Chromosomes and Cell Reproduction

Chapter 6

chromosomesSection 6-1

Bacterial cells divide to reproduce Bacteria reproduce by binary fission Binary fission is a form of asexual

reproduction that produces identical offspring

There are two stages: DNA is copied Bacteria divides

Eukaryotic cells form chromosomes DNA is organized into individual units called

genes A gene is a segment of DNA that codes for a

protein or RNA As a cell preps to divide, DNA is copied and coils

up into chromosomes The two exact copies of DNA that make up a

chromosome are called chromatids The chromatids are attached at a point called a

centromere

Chromosome Number and Structure Affect Development Each human somatic cell (non-sex

cell) has 2 copies of 23 chromosomes Each of the 23 pairs of chromosomes

consist of 2 homologous chromosomes Homologous chromosomes have a

similar size, shape, and genetic content Each homologue comes from each

parent

Chromosomes A somatic cell is diploid, because it contains 2

sets of chromosomes A gamete, which has one set of chromosomes, is

said to be haploid The fusion of 2 haploid cells is called fertilization

and results in a zygote or a fertilized egg

Chromosomes determine sex Autosomes are chromosomes that aren’t

involved in determining gender The sex chromosome is 1 of 23 pairs of

chromosomes and determines gender Y = Boy, X= Girl Males carry XY, females carry XX

Change in Chromosome number

The presence of all 46 chromosomes is essential for normal development

The Cell CycleSection 6-2

The Cell Cycle The cell cycle is a repeating sequence

of cellular growth and division A cell spends 90% of its time in the first

3 phases of the cycle called interphase There are 5 phases of the cell cycle

Cell Cycle1. First Growth (G1) phase: cells grow

rapidly and carry out routine functions. If a cell isn’t dividing, it remains in G1

2. Synthesis (S) phase: DNA is copied. 3. Second Growth (G2) phase: Nucleus

prepares to divide4. Mitosis: nucleus of cell is divided5. Cytokinesis: Cytoplasm divides

Cell Cycle is Controlled The cell has checkpoints and feed back

signals that trigger the next phase of the cell cycle

Proteins control the cell cycle in eukaryotes

Cell Cycle is Controlled Cell Growth (G1) checkpoint: this

checkpoint decides if the cell will divide. Nerve and muscle cells remain in this stage

and never divide DNA Synthesis (G2) checkpoint: DNA

replication is checked by repair enzymes and triggers mitosis

Mitosis Checkpoint: this triggers the exit from mitosis and signals the beginning of growth cycle

When Control is Lost Certain genes make the proteins that

regulate cell growth Mutations can disrupt the regular

function of these proteins Cancer is the uncontrolled growth of

cells It is a disorder of cell division

Mitosis and CytokinesisSection 6-3

In Mitosis, Chromatids are Pulled by Microtubules During mitosis, the nucleus divides

into two nuclei, each contains a complete set of chromosomes

Spindles are cell structures made of centrioles and microtubules that move chromosomes during division

Forming the spindle Animal cells have centrioles that sit

at a right angle to each other Centrioles are replicated to make 2

pairs Centrioles move to opposite poles of

the cell and spindles form Made of microtubules

Plant cells do not have centrioles but still form spindles

Separating chromatids by attaching spindle fibers The spindles attach to the poles of the

cell and to the centromere The chromatids may now separate

The spindle fibers break down by the pole and “reel” the chromatids closer to the pole

Mitosis1. Prophase: chromosomes coil up and become

visible while spindles form2. Metaphase: chromosomes line up at center of

cell and the spindle fibers attach to the chromatids and poles

3. Anaphase: centromeres divide and chromatids move toward the poles

4. Telophase: a nuclear envelope forms around the chromosomes at the pole and spindles disappear

Cytokinesis As mitosis ends, cytokinesis begins Cytoplasm is divided in half and cell membranes

grow to enclose each cell Animal cells use proteins to pinch the membrane

in half Plant cells fuse vesicles together to make a cell

plate and cell walls form on each side to allow cell to split