Chromosomes Honors Biology 2006-2007

Why Cell Division? 2 trillion new cells each day, 25 million a second! Why is it needed? Growth, repair, asexual reproduction, and making gametes New cells come from old cells dividing All of the information stored in DNA must be present in each of the new cells When a cell divides, DNA is copied and distributed Each cell ends up with a complete set Honors Biology 2006-2007

Prokaryotic Cellular Division Bacterial cellular division is not as complex as eukaryotic cellular division. Due to the fact that bacteria have one circular DNA chromosome. Bacterial divide using a process called Binary Fission. Honors Biology 2006-2007

Honors Biology 2006-2007

Binary Fission Bacterial cell division happens in 2 stages: 1. Circular DNA is copied -starting at one point of the circular DNA, the molecule is copied -two copies of the DNA are attached to the interior surface of the cell 2. Bacterial cell splits into 2 separate cells -a new membrane and cell wall form between the DNA copies Honors Biology 2006-2007

Honors Biology 2006-2007

Eukaryotic Cellular Division When eukaryotic cells prepare to divide, their DNA is packaged into tightly wound structures called Chromosomes. Why? Chromosomes are easily split into two. Chromosomes contain Genes. A Gene is a segment of DNA that codes for a protein or RNA molecule. Genes play an important role in determining how a person’s body develops and functions. Honors Biology 2006-2007

Chromosome Structure Honors Biology 2006-2007

DNA Coiling Honors Biology 2006-2007

Chromosome Structure Prior to cellular division, each chromosome is composed of two genetically identical parts called chromatids, held together at a region called the centromere. Honors Biology 2006-2007

Honors Biology 2006-2007

Eukaryotic Cells Continued Each organism has a characteristic number of chromosomes. A typical human cell contains 46 chromosomes. Chromosomes are housed inside the nucleus. Each chromosome must be copied exactly before cellular division can take place. Honors Biology 2006-2007

Types of Chromosomes Humans have 23 pairs of chromosomes. The first 22 pairs of chromosomes are called Autosomes. Autosomes are chromosomes that are not directly involved in determining the gender of an individual. The 23rd pair of chromosomes are called Sex Chromosomes. Sex chromosomes contain genes that determine the gender of the individual. XX in females & XY in males. Honors Biology 2006-2007

The elusive “Y Chromosome” Honors Biology 2006-2007

Homologous Chromosomes The first 22 pairs of chromosomes consists of homologous chromosomes or homologues. Homologous Chromosomes are chromosomes that are similar in size, shape, and genetic content. Each homologue in a homologous pair comes from one of your two parents The 23rd pair isn’t homologous -- sex chromosomes! Honors Biology 2006-2007

Homologous Chromosomes both chromosomes of a pair carry “matching” genes control same inherited characters homologous = same information diploid 2n 2n = 4 eye color (brown?) eye color (blue?) homologous chromosomes double stranded homologous chromosomes Honors Biology 2006-2007

Chromosome Number All of the cells in the body, other than Gametes (sex cells), are Somatic Cells (body cells). Somatic cells contain two sets of chromosomes. Diploid 2n=46 Gametes have one set of chromosomes. Haploid n=23 The fusion of two haploid gametes during fertilization produces a zygote. Zygote = fertilized egg Honors Biology 2006-2007

Fertilization Honors Biology 2006-2007

Karyotypes Each organism has a characteristic number of chromosomes that can be identified by a Karyotype. A karyotype is an arrangement of all chromosomes within a cell by pairs in a fixed order. Honors Biology 2006-2007

Chromosomes of Human Female 23 pairs Honors Biology 2006-2007

Chromosomes of Human Male 23 pairs Honors Biology 2006-2007

Chromosomal abnormalities Incorrect number or amount of chromosomes nondisjunction chromosomes don’t separate properly during meiosis breakage of chromosomes deletion duplication inversion translocation Honors Biology 2006-2007

Nondisjunction Problems with meiotic spindle cause errors in daughter cells tetrad chromosomes do not separate properly during Meiosis 1 sister chromatids fail to separate during Meiosis 2 too many or too few chromosomes 2n n-1 n+1 n Honors Biology 2006-2007

Alteration of chromosome number Honors Biology 2006-2007

Nondisjunction Baby has wrong chromosome number trisomy monosomy cells have 3 copies of a chromosome monosomy cells have only 1 copy of a chromosome monosomy 2n-1 trisomy 2n+1 Honors Biology 2006-2007

Down syndrome Trisomy 21 Effects 3 copies of chromosome 21 1 in 700 children born in U.S. Effects Mental handicap Heart defects Short stature Shorter lifespan Sexually underdeveloped, often sterile Honors Biology 2006-2007

Trisomy 21 Honors Biology 2006-2007

Sex chromosomes Human development more tolerant of wrong numbers in sex chromosome But produces a variety of distinct conditions in humans XXY = Klinefelter’s syndrome male XXX = Trisomy X female XYY = Jacob’s syndrome male XO = Turner syndrome female Honors Biology 2006-2007

Klinefelter’s syndrome XXY male one in every 2000 live births have male sex organs, but are sterile feminine characteristics tall normal intelligence Honors Biology 2006-2007

Jacob’s syndrome male XYY Males 1 in 1000 live male births extra Y chromosome somewhat taller than average Often learning disabilities normal sexual development Honors Biology 2006-2007

Trisomy X XXX 1 in every 1000 live births produces healthy females Why? How many Barr bodies would you expect? Honors Biology 2006-2007

Turner syndrome Monosomy X or X0 1 in every 5000 births Only viable monosomy in humans varied degree of effects webbed neck short stature Normal intelligence Sterile How many Barr bodies would you expect? Honors Biology 2006-2007

Changes in chromosome structure During what process would his happen? Honors Biology 2006-2007

Changes in chromosome structure deletion loss of a chromosomal segment duplication repeat a segment inversion reverses a segment translocation move segment from one chromosome to another Honors Biology 2006-2007

Cri du Chat (Cry of the Cat) Part of Chromosome 5 has been deleted Severe mental handicap Small head with unusual facial features Cry that sounds like a distressed cat Honors Biology 2006-2007

Fragile X Syndrome Part of X chromosome is hanging Most common form of mental handicap 1:1500 males, 1:2500 females Honors Biology 2006-2007