Chapter 6 Section 1 Chromosomes

Formation of New Cells by Cell Division Cell division, also called cell reproduction, occurs in humans and other organisms at different times of their life.

Formation of New Cells by Cell Division The formation of gametes involves yet a special type of cell division. Gametes are an organism’s reproductive cells, such as sperm and egg cells. When a cell divides, the DNA is first copied and then distributed.

Prokaryotic Cell Reproduction Prokaryotes reproduce by a type of cell division called binary fission. Binary fission is a form of asexual reproduction that produces identical offspring.

Prokaryotic Cell Reproduction In asexual reproduction, a single parent passes exact copies of all its DNA to its offspring. Binary fission occurs in two stages: first, the DNA is copied (so that each new cell will have a copy of the genetic information), and then the cell divides.

Prokaryotic Cell Reproduction Eventually, the dividing cell is pinched into two independent cells.

Eukaryotic Cell Reproduction A gene is a segment of DNA that codes for a protein or RNA molecule.

Eukaryotic Cell Reproduction When genes are being used the DNA is stretched out, so that the information it contains can be used to direct the synthesis of proteins.

Eukaryotic Cell Reproduction As a eukaryotic cell prepares to divide, the DNA and the proteins associated with the DNA coil into a structure called a chromosome.

Eukaryotic Cell Reproduction The two exact copies of DNA that make up each chromosome are called chromatids. The two chromatids of a chromosome are attached at a point called a centromere.

Eukaryotic Cell Reproduction The chromatids, which become separated during cell division and placed into each new cell, ensure that each new cell will have the same genetic information as the original cell.

How Chromosomes Number and Structure Affect Development Sets of Chromosomes Homologous chromosomes are chromosomes that are similar in size, shape, and genetic content.

Sets of Chromosomes Each homologue in a pair of homologous chromosomes comes from one of the two parents. The 46 chromosomes in human somatic cells (body cells) are actually two sets of 23 chromosomes.

Sets of Chromosomes When a cell such as a somatic cell contains two sets of chromosomes, it is said to be diploid.

Sets of Chromosomes When a cell, such as a gamete, contains one set of chromosomes, it is said to be haploid.

Sets of Chromosomes The fusion of two haploid gametes – a process called fertilization – forms a diploid zygote. A zygote is a fertilized egg cell.

Sex Chromosome Autosomes are chromosomes that are NOT directly involved in determining the sex of an individual. The sex chromosomes, one of the 23 pair of chromosomes in humans, contain genes that will determine the sex of the individual.

Sex Chromosomes In humans and many organisms, the two sex chromosomes are referred to as the X and Y chromosomes.

Sex Chromosomes Males have an X and a Y chromosome. Females have two X chromosomes.

What do these organisms genetically have in common?

Change in Chromosome Number Humans who are missing even 1 of the 46 chromosomes do not survive. Humans with more than two copies of a chromosome, a condition called trisomy, will not develop properly.

Change in Chromosome Number Abnormalities in chromosome number can be detected by analyzing a karyotype, a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size.

Change in Chromosome Number Changes in an organism’s chromosome structure are called mutations. Breakage of a chromosome can lead to four types of mutations: Deletion mutation Duplication mutation Inversion mutation Translocation mutation