Cell Division and Chromosomes

• Cell division involves chromosomes "boogieing".

Chromosomes

• A chromosome is a condensed form of chromatin.
• Chromosomes are threadlike structures of nucleic acids and protein.
• Chromosomes are always paired with an identical copy of itself.
• Chromosomes consist of chromatids, DNA, and a centromere; sister chromatids are also present.

Types of Chromosomes

• Autosomes are the first 22 pairs of chromosomes in humans.
• Sex chromosomes are the 23rd pair of chromosomes.
• Females have two "X" chromosomes.
• Males have one "X" and one "Y" chromosome.
• The Y chromosome determines that an offspring will be male.

Types of Cell Division

• Asexual reproduction results in offspring that are identical to the parents.
• Examples include budding in yeast and binary fission in bacteria.
• Sexual reproduction results in offspring with a combination of genetic information from two parents, such as in animals and plants.

Numbers of Chromosomes

• Haploid cells contain one set (1n) of genetic information; sperm and egg cells are examples.
• Diploid cells contain two full sets (2n) of genetic information; body cells are examples.
• Two haploid cells unite to form a diploid cell called a zygote.

Why Cells Divide

• Cell division is crucial for growth.
• Needed for reproduction and development.
• Necessary to repair or replace damaged cells.

What Happens During Cell Division

• Exact replication of DNA occurs.
• The two copies of DNA move to opposite ends of the cell.
• The cell divides into two daughter cells.

Cell Cycle

• Interphase prepares cells for Mitosis.
• During interphase, chromosomes appear as threadlike coils called chromatin.
• Interphase accounts for 90% of the cell cycle.
• Is composed of three phases: G1 (Growth), S (DNA Replication), and G2 (prepares to divide).
• Checkpoints during the cell cycle ensure each phase functions correctly.
• An example is checking that DNA and chromosomes are replicating correctly to ensure the correct number of chromosomes.

Mitosis

• One cell divides into two cells.
• both new cells are identical to the original cell.
• All cells undergo mitosis, except sex cells.
• Series of four stages or phases occur during cell division.

Stages of Mitosis

• The acronym PMAT represents the 4 stages of Mitosis:
• Prophase ("BEFORE/PAIR"): chromatin condenses into chromosomes, centrioles replicate, and the nuclear membrane dissolves.
• Metaphase ("MIDDLE"): chromosomes line up in the center of the cell, and centrioles eject spindle fibers that attach to the chromosomes.
• Anaphase ("AWAY/ANTI"): chromosomes are pulled to opposite ends of the cell by spindle fibers, and sister chromatids are separated.
• Telophase ("TWO"): two nuclear membranes form, chromosomes unwind and become chromatin, and daughter cells are still connected.
• Cytokinesis ("Split"): cell membrane or cell wall grows between the nuclei, the separated cells are now two identical daughter cells, each containing 46 chromosomes.

Meiosis

• One cell becomes four cells, called gametes.
• Male gametes are sperm.
• Female gametes are eggs.
• Meiosis is mitosis happening twice without interphase in between.
• It ensures that at fertilization, the chromosome number is correct, and serves in fertilization, when haploid sperm and egg (n) unite into a diploid zygote (2n)
• Homologous chromosomes occur in pairs.
• They have the same genetic information.
• One is inherited from the mother, and one from the father.
• Meiosis Creates Variation: This leads to evolution. Makes offspring unique.

Ways Meiosis Creates Variation

• Law of Segregation: Each gamete contains a single copy of each chromosome
• Law of Independent Assortment: Whether a gamete receives the maternal or paternal chromosome is determined randomly for each pair.
• Crossing Over: Chromosome parts break off and reattach to other chromosomes, which is how a unique chromosome is created.
• A human can make (2)^23 = > 8,000,000 unique gametes.
• Crossing over enables even greater variety

Mitosis vs. Meiosis

• Mitosis creates Diploid Cells (2N).
• These cells have 2 full sets of genetic information.
• Human body cells have 46 chromosomes each (diploid or 2n # is 46).
• This occurs in somatic cells (body cells).
• It makes up the entire organism, except for sperm and eggs.
• Meiosis creates Haploid Cells (N).
• These cells have 1 set of genetic information.
• Human sperm and eggs have 23 chromosomes (haploid or n # is 23).
• Meiosis includes Prophase 1 (tetrad forms, crossing over occurs), Anaphase 1 (homologous chromosomes separate), and Anaphase 2 (sister chromatids separate).
• The rest is just like mitosis.

Chromosomes

• Karyotype is how scientists look at the chromosomes of an individual.
• Karyotype refers to the collection of chromosomes found in an individual's cells.

How Karyotypes are Made

• Take the sample and chemically treat it to stop them in metaphase (usually from blood or amniotic fluid).
• Chromosomes are stained.
• The chromosomes are then photographed.
• Chromosomes are cut out.
• Chromosomes are arranged in pairs from largest to smallest.
• Chromosomes are numbered in 23 sets.

Chromosome Abnormalities

• Nondisjunction.
• Chromosomes fail to separate during anaphase of Meiosis.
• Mutations. -Changes in chromosome structure.

Nondisjunctions

• If a chromosome is missing:
• Most do not survive to be born.
• The condition is called Monosomy.
• Example is Turner's Syndrome (XO).
• If a cell has an extra chromosome:
• The condition in which a cell has an extra chromosome is called Trisomy.
• Example: cell with Trisomy 21 results in Down Syndrome.
• Example is XXY, which results in Klinefelter's Syndrome.

Mutations

• Deletion: part of the chromosome breaks off.
• Duplication: part of the chromosome is repeated.

Cancer

• Occurs with mutations in genes that control normal cell division.

Increasing Cancer Risk Factors

• Sun exposure without sunblock.
• Smoking or being around second-hand smoke.
• Working with known cancer-causing chemicals.
• Eating foods with preservatives like nitrates.
• Cooking foods in plastics containing dioxin.
• Proto-oncogenes are regular genes that code for proteins involved in normal cell growth.
• Oncogenes tell cells to make proteins that cause excessive cell growth and division and are mutant forms of Proto-oncogenes.
• Tumor Suppressor Genes are normal genes whose absence can lead to cancer.
• Genes come in pairs, one from each parent. A defect in one copy will not cause cancer because the other normal copy is still functional.