Chromosomes, The Cell Cycle and Mitosis Cells Chromosomes Genes DNA Organism Nucleus

Chromosome A structure that forms when DNA wraps around proteins and coils up. Chromosomes only form right before the cell is going to divide.

Each half of a chromosome is called a chromatid. Sister chromatids are identical to each other. Chromatids are joined at a centromere. Gene: Segment of DNA found on a chromosome. Each gene codes for a specific trait. Each chromosome has many genes on it. Chromosome Structure

Two Types of Chromosomes Autosomes- chromosomes that do not help determine the gender of the individual (most of the chromosomes in your body) Sex Chromosomes- in humans, they are X and Y; they determine the gender of the individual XX = Female XY = Male

Two types of Cells Gametes- the reproductive cells (sperm for males, eggs for females). Somatic Cells- all of the cells in the body that are not gametes. Ex – skin cells, muscle cells, brain cells

Chromosome Number in Cells Different cells can have different numbers of chromosomes depending on what type of cell they are. Diploid VS Haploid Diploid (2N)– two sets of chromosomes (one from mom, one from dad) Somatic cells are diploid Haploid (N or 1N)– one set of chromosomes Gamete cells are haploid

Different organisms have different #’s of chromosomes. Ex: Drosophila melanogaster (fruit flies) : 8 Homo Sapiens (human): 46 Podocnemis uuiilis (turtle): 28

Karyotype - A karyotype is a picture of all of the chromosomes in the cell of an individual, taken right before the cell divides. Humans: 22 Pairs of Autosomes, 1 Pair of Sex Chromosomes Homologous Chromosomes

MUTATIONS Point Mutations (occur in the DNA on the genes) A. Addition – adding on of extra nucleotides A B C D  A B B C D B. Deletion – subtraction of nucleotides A B C D  A C D C. Substitution – a different nucleotide is put in place A B C D  A B F D D. Translocation – nucleotides are switched around A B C D  C D A B

Chromosomal Mutations (occur in the genes on the chromosome)Chromosomal MUTATIONS Some genetic disorders are characterized by having too many or too few chromosomes.

Trisomy 21 Also known as “Down’s Syndrome” Characterized as having an extra chromosome on the 21 st pair. Affects approximately 800 in 1,000 births Maternal Age Risk at birth 15 to 24 years 1 out of to 29 years 1 out of years 1 out of years 1 out of (and older) 1 out of 25

Klinefelter’s Syndrome Characterized as a male with an extra “X” chromosome (XXY) The condition exists in roughly 1 out of every 500 to 1,000 males Extra “X” causes slight feminization (including small penis, tall physique, enlarged breast tissue and infertility)

Turner Syndrome Characterized as a female with only one “X” chromosome (XO) Turner syndrome occurs in about 1 out of 2,000 live births Symptoms include webbed neck, drooping eyelids, short height and infertility

THE CELL CYCLE All somatic cells undergo the cell cycle in order to prepare for cell division (mitosis) Mitosis is a form of asexual reproduction. The cells that form as a result of mitosis are clones (identical copies) of the original cell. The reasons for mitosis are: Growth of the organism Repair of cells or replacement of “dead” cells

Stages of the Cell Cycle  G1 Phase  S Phase  G2 Phase  Mitosis  Cytokinesis

G1 (growth 1) Phase  First growth stage  Cell increases in size  Happens after cell division  Normal cell processes are occurring

Synthesis (S) Phase Copying of all DNA DNA is replicated Chromosomes duplicated Chromosomes needs to be copied before a cell divides, so that each new cell has the correct amount of DNA and the correct number of chromosomes.

G 2 (growth) Phase The time between DNA synthesis (S) and mitosis (M). Cell continues growing. Proteins needed for cell division (mitosis) are produced.

Interphase – Resting Stage Interphase is the first 3 phases of the cell cycle together G1, S and G2 Cells carrying on normal activities  Chromosomes aren’t visible (not coiled up).  Normal cell metabolism and processes are occurring.  Occurs before mitosis. Includes phases of cell cycle “getting ready” for mitosis

Interphase

Mitosis (M) and Cytokinesis Phases  Cell growth & protein production have stopped.  The cells energy is used to make 2 daughter cells (splitting of original cell into 2).  Mitosis – division of the nucleus into 2 nuclei in one cell.  Cytokinesis – division of cytoplasm, resulting in 2 new cells.

Cell Cycle Checkpoints The cell has checkpoints to ensure that each phase was completed correctly. G1: making sure the cell has grown enough and is able to divide. S/G2: making sure the DNA copied correctly and that all processes are ready for division. Mitosis (and Cytokinesis): making sure the cell divided correctly. Checkpoints not working and the cell dividing uncontrollably can lead to cancer.

Stages of Mitosis  Prophase  Metaphase  Anaphase  Telophase

PROPHASE  DNA coils tightly & becomes visible as chromosomes.  Nuclear membrane disappears  Centrioles migrate to poles  Centrioles: organelles that produce spindle  Spindle fibers begins to form

Prophase - under the microscope

METAPHASE  Spindle fibers from centrioles attach to each chromosome.  Chromosomes line up in the middle of the cell.

Metaphase – under the microscope

ANAPHASE Chromosomes are already separated into sister chromatids at the centromere. o Spindle fibers shorten and each chromatid is pulled to the opposite end of cell. chromatid

Anaphase – under the microscope

TELOPHASE  Separation of chromosomes into chromatids is completed  Nuclear membrane reforms.  Now have two nuclei in one cell  Chromosomes uncoil

Telophase – under the microscope Plant Animal

CYTOKINESIS  Cytoplasm division  Occurs after chromosomes separate and two nuclear envelopes reappear.  Forms two, identical daughter cells (they are identical to the original cell)

Check your chromosome numbers!!! Human Cell: Before G1 – 46 chromatids After S – 92 chromatids (46 chromosome pairs) After mitosis – 2 cells each with 46 chromatids!! Daughter cells are identical to the parent cell!!!