Biology 6.1 Chromosomes Chromosomes

Key ideas we will cover today...  Students will... ○ Differentiate between a gene, a DNA molecule, a chromosome, and a chromatid ○ Differentiate between homologous chromosomes, autosomes, and sex chromosomes ○ Compare haploid and diploid cells ○ Predict how changes in chromosome number or structure can effect development

Formation of New Cells by Cell Division  About 2 trillion cells are produced by an adult human body every day.  That is about 25 million new cells per second. These new cells are formed when older cells divide.  Cell division, also called cell reproduction, occurs in humans and other organisms at different times in life.  The type of cell division differs depending on the organism and why the cell is dividing.

Formation of New Cells by Cell Division  For example, bacterial cells undergoing reproduction, divide by one type of cell division.  Eukaryotic organisms undergoing growth, development, repair, or asexual reproduction divide by a different type of cell division.  And the formation of gamates involves a third type of cell reproduction.  Gamates are an organism’s reproductive cells, such as sperm or egg cells.

Formation of New Cells by Cell Division  Regardless of the type of cell division, all of the information stored in the molecule DNA must be present in each of the resulting cells.  When a cell divides, the DNA is first copied and then distributed.  Each cell ends up with a complete copy of the DNA.

Prokaryotic Cell Reproduction  A prokaryote’s single DNA molecule is circular and is attached to the inner cell membrane.  Prokaryotes reproduce by a type of cell division called binary fission.

Binary Fission  Binary Fission is a form of asexual reproduction that produces identical offspring.  In asexual reproduction, a single parent passes exact copies of all it’s DNA to it’s offspring.  Binary Fission occurs in two stages: first, the DNA is copied and second, the cell divides.

Binary Fission The prokaryote divides by: 1. adding a new cell membrane to a point on the membrane between the two DNA copies. 2. As new material is added, the growing cell membrane pushes inward in the middle, like a long balloon being squeezed near the center. 3. A new cell wall forms around the new membrane. Eventually the dividing membrane is pinched into two independent cells. 4. Each cell contains one of the circles of DNA and is a complete functioning prokaryote.

Eukaryotic Cell Reproduction  The vast amount of information encoded in DNA is organized into units called genes.  A gene is a segment of DNA that carries the code to create a protein or RNA molecule.  A single molecule of DNA has thousands of genes lined up like train cars.

Eukaryotic Cell Reproduction  Genes play an important role in determining how a person’s body develops and functions.  When genes are being used, the DNA is stretched out so that the information it contains can be used to direct the creation of proteins.

Eukaryotic Cell Reproduction  As eukaryotic cells prepare to divide, the chromosomes (the DNA and associated proteins) coil tighter as the cell prepares to divide.  Before the DNA coils up, the DNA is copied.  The two exact copies of DNA that make up each chromosome are called chromatids.

Eukaryotic Cell Reproduction  The two chromatids of a chromosome are attached at a point called a centromere.  The chromatids become separated during cell division and placed into each new cell.

How Chromosome Number and Structure Affect Development  Somatic cells are any cell other than a sperm or egg cell.  Each human somatic cell normally has two copies of 23 different chromosomes for a total of 46 chromosomes.  The 23 chromosomes differ in shape, size, and sets of genes.

How Chromosome Number and Structure Affect Development  Each of the 23 pairs of chromosomes consists of two homologous chromosomes.  Homologous chromosomes are chromosomes that are similar in shape, size, and genetic content.  Each homologue in a pair comes from one of the two parents.

How Chromosome Number and Structure Affect Development  Thus, the two sets of 46 chromosomes in human somatic cells are actually two sets of 23 chromosomes.  One set comes from the mother, the other from the father.  When a cell, such as a somatic cell, contains two sets of chromosomes, it is said to be diploid.

How Chromosome Number and Structure Affect Development  When a cell, such as a gamate cell (sperm or egg cell) contains only one set of chromosomes, it is said to be a haploid.  The fusion of two haploid gametes, a process called fertilization, forms a diploid zygote.  A zygote is a fertilized egg cell, the first cell of a new individual.

Chromosome Number and Structure  As seen in the table at right, different organisms have different numbers of chromosomes.  The number of chromosomes within a cell is constant within that species. Humans always have 46 and dogs always have 78.  Many plants have far more chromosomes; some ferns have more than 500.

Sex Chromosomes  Of the 23 chromosomes in human somatic cells, 22 pairs are called autosomes.  Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual.  The 23rd pair contains the genes that will determine the gender of the new organism and is called the sex chromosome.  This pair of chromosomes are called the X and Y chromosomes. (XY) (XY)

Sex Chromosomes  The genes that cause a fertilized egg to develop into a male are located on the Y chromosome.  In human males, the sex chromosomes are made up of one X chromosome and one Y chromosome (XY).  The sex chromosomes in human females consist of two X chromosomes (XX)  Because a female can donate only an X chromosome, the sex of an offspring is determined by the male who can donate either an X or a Y. (XY)

Change in Chromosome Number  Each of a persons 46 chromosomes has thousands of genes. The presence of all 46 chromosomes is essential for normal development to occur.  Sometimes a human will have more than two copies of a chromosome in a cell which will result in abnormal development.  Abnormalities in chromosome number can be detected using a karotype, a photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size.

Change in Chromosome Number  A karotype from an individual with an extra copy of chromosome 21 is shown at right.  This produces a condition call Down Syndrome or Trisomy 21. Traits of Trisomy 21 include... Short stature, a round face with upper eyelids that cover the inner corners of the eyes, and varying degrees of mental retardation are characteristics of people with down syndrome. Down syndrome karotype

Down Syndrome  In mothers younger than 30, Down syndrome occurs in about 1 in 1500 births.  In mothers 37 and older, the incidence doubles to 1 in 290.  In mothers over 46, the risk is as high as 1 in 46.

Down Syndrome  This is because, when a female is born, all the eggs she will ever produce are already present in her ovaries; unlike males who can produce new sperm at any time.  As a female ages, her eggs can accumulate an increasing amount of damage.

Prenatal Testing  In prenatal testing, the cells of the fetus are tested for normal chromosome number and cell structure by a procedure called fetal karyotyping.  Through this process, the doctor can check for any abnormalities such as Down Syndrome.

Mutations  Changes in an organism’s chromosome structure is called a mutation.  Breakage of a chromosome can lead to four types of mutation Deletion mutation: a piece breaks off completely and the new cell will lack certain genes Duplication mutation: a chromosome fragment attaches to it’s paired homologous chromosome which will than carry two copies of the same gene Inversion mutation: a piece reattaches to the original chromosome but in an inverted position If the piece reattaches to another chromosome, not of it’s homologous pair, this becomes a translocation mutation.

Computer Lab:  Go to the Computer Lab: research on the internet and create a one page written report on one of the following topics done in Word. Chromosomes Down Syndrome Prenatal testing Mutations