Biology 6.1 Chromosomes

1. Biology 6.1 Chromosomes Chromosomes

2. 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

3. 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.

4. 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.

5. 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.

6. 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.

7. 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.

8. Binary Fission The prokaryote 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 in the middle, like a long balloon being squeezed near the center. • A new cell wall forms around the new membrane. Eventually the dividing membrane is pinched into two independent cells. • Each cell contains one of the circles of DNA and is a complete functioning prokaryote.

9. 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.

10. 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 creationof proteins.

11. 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.

12. 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.

13. 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.

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

15. 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.

16. 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.

17. 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.

18. 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)

19. 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)

20. 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.

21. 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 Syndromeor 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

22. 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.

23. 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.

24. 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.

25. 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.

26. 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