Mitosis and Cytokinesis Section 5.2

Objectives SWBAT describe the structure of a chromosome. SWBAT follow chromosomes through the processes of mitosis and cytokinesis.

Vocabulary Chromosome Histone Chromatin Chromatid Centromere Telomere Prophase Metaphase Anaphase Telophase

Chromosomes A chromosome is one long, continuous, thread of DNA. It consists of genes along with “regulatory (for example start and stop instructions for gene replication)” information. Humans have 46 chromosomes each (2 pairs of 23 chromosomes). The DNA you have in each cell, if stretched out, would measure about 10 feet in length.

Human Karyotype What does diploid mean? What does haploid mean?

Salmon and Field Mouse Karyotypes

Chromosomes and Histones DNA wraps around proteins called histones that help to organize and condense it. During interphase, DNA is loosely organized (looks like spaghetti. During mitosis, however, the DNA is tightly condensed.

Chromosomes and Histones The changes in DNA organization – from loose and spaghetti-like to very condensed during mitosis – are important for allowing cells to carry out their functions. For example: during interphase, DNA needs to be available so genes can be copied and transcribed to make proteins and other molecules. During mitosis, DNA does not need to be transcribed but it does need to be divided (hence condensed is better than stringy to avoid “DNA tangles”).

Chromosomes and Histones DNA double helix DNA and histones Chromatin Supercoiled DNA Each continuous double-stranded DNA molecule makes one chromosome. The DNA wraps around the histones at regular intervals – looking like beads on a string.

Chromosomes and Histones During interphase (when DNA is in its “spaghetti phase”), the loose combination of DNA and proteins is called chromatin. When interphase ends and prophase of mitosis begins, the chromatin is condensed. It coils more tightly around the histones until it forms “small thick rods” – like we saw in the karyotypes.

Chromatids REMEMBER: each chromosome was duplicated in S-Phase. So now, in mitosis, we have identical chromosomes that are condensed and attached to one another at a region called the centromere. One half of a duplicated chromosome is called a chromatid. The two identical chromatids are called sister chromatids. telomere chromatid centromere telomere

Telomeres Telomeres are at the ends of DNA molecules. They are made up of repeating nucleotides (nitrogen bases) that do not form genes. The purposes of telomeres are: to keep chromosomes from accidentally attaching to one another; and to help prevent the loss of genes. A short length of nucleotide is lost every time a chromosome is copied. It is important that the nucleotides are lost from telomeres and not from the genes themselves.

Prophase Chromatin condenses into tightly coiled chromosomes (two sister chromatids). The nuclear membrane breaks down and the nucleolus disappears. The centrioles and centrosomes migrate to opposite sides of the cell – producing spindle fibers (microtubules).

Metaphase Spindle fibers attached to protein structure on the centromere of each chromosome. The chromosomes are aligned along the equator of the cell.

Whitefish Blastula Metaphase

Anaphase The sister chromatids are separated as the spindle fibers shorten - pulling them apart.

Whitefish Blastula Anaphase

Telophase A complete set of identical chromosomes are now in the developing daughter cells. The nuclear envelope begins to appear. The chromosomes uncoil and become spaghetti-like again. The daughter cells complete cytokinesis and separate completely.

Whitefish Blastula Telophase