Cell Division and Genetics – Mechanisms for a Knit of Identity and Thread of Distinction

DNA and Its Faithful Replication – The Knit of Identity

Because DNA stores genetic information and is faithfully replicated, information is passed largely unaltered from cell-to-cell, generation-to- generation.

Proteins and Their Production – The Primary Reason for DNA

Cell Division DemandsCoordination of DNA Replication, Mitosis and Cytokinesis

What’s so important about cell division?

Cell division requires coordinated division of chromosomes (mitosis) …..

…… and division of the cytoplasm (cytokinesis).

DNA Replication – Simple in Principle, Complicated in Practice

DNA is Packaged into Chromosomes

DNA in the cell is virtually always associated with proteins.

The packaging is impressive – 2 meters of human DNA fit into a sphere about 0.000005 meters in diameter.

chromatin

duplicatedchromosome

The Link Between DNA Replication and Chromosome Duplication

DNA is Condensed into Visible Chromosomes Only For Brief Periods in the Life of a Cell

95% of the time, chromosomes are like this.

Easily visible chromosomes are apparent perhaps 5% of the time in an actively growing cell and less in a non-growing cell.

A Karyotype is an Arranged Picture of Chromosomes At Their Most Condensed State

A normal human karyotype

Boy or girl?

Note that almost all chromosomes come in homologous pairs.

From Birth to Rebirth, a Cell Progresses Through Characteristic Stages That Constitute the Cell Cycle

In multicellular organisms like us, progress through the cell cycle is carefully regulated.

Cancer Is One Outcome of A Runaway Cell Cycle

Licentious division - prostate cancer cells during division.

The Knit of Identity - Mitosis Precisely and Evenly Divides Duplicated Chromosomes

Precisely dividing the duplicated chromosomes has the consequence of providing each new cell with an identical and complete set of genetic instructions.

interphase prophase metaphase

Mitosis Precisely and Evenly Divides Duplicated Chromosomes

Cytokinesis is the process of cell division and it is distinct and separable from mitosis.

Mitosis in Action

Blue shows DNA, green shows spindle fibers.

In Animal Cells, a Cleavage Furrow Forms and Separates Daughter Cells

Cleave furrow in a dividing frog cell.

The Plant Cell Wall Forces Cytokinesis to Play by Different Rules

Part 1: Cell Division

• Cell division is the cornerstone of life

• Genome: an organisms complete set of genetic material (DNA)

Prokaryotic Chromosomes

Bacteria & viruses

A single DNA (or RNA in some virusis) moleucle that contains all the genetic information for the individual.

Chromosomes (colored bodies)

Prokaryotes have a single, circular chromosome

– Prokaryotes (bacteria) reproduce through cell division called Binary fission

– Circular chromosome (DNA) is replicated

– Replicated chromosomes are attached to the cell membrane at nearby sites

– As membrane expands, the copies separate

– New cell wall forms between copies, cell splits

Eukaryotic Chromosomes

DNA & protein structures with only part of the DNA information. All of the chromosome together make up the genome.

Mitosis

– Eukaryotes use cell division to create new cells

– MITOSIS: division of the nucleus. Responsible for Growth, Maintenance and Repair

Type of Asexual cell division

Chromosomes (colored bodies) • Chromosome made of a DNA and protein complex:

chromatin. Following DNA replication, a chromosome contains two sister chromatids attached by a centromere

Human Chromosomes: 46 – 46 chromosomes (2n = 46)

• 2 sets of 23 chromosomes (diploid)

– n = number of chromosomes in a set

– Fundamental number, haploid number

– n = contribution from each parent

– 23 = 1 sex chromosome, 22 autosomes

Mitosis and Interphase Alternate

• The cell cycle

– Cells are in interphase from 75-90% of the time

– G1 = Gap 1 (cell growth)

– S = chromosome synthesis (duplication)

– G2 = Gap 2 (cell growth)

– M = Mitosis

Chromosomes cont.

Homologous pair (2n, diploid) in parent cell

•Chromosomes are replicated during S-phase of cell cycle. Chromosomes and copies are separated during mitosis.

One of each pair to daughter cell

Copies

Mitosis is a Continuum

Prophase Chromatin coils into visible chromosomes

Under a light microscope, only the nuclear envelope (with nucleoli) and a tangle of chromatin are visible

Pro-metaphase (late prophase)• Nuclear envelope breaks

• Microtubules attach centromere to centrioles

Metaphase Chromatids align on a plane at cell’s equator

Anaphase • Chromatids separate simultanously

• Sister chromatids become daughter chromosomes

Telophase Daughter chromosomes stop moving

Chromosomes uncoil, nucleus and nucleoli reformresulting in two new, identical cells.

Cytokinesis • Division of cell cytoplasm after mitosis is known as

cytokinesis

Phases of the Cell Cycle

• The cell cycle consists of– Interphase– Mitosis– Cytokinesis

INTERPHASE

G1

S(DNA synthesis)

G2Cyt

okin

esis

Mito

sis

MITOTIC(M) PHASE

Figure 12.5

• Heredity– Is the transmission of traits from one generation to the

next

• Variation– Shows that offspring differ somewhat in appearance

from parents and siblings

Figure 13.1

• Genetics– Is the scientific study of heredity and hereditary

variation– Offspring acquire genes from parents by inheriting

chromosomes

Inheritance of Genes• Genes

– Are the units of heredity– Are segments of DNA

• Each gene in an organism’s DNA– Has a specific locus (location) on a certain

chromosome

• We inherit– One set of chromosomes from our mother and one

set from our father

Comparison of Asexual and Sexual Reproduction

• In asexual reproduction (mitosis)– One parent produces

genetically identical offspring by mitosis

Figure 13.2

Parent

Bud

0.5 mm

• In sexual reproduction– Two parents give rise to offspring that have unique

combinations of genes inherited from the two parents

• Genes are located on Chromosomes

• Individuals inherit two sets of genes (chromosome) – one from each parent

• These “Matching” chromosomes are called Homologous chromosomes, because they carry genes for the “same” (homo) traits– Have genes for the same characteristics– Are also be called autosomes

• Sex chromosomes– Are distinct from each other and carry genes for

different triats– We all inherit two sex chromosomes, one from each

parent– represented as X and Y – Determine the sex of the individual

• XX = female,

• XY = male

• A diploid cell (2n)– Has two sets of each of its chromosomes– Human have 46 chromosomes (2n = 46)

• We have a “Problem” in sexual reproduction

– How can we have 46 chromosomes in our cells, combine a cell from a father and a mother, and still have only 46 chromosomes in the offspring’s cells?

46 (father) + 46 (mother) = 46 (offspring)

– There must be a way to reduce the number of chromosomes in the parents to half the number (23 for humans)

Behavior of Chromosome Sets in the Human Life Cycle

• At sexual maturity– The ovaries and testes produce special haploid (n)

cells by meiosis– The cells are called gametes– Gametes, sperm and egg cells are haploid cells,

containing only one set of chromosomes– In humans, the haploid (n) number is = 23, ½ of our

diploid number (2n)

• Meiosis reduces the number of chromosome sets from diploid to haploid

• Meiosis– Takes place in two sets of divisions, meiosis I and

meiosis II

• Meiosis I– Reduces the number of chromosomes from diploid to

haploid

• Meiosis II– Produces four haploid daughter cells

An Overview of Meiosis

Figure 13.7

Interphase

Homologous pairof chromosomesin diploid parent cell

Chromosomesreplicate

Homologous pair of replicated chromosomes

Sisterchromatids Diploid cell with

replicatedchromosomes

1

2

Homologous chromosomes separate

Haploid cells withreplicated chromosomes

Sister chromatids separate

Haploid cells with unreplicated chromosomes

Meiosis I

Meiosis II

• Meiosis I

– Reduces the number of chromosomes from diploid to haploid

• Meiosis II

– Produces four haploid daughter cells

• The haploid reproductive cells are the gametes

• Meiosis only occurs in the sex organs – Males testes to produce sperm– Females ovaries to produce eggs

• Gametes are not produced until the individual reaches puberty and special hormones “kick” in and start the process– Males testosterone– Females estrogen & progesterone

Newborn (2n)

Growth Development Maintenance

Repair

Gametogenesis

Adult (2n)

Growth Development Maintenance

Repair

Mitosis

GametesEgg (n)Sperm (n)

Meiosis

Zygote (2n)

Embryo (2n)

MitosisGrowth

Development Maintenance

Repair

MitosisGrowth

Mitosis

• Reshuffling of genetic material in meiosis– Produces genetic variation

Origins of Genetic Variation Among Offspring

• In species that produce sexually– The behavior of chromosomes during meiosis and

fertilization is responsible for most of the variation that arises each generation

Independent Assortment of Chromosomes

• Homologous pairs of chromosomes– Orient randomly at metaphase I of meiosis

• In independent assortment– Each pair of chromosomes sorts its maternal and paternal homologues

into daughter cells independently of the other pairs

Figure 13.10

KeyMaternal set ofchromosomes

Paternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubule attached tokinetochore

Tetrads line up

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicate

Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

• Interphase and meiosis I

Figure 13.8

TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still double

Haploid daughter cellsforming

Figure 13.8

Telophase I, cytokinesis, and meiosis II

Figure 13.9

MITOSIS MEIOSIS

Prophase

Duplicated chromosome(two sister chromatids)

Chromosomereplication

Chromosomereplication

Parent cell(before chromosome replication)

Chiasma (site ofcrossing over)

Prophase I

Tetrad formed bysynapsis of homologouschromosomes

Metaphase

Chromosomes positioned at themetaphase plate

Tetradspositioned at themetaphase plate

Metaphase I

Anaphase ITelophase I

Haploidn = 3

MEIOSIS II

Daughtercells of

meiosis I

Homologuesseparateduringanaphase I;sisterchromatidsremain together

Daughter cells of meiosis II

n n n n

Sister chromatids separate during anaphase II

AnaphaseTelophase

Sister chromatidsseparate duringanaphase

2n 2n

Daughter cellsof mitosis

2n = 6

MEIOSIS I

• A comparison of mitosis and meiosis

A Comparison of Mitosis and Meiosis

• Meiosis and mitosis can be distinguished from mitosis by three events in Meiosis l– Crossing over: homologous chromosomes physically connect

and exchange genetic information

– paired homologous chromosomes (tetrads) are positioned in the middle of the cell

– At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell

In anaphase II of meiosis, the sister chromatids separate