DNA and The Cell Cycle

Forms of DNA• DNA + Proteins = Chromosomes• Most of the time, DNA is “unspooled” into

loose strands – called chromatin– can be used to provide instructions in this form.

• Before a cell divides, the DNA winds around histone proteins and becomes visible as “chromosomes”

• The chromosomes can be counted in this form.

Structure of DNA

Chromosome

Supercoils

Coils

Nucleosome

Histones

DNA

double

helix

Chromosome number

• Eukaryotic organisms have a specific number of different types of chromosomes.

• They have two of each type – so chromosomes come in pairs.

• Cells with chromosomes present in pairs are said to be diploid (2N)– one of each is haploid

Chromosome identity

• Each chromosome type can be identified by shape and size.

• If stained, characteristic patterns of bands can be seen as well.

• A karyotype is a display of all of an individual’s chromosomes arranged by type.– Can be used to identify major genetic disorders.

Chromosome number • Each organism has a different number of

chromosomes:Camel 70 Chicken 78 Opossum 22 Housefly 22 Bat 44 Corn 24Lentil 14 Rice 24Goat 60 Barley 14 Apple 34 Lettuce 12

HUMANS HAVE 46! 2 each of 23 different pairs

How many chromosomes?• In a diploid cell, one set of chromosomes

comes from the mother and another from the father.

• Each human gets 23 from each parent (46 total)– 44 are autosomes (general body characteristics)

and two are sex chromosomes (determine sex and carry general characteristics)

Being Diploid

• The two corresponding chromosomes are called homologous chromosomes. Homologous chromosomes need not be genetically identical.– For example, a gene for eye color at one locus

(location) on the father chromosome may code for green eyes, while the same locus on the mother chromosome may code for brown.

Before Cell Division, each chromosome must replicate!

• Individual chromosomes replicate and form sister chromatids.

• Each sister chromatid is destined for one of the two resulting daughter cells

• Sister chromatids together are considered one chromosome.

• After separation, the two independent copies are sister chromosomes.

• Each sister chromosome goes to a different cell.

After Replication Chromosomes

The Cell Cycle• Sequence of events a cell

goes through as it grows and divides

• (G1 Phase) Cell grows and synthesizes proteins and new organelles

• (S Phase) Chromosomes replicate

• (G2 Phase) Organelles and molecules used in cell division are produced

• (M Phase) mitosis (chromosomes) and cytokinesis (cytoplasm) are divided.

Regulation of Cell Cycle• Not all cells move through cell cycle at same

rate–Bone marrow cells/skin cells continuous

division–Nerve and muscle cells seldom or never

http://nobelprize.org/educational_games/medicine/2001/cellcycle.html

Cycle Regulators - Internal

• Special proteins called cyclins and cyclin-dependent kinases.

• High concentrations of cyclin influence a cell to divide.

• proteins that respond to internal stimuli: cell cycle checkpoints!– Ex. Cell will not enter mitosis until all

chromosomes are replicated

External Regulators

• Factors that respond to external stimuli–Cell will begin to divide rapidly after injury–When dividing cells come in contact with

adjacent cells, division will slow– Embryos – all cells go through cell cycle

quickly to produce growth