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Transcript
Cell Reproduction Unit Part 1: Overview Objectives • Be able to explain the difficulties a cell faces as it increases in size • Be able to compare and contrast sexual & asexual reproduction • Be able to describe the structure of a chromosome • Be able to identify the differences in structure between prokaryotic chromosomes and eukaryotic chromosomes • Be able to explain the differences between sex chromosomes and autosomes Limits to Cell Size • • • Nearly all cells grow by increasing in size At some point, they divide There are two main reasons cells divide rather than continue growing: – Information Overload • – The larger a cell becomes, the more demands the cell places on its DNA Exchanging Materials • A larger cell is less efficient in moving nutrients and waste materials across the cell membrane Surface Area to Volume Ratio • The size of a cell is limited by the relationship of the cell’s outer surface area to its volume, or its surface area–to–volume ratio • As a cell grows, its volume increases much faster than its surface area does • Why is this important to cells? • This trend is important because the materials needed by a cell, such as nutrients and oxygen and the wastes produced by a cell, such as carbon dioxide must pass into and out of the cell through its surface. Division of the Cell • Before a cell becomes too large it divides into two “daughter” cells • Cell Division: is the process by which a cell divides into two new daughter cells Cell Division & Reproduction • All organisms produce new organisms like themselves • During reproduction, organisms transmit hereditary information Two Types of Reproduction • Asexual Reproduction: produces new organisms that are exactly the same as the original organism – Examples: strawberries, amoebas, bacteria, starfish, onions etc. • Sexual Reproduction: produces a genetically unique organism from the combining of the parents’ DNA – Examples: humans, tigers, some plants, whales, etc. Compare & Contrast Asexual Reproduction Sexual Reproduction • Needed for the survival of a species • Happens faster • Offspring are exact copies of parents • Lack of genetic diversity • Needed for the survival of a species • Takes longer • Offspring are a combination of parents DNA • Lots of genetic diversity Visual Analogy Activity • In the assigned groups, discuss the following and be prepared to share with the class. • How could cell growth create a problem that is similar to a traffic jam? • How does a cell solve the “traffic jam” like a growing city would? • Why are cells concerned with “traffic jams”? Facts about DNA • It is a long, thin molecule that stores ________ ______________. • It is made of nucleotides • Nucleotides are made of: a sugar, a phosphate, and a nitrogen base • In human cells, its consists of approx. 3 billion nucleotides • In eukaryotes, DNA is found in the _________. • In prokaryotes, DNA is found in the ___________. Chromosomes • Chromosomes are rod-shaped structures made of DNA & proteins • The DNA & proteins are tightly coiled together • The proteins that eukaryotic DNA wraps around are called histones • Histones help maintain the shape of a chromosome & keep it tightly packed Chromosome Structure of Eukaryotes Chromosome Nucleosome DNA double helix Coils Supercoils Histones Chromosome Structure • Each chromosome consists of 2 identical halves called ___________ • The 2 chromatids are held together by a ______________. Chromosome Structure • Normally DNA is not so tightly coiled into chromosomes • Chromosomes only appear right before cell division • When cell division is not occurring, DNA is less tightly coiled as __________. • Why is the DNA not kept tightly packed all the time? • Because it is easier to copy (transcribe) in its chromatin form, which happens before division occurs Chromosome Number • Every species has a characteristic number of chromosomes in each cell • Human & animal chromosomes are categorized into 2 groups: ____________ & _______________. Sex Chromosomes & Autosomes • Sex chromosomes: are the chromosomes that determine the sex of an organism – In humans, they are either X or Y – Females are XX – Males are XY • Autosomes: are all the other chromosomes of an organism – Humans have 44 autosomes (22 pairs) Chromosome Numbers • Every organism produced by sexual reproduction has 2 copies of each autosome • One copy comes from _____ and the other comes from ______. • The 2 copies of each autosome are called homologous chromosomes – Meaning they are the same size, shape and carry the same traits Human Karyotype • A karyotype is pictorial diagram showing all the chromosomes of an organism Chromosome Numbers: Diploid vs. Haploid Cells • Diploid cells: have 2 sets of homologous chromosomes – All human cell except the sex cells are diploid – Abbreviated 2n – In humans, 2n = ____ or ____ pairs • Haploid cells: have only 1 set of chromosomes – Sperm & egg cells are haploid – Abbreviated 1n – In humans, 1n = ____