Chromosomes & Cell Division Notes

About two trillion cells are produced by an adult human body every day!

Cell DivisionOccurs in humans and other organisms at different times in their lives.Also known as cell reproduction.

Why do cells divide?Healing a wound (repair)Growth and developmentFormation of gametesBacterial cell division (binary fission – form of

asexual reproduction)

All resulting cells must have DNA

DNA (deoxyribonucleic acid) Stores info and tells the cells when to make

proteins, and what kind to make.

A Gene is a segment of DNA.A single DNA molecule is composed of thousands of genes linked up like train cars.Genes tell how a person’s body develops and

functions.DNA coil up into a chromosome.

Forms of DNA(1) Chromatin - genetic material in non-dividing cell’s

nucleus; DNA in thin, non-coiled strands. DNA is in this form 99% of the time because cells are

generally not dividing. DNA must be in this form in order to be copied

(replicated).

(2) Chromosomes - genetic material in dividing cell’s nucleus; DNA in coiled, rod-shaped form.

DNA must be in chromosome form in order for the cell to divide or reproduce.

ChromosomesChromatids - one of two duplicated, joined,

identical parts of a chromosome; found after DNA replication but before cell division.

Centromere – Point at which sister chromatids are attached

Gene

Gene

Centromere

Chromatid

Chromosome

DNA supercoil

Somatic CellAny cell that is not a gamete (sperm or egg)

Each human somatic cell normally has two copies of 23 different chromosomes.

Making a total of 46 chromosomes

Each of the 23 pairs consists of two chromosomes that are similar in size, shape, and genetic content.This similarity is known as homologous chromosomes.

Every species has a characteristic number of chromosomes which is different from other species.

Example: nematode worm 4 protozoan 600 human 46 (23

pairs)Thus the 46 chromosomes in human somatic

cells are two sets of 23 chromosomes – one set of 23 from the mother, and one set of 23 from the father.

When a cell, such as a somatic cell, contains two sets of chromosomes (46 total) it is called diploid. (2n)

When a cell, such as a gamete cell, contains one set of chromosomes (23 total) it is called haploid (n)

The fusion of two haploid gametes (known as fertilization) forms a diploid zygote.This is a fertilized egg cell, the first cell of a

new individual.

Sperm Cell n = 23 Egg cell

n = 23

Zygote 2n = 46

Hap

loid

cells

!

Dip

loid

cel

ls!

Changes in chromosome numberThe presence of all 46 chromosomes is

essential for normal development, and function.

Humans with only one copy of a particular chromosome, called monosomy, or humans with more than two copies of a chromosome, called trisomy, will not develop properly.

Abnormalities in chromosome number can be detected by analyzing a karyotype, which is a photo of the chromosomes arranged by size.

Monosomy 21

Trisomy 21

Autosomes are…chromosomes that are not directly involved in

determining the sex of an individual.

GametesA gamete is an organism’s reproductive cell.Two examples:

EGGSPERM

All cells other than gamete cells are somatic cells!

Sex chromosomesOne of the 23 pairs of chromosomes in

humans that contain the genes that will determine the sex of the individual.

The two sex chromosomes are referred to as the X and the Y chromosomes.

The genes that causes a fertilized egg to become a male are located on the Y chromosome.This any individual with a Y is male, and any

individual without a Y is a female.

Sex chromosomesMale Female

“Y” makes the guy!