Chapter 6: Chromosomes and Cell Reproduction

Biology II

Cell Division Cell division occurs at different times

in an organism’s life– Growth and development– Repair– Reproduction

Gametes – reproductive cells (egg/sperm)

– When a cell divides, DNA is first copied and then distributed so that each cell ends up with a copy of DNA

Prokaryotic Cell Division Prokaryotes have single circular DNA

molecule Reproduce by binary fission

– Form of asexual reproduction that produces identical offspring

– Occurs in 2 stages DNA is copied Cell divides by adding a new cell membrane to a

point on membrane between the 2 DNA copies

Genes Information encoded in DNA is organized into

units called genes– Segment of DNA that codes for a protein or RNA

molecule Single DNA molecule

has 1000s of genes linked together

Genes play important role in determining how an organism’s body develops and functions

When genes are being used, the DNA is stretched out so that its information can be used to direct protein production

Eukaryotic Cell Division As eukaryotic cell prepares to divide,

chromosomes become visible – DNA and proteins associated with DNA

Before DNA coils, DNA is copied, forming chromatids– 2 exact copies of DNA

that make up each chromosome– Attached at a point called a centromere

Chromatids separate during cell division and are placed into each new cell, ensuring that each new cell will have same genetic information as the original cell

Homologous Chromosomes Each human somatic (body) cell has 2

copies of 23 chromosomes (total of 46 chromosomes) – Each pair is made up of

homologous chromosomes Similar in size, shape, and

genetic content Each homologue comes from

one of the 2 parents

– Differ in size, shape, and sets of genes– Each contains 1000s of genes

Sets of Chromosomes All cells in the body, other than

gametes, are somatic cells– Said to be diploid (2n), since they contain

2 sets of chromosomes Symbol “n” is used to represent 1 set of

chromosomes (human diploid #: 2n = 46)

– Gametes are said to be haploid (n), since they contain only one set of chromosomes

Fusion of 2 haploid gametes, known as fertilization, forms a diploid zygote– Fertilized egg cell

Numbers of Chromosomes Each organism has a characteristic

number of chromosomes– Number of

chromosomes remain constant within each species

Sex Chromosomes Humans have 22 pairs of autosomes and 1 pair

of sex chromosomes– Autosomes – chromosomes that are not directly

involved in determine the gender of an individual– Sex chromosomes – contain genes that will

determine sex of individual Referred to as X and Y chromosomes in humans and many

other organisms Sex of offspring is determined by male

– XX: Female– XY: Male

In some insects (grasshoppers), females are XX and males are XO (O = absence of chromosome)

In birds, moths, and butterflies, male is XX and female is XO

Karyotypes Presence of normal number of

chromosomes is necessary for normal development and function– Abnormalities in chromosome number

can be detected by analyzing a karyotype

Photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size

Trisomy and Nondisjunction

Trisomy is a condition in which humans have more than 2 copies of a chromosomes– Occurs if one or more chromosomes fail to separate

properly, an event known as nondisjunction– Trisomy 21 is known as Down’s syndrome

Characterized by short stature, round face with upper eyelids that cover inner corners of eyes, and varying degrees of mental retardation

More likely in pregnancies of older women because eggs can accumulate increasing amount of damage over time

Change in Chromosome Structure

Changes in organism’s chromosome structure are called mutations– Breakage of chromosomes can lead to 4

types of mutations Deletion mutation – piece of chromosome

breaks off completely Duplication mutation – chromosome fragment

attaches to its homologous chromosome Inversion mutation – chromosome piece

reattaches to original chromosome but in reverse orientation

Translocation mutation – chromosome piece reattaches to a nonhomologous chromosome

Section 6-2The Cell Cycle

The Cell Cycle Cell division more complicated in

eukaryotes because it involves dividing chromosomes, cytoplasm, and other organelles– Cell cycle: repeating

sequence of cellular growth and division during life of an organism

90% of cell’s time is spent in first of 3 phases, collectively called interphase

Cell enters last 2 phases only if it is about to divide

Stages of Interphase First growth phase (G1): cell grows rapidly

and carries out routine functions– Cells remain in this phase until they prepare to

divide– Some cells (nerve/muscle cells) never divide

Synthesis phase (S): cell’s DNA is copied– At end of this phase, each chromosome consists

of 2 chromatids attached at a centromere Second growth phase (G2): preparations

are made for nucleus to divide and microtubules are rearranged in preparation for mitosis

Stages of Cell Division Mitosis: process during cell division in

which nucleus of cell is divided into 2 nuclei– Each nucleus ends up with the same number

and kinds of chromosomes as original cell Cytokinesis: process during cell division in

which cytoplasm divides Recall: Mitosis and cytokinesis produce

new cells identical to the original cells– Allows organisms to grow,

replace damaged tissues, or to reproduce asexually

Control of Cell Cycle Cells know when to divide based on a series

of three main checkpoints at which feedback signals from cell can trigger or delay the next phase– Cell growth (G1) checkpoint: makes

decision whether cell will divide Are conditions favorable for division? Is cell healthy and large enough? If so, certain proteins will stimulate cell

to begin synthesis phase

– DNA synthesis (G2) checkpoint: DNA replication is checked by DNA repair enzymes

– Mitosis checkpoint: triggers exit from mitosis and signals beginning of G1 phase

Cancer Specific genes contain information to

make proteins that regulate cell growth and division– If one of these genes is mutated, protein

may not function and regulation of cell growth and division may be disrupted

Cancer: uncontrolled growth of cells

– Some mutations cause cancer by overproducing growth-promoting molecules, speeding up cell cycle

– Others cause cancer by inactivating control proteins that normally act to slow or stop cell cycle

Section 6-3

Mitosis and Cytokinesis

Mitosis and the Spindle

During mitosis, the chromatids of each chromosome are physically moved to opposite sides of dividing cell– Structures called spindles are involved in

moving these chromosomes

Made up of centrioles and individual microtubule fibers

Spindle Formation Organelles that organize the assembly of

the spindle are called centrosomes– Found at each of the cell’s poles

In animals, a pair of centrioles is found inside each centrosome– Like spindle fibers,

centrioles are made of microtubules

Each spindle fiber consists of an individual microtubule

Each centriole is made of nine triplets of microtubules arranged in a circle

– Though plant cells do not have centrioles, they form a spindle almost identical to that of an animal cell

Separation of Chromatids As cell prepares to divide, microtubules

in spindle extend out toward opposite poles of cell– Once microtubules attach to centromeres

and poles, the 2 chromatids (now called chromosomes) can be separated

As the paired chromatids separate, they move toward opposite poles along paths described by microtubules to which they are attached– Chromatids draw closer to poles of cell as spindle

microtubules break down and become shorter– Once breakdown of the spindle is complete, each

pole has one complete set of chromosomes

Stage 1: Prophase

Stage 1 of mitosis is called prophase During this stage:

– Chromosomes coil up, becoming visible

– Nuclear envelope dissolves

– Spindle forms

Stage 2: Metaphase

The second stage of mitosis is called metaphase

During this stage:– Chromosomes move

to center of cell and line up along equator

– Spindle fibers link chromatids of each chromosome to opposite poles

Stage 3: Anaphase The third stage of mitosis is called

anaphase. During this stage:

– Centromeres divide– Separated chromatids

(now called chromosomes) move toward opposite poles as spindle fibers attached to them shorten

Stage 4: Telophase The last stage of mitosis is called

telophase. During this stage:

– Nuclear envelope forms around chromosomes at each pole

– Chromosomes (now at opposite poles) uncoil

– Spindle dissolves as spindle fibers break down and disappear

Stages of Cell Cycle

Cytokinesis

As mitosis ends, cytokinesis begins.– Cytoplasm of cell is divided in half– Cell membrane

grows to enclose each cell, resulting in formation of 2 separate but genetically identical cells

Cytokinesis: Plant vs. Animal Cells

Cytokinesis varies according to cell type:– In animals (lack cell walls),

cells are pinched in half by belt of protein threads

– In plants (have cell walls), vesicles formed by Golgi apparatus fuse at midline of dividing cell to form a cell plate

A new cell wall then forms on both sides of cell plate, separating the plant cell into 2 genetically identical cells