UNIT 7: Cell Division

UNIT 7: Cell Division

BIG IDEA: Cell division is the process in the body to create

more cells and gametes

Chromosome Structure• Inside your nucleus are 3 billion

nucleotides of DNA• In order to fit it is coiled and packed

into a complicated but organized structure

Chromosome Structure• Histones-Proteins that DNA are wrapped around

Chromosome Structure• Chromati

n-Less tightly coiled DNA wrapped around a histone

Chromosomes• Rod shaped structures made up of

DNA and histones• Two identical halves are called

chromatids• Point holding together the

chromatids is called a centromere

Chromosomes• Structure in cells

that undergo division and that contain hereditary information of the organism

• In mitosis the chromosomes that undergo cellular division are called autosomes

Chromosomes• Draw a

chromosome and label the centromere and the chromatids

How many chromosomes are present in an autosomal cell?

• 23 pairs or 46 chromosomes

• Refers to DIPLOID: meaning that a cell possesses 2 complete sets of chromosomes

Why are there two sets of chromosomes?

– 1 set is from your mother and 1 set is from your father

– This leads to genetic variation because genetic information on chromosomes from your parents can be different

• Example: both chromosomes can code for eye color except one can be for blue eyes and one for green eyes

Blonde Hair Black Hair

Genetic Variation

Why is genetic variation beneficial?

• Causes a diverse collection of genes

• Increase in survival

MITOSIS• Cell division that produces two

identical diploid daughter cells• Every body cell has a life span called

the cell cycle so cells must divide to replace themselves

• Cells also divide in order for organisms to grow

CELL CYCLE• Cycle that a cell goes through in order to make new cells

• If an organism is unicellular, than a new organism is created via the cell cycle

Asexual Reproduction• Reproduction with only one parent

• offspring are identical to parents

Sexual Reproduction• Requires 2 parents• New individual is produced by

the union of the nuclei of two specialized cells (sperm & egg)

• We all start off as a single cell-the cell cycle and division made you what you are today

Cell Cycle Consists of Two Parts:

• Mitosis:– Division of the

nucleus to form two identical nuclei

– REMEMBER: Hereditary material is found in the nucleus

• Cytokinesis–Division of the

cytoplasm of the cell after mitosis

INTERPHASE

Interphase• Stage when a cell

is between mitotic cycles:

A. G1- cell growsB. S- DNA replicationC. G2- Organelle replication

MITOSIS

Prophase• Chromosome

become visible• Centrioles

move to opposite poles of the cells

• Microtubules form spindles

• Spindle fibers become attached to centromeres

• Nuclear membrane disappears

• Nucleolus disappears

Prophase:

Metaphase• Chromosomes

attach to the microtubule spindles

• Chromosomes line up on the equator of the cell

Anaphase• Centromeres

double• Chromosomes(sis

ter chromatids) move to opposite poles

• Division (cleavage furrow) appears

Telophase• Begins when chromosomes reach the poles

• Chromosome uncoil and become invisible

• Spindle disappears• Nuclear membrane forms• Nucleioli reappears

Telophase

Cytokinesis in Animal Cells• Begins during late anaphase

• Cytoplasm is pinched off by cell membrane

Mitosis & Cytokinesis in Plant Cells

• Main events are the same• Plants do not have centrioles or

asters• Cytokinesis cannot occur by

pinching off of membrane a cell plate forms between the new cells during anaphase or telophase

Cytokinesis

End with two identical daughter cells

Cells that divide rapidly• Plants • Animals

Cells that seldom or never divide

• Xylem (part of a plant)

• Nerve Cells• Muscle Cells

Meiosis• Remember-mitosis is cell division in

autosomes (all cells except sex cells)

• Meiosis is the creation of haploid sex cells for reproduction

What is the function of a haploid cell?Used in sexual

reproduction

EX: sperm for males & egg for females

During fertilization haploid sperm and haploid egg combine to form a diploid cell

How does meiosis begin?• Once diploid cells in reproductive

organs have replicated DNA and they are no longer in interphase meiosis starts

(i.e. Prophase I and Prophase II)

Stages of Meiosis: Same as mitosis EXCEPT there are two for each stage

Prophase I • Chromosome condense from

chromatin• Nuclear Membrane Disappears• Spindle fibers develop on which

chromosomes group together• Homologous chromosomoes align

Prophase I continued• The pairs of chromosomes are called

a tetrad and the pairing up is called synapsis

• Chromatids may break off and attach to adjacent chromatids-called crossing over.

• Exchange of genetic material between parental chromosomes creates genetic recombination

Metaphase I• Duplicated chromosomes line up in

the middle of the cell.

Anaphase I• Chromosomes move to opposite poles

Telophase I• One set of each chromosome ends up

in each new nucleus and the cytoplasm divides

Now these two cells will go through the phases again

At this point we have two DIFFERENT diploid cells

Prophase II• Spindle fibers form• Chromosomes move toward the

middle of the spindle

Metaphase II• Spindle fibers attach to centromere of

chromosomes and chromosomes are in the middle of the cell.

Anaphase II• Chromosomes separate and move

toward opposite ends of the cells

Telophase II• Daughter cells divide to form 4 haploid

cells• Chromosomes return to their

interphase or “resting” form

At the end of meiosis…• Result in 4 cells

• Either sperm or egg cells

• Each cell possesses half the number of chromosomes of the parent (or 23 chromosomes)

Why is the number of chromosomes reduced by half?

• Cells have divided two times but DNA was only replicated once

Review: Diploid cells Haploid Cells

In meiosis, haploid daughter cells are created from diploid parent cells

Chromosome Errors in Cell Division

• Deletion-loss of a piece of a chromosome

• Inversion-A chromosomal segment breaks off, flips around and reattaches

Chromosome Errors in Cell Division

• Translocation-a piece of a chromosome breaks off and reattaches to a nonhomologous chromosome

• Nondisjunction-a chromosome does not separate from its homologue during meiosis