Chromosomes and Cell Reproduction

Chapter 6

Chromosomes

DNA must be present in any new cell that is formed so it must be copied and distributed so each cell has a complete set

Prokaryotic Cell ReproductionSingle circular DNA attached to inner cell membrane

Binary Fission – asexual reproduction producing identical offspring

2 Stages of Binary Fission

Stage 1: DNA is copied

Stage 2: Cell divides by adding a new cell membrane between 2 DNA copies

Squeezes in middle

New cell wall forms until pinched in 2

Eukaryotic Cell Reproduction

DNA is organized into genesThese are segments that code for a protein of RNA molecule

Thousands of genes in DNA

When genes are being used the DNA is stretched out

Chromosomes

Chromosomes are the DNA and the proteins associated with it

As a eukaryote prepares to divide, chromosomes become visible (DNA is copied before this)

Chromatids – 2 exact copies of each

Chromosome attached at point called centromere

During cell division chromatids separate and are placed into new cells

Each Human somatic cell (anything besides sperm and egg) has 2 copies of 23 different chromosomes = 46 chromosomes

These 23 pairs are different in size, shape, and the genes they carry

Sets of Chromosomes

23 pairs made of 2 homologous chromosomes (a.k.a. homologues) which are similar in size and shape, and genetic content

Each Homologue comes from 1 parent

Again, you have 46 chromosomes, 23 from each parent

Diploid or Haploid?

Diploid – somatic cells contain 2 setsHaploid – gametes contain 1 setn = 1 set of chromosomes

n = 23 = human haploid #; gametes2n = 46 = human diploid #; somatic cell

Fertilization is when 2 haploid gametes fuseForms a diploid zygote (fertilized egg), the first cell of an individual

Each organism has a characteristic number of chromosomes

Some may have 1 pair, some plants have over 500

Sex Chromosomes

23 chromosome pairs in human somatic (body) cells22 of these are autosomes, not directly involved in determining genderThe other pair are sex chromosomes that contain genes that determine genderHumans have X and Y chromosome

Y chromosomes have genes that cause fertilized egg to develop into a male (XY)

Females can only donate X so sex is determined by man

Sex chromosomes are different in other organisms

Male grasshopper = X0 (0 = missing)

Female grasshopper = XX

Change in Chromosome NumberAll 46 chromosomes are needed for normal development and function

If you have more than 2 copies of a chromosome – trisomy – don’t develop properly

KaryotypePhoto of chromosomes in a dividing cell that shows them in order by size

Change in Chromosome Structure

These are called mutations

Breaking of chromosome in 4 different ways

Deletion

Duplication

Inversion

Translocation

Deletion

Piece breaks off completely

Certain genes are missing in new cell

Usually fatal to zygote

Duplication

Chromosome fragment attaches to homologous chromosome giving it 2 copies of certain genes

Inversion

Chromosome reattaches to original but in reverse order

Translocation

Reattaches to nonhomologous chromosome

6.2 The Cell Cycle

A repeating sequence of growth and division

90% of the time cells are in 1st three phases called Interphase

Last 2 phases only when it’s about to divide

5 Phases of Cell Cycle:Phases 1 through 3 = Interphase

G1 = 1st Growth PhaseCell grows rapidlyCarries out normal functionsMajor portion of lifeStay here if not dividing

S = Synthesis PhaseDNA is copiedAt end each chromosome is 2 chromatids attached by/at centromere

G2 = 2nd Growth PhasePrepares for nucleus to divideMicrotubules are rearranged preparing for next stage = Mitosis

MitosisNucleus divides into 2 nuclei with same number and kind of chromosomes

CytokinesisCytoplasm divides

These 2 stages produce new cells identical to original and allow growth, repairs and in some cases asexual reproduction

Control of Cell Cycle

Feedback information tells cell what to do

Checkpoints/Inspection PointsFeedback can trigger next phase or delay the next phase

The cycle is controlled by many proteins and has 3 principal checkpoints

G1 Checkpoint - Growth

Decides whether cell will divide

Proteins will stimulate cell to begin S phase if conditions are right

G2 Checkpoint – DNA Synthesis

DNA replication is checked by DNA repair enzymes

Once passed, proteins trigger mitosis

Mitosis Checkpoint

Triggers an exit from mitosis

Cancer

Uncontrolled growth of cells

May promote growth-promoting molecules

May inactivate off/slow switch

6.3 Mitosis and Cytokinesis

When cells divide the chromatids on each chromosome are moved to opposite sides with the help of a spindle

A spindle is a cell structure made up of both centrioles and individual microtuble fibers

Forming The SpindleCentrosomes found at cells poles organize assembly of spindleIn animal cells there are a pair of centrioles found inside each centrosome (plants don’t have centrioles)Centrioles and spindle fibers are both made of microtubules (hollow tubes of protein)Each spindle fiber is made of 1 microtubuleEach centriole is made of 9 triplets of microtubules arranged in a circle

Separation of Chromatids by Attaching Spindle Fibers

Some microtubules interact, others attach to centromere

2 sets of microtubules extend out from poles

When poles and centomeres are attached the 2 chromatids can be separated

Once separated called chromosomes

One chromosome goes to one pole while the other is pulled towards the other

They move along microtubule paths and move closer to poles as microtubules are broken down

Mitosis – 4 StagesProphase

Chromosomes coil and become visibleNuclear envelope dissolves

MetaphaseChromosomes line up along equatorChromatids linked to poles by spindle fibers

AnaphaseCentromeres divideChromosomes move toward poles

TelophaseNuclear envelope reformsChromosomes uncoilSpindle dissolves

CytokinesisCytoplasm divided in halfCell membrane grows to enclose each cellAnimal Cells – pinched in half by belt of protein threadsPlant Cells – Golgi apparatus forms vesicles that fuse at midline forming cell plate (cell wall in middle of cell)Each offspring is equal in size, amount of cytoplasm, number of organelles and has an identical copy of chromosomes