Section 6-1 Chromosomes

Cell division is the same as reproduction of the cell. Gametes – an organism’s reproductive cells Females – eggs Males – sperm Regardless of the type of cell division occurring, all information that is stored in DNA must be present in the resulting cell.

DNA stores the information that tells cells which proteins to make and when to make them. This information directs a cell’s activities and determines its characteristics. Thus, when a cell divides, the DNA is first copied then distributed. Each cell ends up with a complete set or copy of the DNA.

Bacterial Cells Divide to Reproduce First remember… Bacterial DNA is  single-stranded  circular  attached to the inner cell membrane (no nuclear envelope).

Bacterial Cells Divide to Reproduce Bacteria reproduce by a type of cell division called BINARY FISSION. Binary Fission – form of asexual reproduction that produces identical offspring Asexual Reproduction – a single parent passes exact copies of all its DNA to its offspring

Binary Fission Occurs in Two Stages 1.DNA is copied so each cell has a copy of the genetic information. 2.Cell divides – bacterium divides by adding a new cell membrane to a point on the membrane between the two DNA copies. As new material is added, the growing cell membrane pushes inward and the cell is constricted in the middle. It will be pinched into two cells.

Eukaryotic Cells Form Chromosomes Before Cell Division Gene – segment of DNA that codes for a protein or RNA molecule. How DNA is organized. Thousands of genes are on a single DNA molecule. Genes play an important role in determining how a person’s body develops and functions.

As eukaryotic cells prepare to divide, the DNA and proteins associated with DNA coil into a structure called a chromosome. Before the DNA coils up, the DNA is copied. The two exact copies of DNA that make up each chromosome – chromatids Centromere – the point that the two chromatids of a chromosome are attached

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.

Chromosome Number and Structure Affect Development Each human somatic cell contains 23 different chromosomes; two copies each, totaling 46 chromosomes The 23 different chromosomes differ in size, shape, and genes. Each chromosome contains thousands of genes – genes determine body development and functions

Sets of Chromosomes Each of the 23 pairs of chromosomes consists of two homologous chromosomes or homologues. Homologous chromosomes – similar in size, shape, and genetic content.

Each homologue in a pair of homologous chromosomes comes from one of the two parents. Thus, 46 chromosomes in human somatic cells are actually 23 pairs of chromosomes. One set comes from the mother and the other comes from the father. All cells in the body other than the gametes are SOMATIC CELLS.

SOMATIC CELLS – contain 2 sets of chromosomes (46 chromosomes or 23 pairs). These cells are DIPLOID. Diploid cells are 2n. So… In humans, 2n = 46. GAMETES – contain 1 set of chromosomes (23 chromosomes). These cells are HAPLOID. Haploid cells are n. So… In humans, n = 23. Fusion of 2 haploid gametes (fertilization) forms a diploid zygote. Zygote – a fertilized egg cell, or the first cell of a new individual.

Each organism has a characteristic number of chromosomes. It stays constant in each species.

Chromosomes Determine Your Sex Of the 23 pairs of chromosomes in somatic cells, 22 pair are autosomes and I pair are sex chromosomes. AUTOSOMES – chromosomes not directly involved in determining the sex or gender of an individual. SEX CHROMOSOMES – contains genes that will determine the sex of an individual.

Often sex chromosomes are referred to as X (female) and Y (male). Genes that cause a fertilized egg to develop into a male are on the Y chromosome. XX – female; XY – male The mother always gives an X chromosome. The father will give either an X or Y chromosome, so he determines the offspring’s gender. Some species have no Y chromosome, so the males are characterized by XO.

Change in Chromosome Number The presence of all 46 chromosomes essential for normal developmental function. Humans missing one of the 46 chromosomes do not survive. Humans with more than two copies of a chromosome will not develop properly (TRISOMY).

KARYOTYPE – a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size. It will detect abnormalities in chromosome number.

Down Syndrome Down Syndrome occurs when there is an extra copy of chromosme 21. This is also known as Trisomy 21. People with this have short stature, a round face with upper eyelids that cover the inner corners of the eyes, and varying degrees of mental retardation.

Down Syndrome Karyotype

What can cause an individual to have an extra copy of a chromosome? When a sperm and an egg cell form, each chromosome and its homologue separate – DISJUNCTION. If one or more chromosomes fail to separate properly (NONDISJUNCTION), one new gamete ends up receiving both chromosomes, and the other one does not get one. If the gamete with both copies undergoes fertilization, trisomy results.

Change in Chromosome Structure Mutations – changes in chromosome structure. Breakage of a chromosome can lead to one of four types of mutations.

Four Types of Mutations 1.Deletion – piece of chromosome breaks off completely. 2.Duplication – Chromosome fragment attaches to its homologous chromosome, which will then carry 2 copies of a certain set of genes 3.Inversion – chromosome piece reattaches to the original chromosome but in a reverse orientation. 4.Translocation – chromosome piece reattaches to a nonhomologous chromosome.

HOMEWORK SECTION 6-1 Review Questions #1-#6