Chromosome Structure

Sexually Reproducing Organisms: Chromosomes occurs in pairs (One

from the mother and 1 from the father)

These pairs are called homologous chromosomes Carry directions/alleles for traits but they do not always carry same version for the same trait.

Organism – Kingdom Body/Somatic Cell (2n)Sex Cell/Gamete/Eggs & Sperm (n)

All Bacteria 1 0

Yeast – Fungi 32 16

Bread mold – Fungi 8 4

Potato – Plant 48 24

Tobacco – Plant 48 24

Corn – Plant 20 10

Adder’s Tongue Fern – Plant

1,262 631

Chicken 78 39

Dog 78 39

Human 46 23

Chimpanzee 48 24

Cell Types and their Chromosomes Number

Cell type w/examples

Cell type alternate names

Sets of chromosomes per cell

Naming sets of chromosomes

 Body Cells

kidney, bone, liver, muscle

 

    

  or

 Sex Cells

eggs, sperm  

* * *

    

or

Chromosome Number

A cell that contains both sets of homologous chromosomes are Diploid “Two Sets” Diploid=2N

Cells containing One set of chromosome are Haploid

“One Set”’ Haploid=1N Sex Cells ONLY!

Sex Cell Reproduction All living things can reproduce.

Reproduction occurs in one of two ways: Sexually

The Union of two cells, each from a different parent and containing one set of chromosomes

To produce the first cell of a new organism, having TWO full sets of chromosomes

AsexuallyProcess by which a single parent reproduces

by itself (Ex. Budding)

MeiosisTakes place in the Gametes of an organism

People have a Chromosome count of 46

When an egg joins a sperm the count must stay at 46 to remain human

So, the egg can only have 23 chromosomes, and the sperm can only have 23 chromosomes

But, the integrity of the organism must be maintained.

How does this happen?

During Meiosis gamete (sex) cells undergo a “double division”, maintaining the DNA, but reducing the chromosomal count to 23

+ =

Sperm (23) + Egg (23) = Fertilized Cell (46)

Original Gamete Metaphase Anaphase Telophase Cytokinesis

2 Daughter Cells Metaphase 2 Anaphase 2 Telophase 2 Cytokinesis – 4 Gametes

Phases of Meiosis

Meiosis involves two divisions, meiosis 1 and meiosis II By the end of meiosis II, the diploid cell that entered meiosis has become 4 haploid cells

Phases of Meiosis I

Phases of Meiosis Cells undergo a

round of DNA replication, forming duplicate chromosomes.

Interphase 1

Phases of Meiosis Each chromosome

pairs with its corresponding homologous chromosome to form a tetrad.

There are 4 chromatids in a tetrad.

MEIOSIS I Prophase I

Phases of MeiosisWhen homologous chromosomes form

tetrads in meiosis I, they exchange portions of their chromatids in a process called crossing over.

Crossing-over produces new combinations of alleles.

Phases of Meiosis Spindle fibers

attach to the chromosomes.

MEIOSIS I Metaphase I

Phases of Meiosis MEIOSIS I Anaphase I The fibers pull

the homologous chromosomes toward opposite ends of the cell.

Phases of Meiosis MEIOSIS I Telophase I

and Cytokinesis

Nuclear membranes form.

The cell separates into two cells.

The two cells produced by meiosis I have chromosomes and alleles that are different from each other.

Phases of Meiosis

Meiosis IIThe two cells produced by meiosis I now

enter a second meiotic division.Unlike meiosis I, neither cell goes through chromosome replication.

Each of the cell’s chromosomes has 2 chromatids.

Phases of Meiosis Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

MEIOSIS IIProphase II

Phases of Meiosis The chromosomes

line up in the center of cell.

MEIOSIS II Metaphase II

Phases of Meiosis The sister

chromatids separate and move toward opposite ends of the cell.

MEIOSIS II Anaphase II

Phases of MeiosisMeiosis II results in four haploid (N) daughter cells.

Cytokinesis: results in formation of: 4 sperm cells in

males1 Egg and 3 polar

bodies in females

MEIOSIS II Telophase II and

Cytokinesis

How is Meiosis different from Mitosis?

Comparing Mitosis and Meiosis

Mitosis results in the production of two genetically identical diploid cells.

Meiosis produces 4 genetically different haploid cells.

Comparing Mitosis and Meiosis

MitosisCells produced by mitosis have the same number of chromosomes and alleles as the original cell.

Mitosis allows an organism to grow and replace cells.

Some organisms reproduce asexually by mitosis.

Comparing Mitosis and Meiosis Meiosis

Cells produced by meiosis have half the number of chromosomes as the parent cell.

These cells are genetically different from the diploid cell and from each other.

Meiosis is how sexually-reproducing organisms produce gametes.

  Mitosis Meiosis

What is the number of divisions of the nucleus?    

What is the number of daughter cells produced?    

Are the daughter cells identical or not identical to one another?

   

Are the daughter cells identical or not identical to the parent cell?

   

Do the homologous chromosomes pair up to form a tetrad?

   

Is the parent cell haploid or diploid?    

Are the daughter cells haploid or diploid?    

Is this the nuclear division for somatic or sex cells?