Chapter 8 9 Chromosomes and Cell Division DNA

Chapter 8 -9: Chromosomes and Cell Division

• DNA containing a cell’s genetic code • At the beginning of cell division, DNA makes a copy of itself and then forms chromosomes

Chromosomes • The single set of DNA in the cell must be replicated before division • Each daughter cell will get 1 full set of DNA

Chromosomes • Visible form of DNA that are only around during cell division • When not dividing DNA is spread throughout the nucleus

Chromosomes • Each chromosome is made of 2 sister chromatids centromere: connects the sister chromatids near the middle

Chromosomes Different organisms have a different # of chromosomes • Fruit flies: 8 • Humans: 46 • Carrots: 18

Some chromosomes are circular, others are linear.

Prokaryotes vs. Eukaryotes q In prokaryotes, genetic information is carried in a single, circular chromosome. q q This strand of DNA is attached at one site to the cell membrane. Eukaryotes have much more DNA. In eukaryotes, genetic information is organized into linear chromosomes. q Eukaryotic chromosomes float freely in the nucleus. q

Prokaryotes and some unicellular eukaryotes divide by binary fission. q. The circular chromosome duplicates itself and the cell splits into two identical new cells.

A time for everything: the Cell Cycle.

Interphase G 1 phase S phase G 2 phase

G 1 phase • cell growth • make organelles • make proteins

S phase • DNA is copied Once cell enters S phase it MUST divide

G 2 phase • Make more organelles • Make proteins needed for cell division

M Phase Mitosis Prophase Metaphase Cytokinesis Anaphase Telophase

M phase Mitosis + cytokinesis = M phase • Lasts several minutes to several days – depending on the type of cell

Mitosis has just one purpose: q To make genetically identical cells. 1. Growth 2. Replacement

Growth • You began life as a single cell (zygote – a fertilized egg), but there are now more cells in your body than stars in the Milky Way.

Reminder: Sister Chromatids A chromosome and its identical replicated copy, joined at the centromere.

Cytokinesis Separating of the cytoplasm Occurs at the end of Telophase. Results in two genetically identical daughter cells

Cytokinesis in plants vesicles containing cell wall material gradually fuse to form a cell plate.

Cancer Cells Ignore signals to stop dividing Divide indefinitely

Immortal cells can lead to cancer The average human cell can copy itself about 50 times before it dies.

Telomeres q The telomere is like a protective cap at the end of the DNA. q Every time a cell divides, the telomere gets a bit shorter. q Cells that rebuild the telomere with each division can become cancerous

Cancer Treatment – Radiation therapy disrupts cell division. – Chemotherapy involves drugs that disrupt cell division. Examples • Taxol: freezes the mitotic spindle after it forms and keeps it from functioning. • Vinblastine: prevents the mitotic spindle in the first place.

Why do the treatments have so many side effects? Chemotherapy interferes with other areas of rapid cell division in the body. 1. Bone Marrow divides rapidly producing immune system cells, Red Blood cells and platelets. 2. Hair follicles divide rapidly

• Cancer cells are often grown in culture for study. They can continually divide as long as there is a supply of nutrients. Figure 8. 10

Non gamete cells are diploid (2 n) Sperm and egg cells are haploid (n) gametes.

Meiosis vs. Mitosis Asexual – Results in genetically identical daughter cells – Diploid (2 n) to diploid (2 n) Meiosis Sexual reproduction – Results in genetic diversity – Diploid (2 n) to haploid (n)

Sexual reproduction requires gametes made by meiosis. Homologous Chromosomes carry the same sequences of genes and control the same inherited characteristics.

Meiosis: Step-by-step Meiosis only occurs in gamete production

Cells undergoing meiosis divide twice Meiosis I: The homologues are separated, but the sisters stay together. Meiosis II: Each of the two new cells divides again, separating the sister chromatids into two even newer cells

1. Prophase I q The most complex of all of the phases of meiosis q Crossing over, an important source of variability.

Crossing over: an important source of variation.

2. Metaphase I q Each pair of homologous chromosomes moves to the equator of the cell. They line up in groups of 4 called tetrads

3. Anaphase I is the phase in which the genetic material is halved, the alternative versions of genes are separated.

4. Telophase I and Cytokinesis The result is 2 haploid cells, each with both copies of the chromosome (sister chromatids).

Meiosis II Separating the sister chromatids

8. Telophase II q The cytoplasm then divides. q The result is 4 haploid cells, each with a unique set of traits.

Mitosis provides growth, tissue repair, and asexual reproduction. Meiosis yields haploid cells for sexual reproduction.

3 Sources of Genetic Variation variation is important for evolution.

Origins of Genetic Variation (Alleles come from two parents) • For any species, the total number of possible unique gamete combinations is 2 n, where n is the haploid number. • For humans (n = 23), so 223 (8 million). – One egg has 8 million possibilities. – One sperm has 8 million possibilities. – Together that equals 64 trillion possible combinations for a fertilized egg.

Male and Female in Humans Sex Chromosomes carry information that directs growth as a male or a female. Male: Y chromosome is present (XY) Female: absence of Y chromosome (XX)

Down syndrome can be detected before birth karyotypes reveal an individual’s entire chromosome set. q karyotype a display of an individuals’ complete set of chromosomes

Down syndrome is also known as Trisomy 21 and is a result of nondisjunction.

Nondisjunction q the unequal distribution of chromosomes during meiosis q error of cell division that creates a gamete with zero copies or double copies of a chromosome rather than a single copy

Abnormal Numbers of Sex Chromosomes • Nondisjunction Also affects the sex chromosomes. A man with an extra X chromosome (Klinefelter syndrome) will develop feminine qualities. A female is missing her second X chromosome (Turner syndrome) and will develop masculine qualities.