules 

Aster microtubules

Polar microtubules

Kinetochore microtubules

Aster microtubules

Important for positioning of the spindle apparatus 

Polar microtubules

Help to push the poles away from each other 

Kinetochore microtubules

attached to the kinetochore, which is bound to centromere of each individual chromosome

Prophase

Nuclear envelope dissociates into small vesicles

Chromatids condense into more compact structures

centrosomes begin to separate

Prometaphase

centrosome move to opposite ends of the cell forming the spindle poles

Spindle fibers interact with the sister chromatids; spindle apparatus forms 

kinetochore microtubules grow from the two poles 

Prometaphase after kinetochore microtubules grow from the two poles 

if they contact a kinetochore, sister chromatid is captured

if not the microtubule depolymerizes and retracts to the centrosome

Two kinetochores on a pair of sister chromatids are attached to kinetochore microtubules on opposite poles

Metaphase

Pair of sister chromatids align themselves along a plane called the metaphase plate

Each pair of chromatids(dyad) is attached to both poles by kinetochore microtubules 

Anaphase

connection holding sister chromatids together is broken

each chromatid, now an indivdual chromsome, is linked to only one pole

As Anaphase proceeds

Kinetochore microtubules shorten

Chromosomes move to opposite poles

Polar microtubules lengthen

Poles themselves move further away from each other 

Telophase

Chromosomes reach their respective poles and decondense

Nuclear membrane reforms to form two separate nuclei 

Entire process of mitosis steps

Interphase

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Interphase basic

chromosomes are extended and uncoiled, forming chromatin 

prophase basic

chromosomes coil up and condense; centrioles divide and move apart

prometaphase basic

chromosomes are clearly double structures; centrioles reach the opposite poles; spindle fibers form 

Metaphase basic

Centromeres align on metaphase plate

Anaphase basic

Centromeres split and daughter chromosomes migrate to opposite poles

Telophase basic 

Daughter chromosomes arrive at the poles; cytokinesis commences 

What is mitosis followed by

cytokinesis 

Cytokinesis in animals

Formation of a cleavage furrow

Cytokinesis in plants

Formation of a cell plate

Outcome of mitosis and cytokinesis

produce two daughter cells that have same number and complement of chromosomes as mother cell

Outcome of mitotic cell division

two daughter cells are genetically identical to each other

ensures genetic consistency

development of multicellularity relies on repeated process of mitosis and cytokinesis 

Meiosis

Parents(diploids) make gametes with half the amount of genetic material(haploid)

gametes fuse with each other during fertilization to create a new diploid individual

A diploid human cell

contains 46 chromosomes

A human gamete

contains only 23 chromosomes

Gametes

1n

diploid cells

2n

During meiosis

haploid cells are produced from diploid cells

chromosomes must be correctly sorted and distributed to reduce the chromosome number to half its original value

Meiosis 1 and 2 phases

Prophase

Prometaphase

Metaphase

Anaphase

Telophase

Meiosis vs Mitosis 

both begin after a cell has advanced through interphase

meiosis involves two successive divisions to reduce chromosome content

Prophase of meiosis 1

chromosomes begin to condense

Homologous chromosomes pair together in a process called synapsis to form a bivalent

DNA is exchanged between the homologous pair at various loci, in a process called crossing over. Number of chiasmata varies 

synapsis

Homologous chromosomes pair together

bivalent

two pairs of sister chromatids (also known as a tetrad containing four chromatids or monads)

crossing over

DNA is exchanged between the homologous pair at various loci, or chiasmata

Prometaphase of Meiosis 1

Nuclear envelope is completely broken apart

spindle apparatus formation is complete

spindle apparatus formation is complete in prometaphase of meiosis 1

Spindle fiber microtubules link sister chromatids to each pole by attaching to kinetochore proteins located at the centromeres of each sister chromatid

NOTE: each pair of sister chromatids is only linked to one pole, which is different from mitosis 

Metaphase 1 Meiosis 1

Bivalent align along the metaphase plate

Random arrangement of homologs along the plate is result in independent assortment and provides a mechanism to promote genetic diversity

Anaphase 1 Meiosis 1

Two pairs of sister chromatids within a bivalent separate from each other

Spindle fibers attached to each homolog shorten, pulling each to opposite poles

Meiosis 1 Telophase 1

Sister chromatids have reached respective poles

DNA decondenses back into chromatin

The nuclear envelope reforms around each haploid set of DNA

A cleavage furrow forms to divide the cell in half, marking the beginning of cytokinesis

Two non-identical daughter cells result from the division

Meiosis 2

Meiosis 1 is followed by cytokinesis and then meiosis 2