LectureLecture 1 1

Chapter I: Cellular Basis of Heredity 

The cell

The cell is the functional basic unit of life

There are two types of cells: eukaryotic and prokaryotic. Prokaryotic cells are usually independent, while eukaryotic cells are often found in multicellular organisms.

Organisms that reproduce Sexually are made up of two different types of cells.

1. Somatic Cells are “body” cells and contain the normal number of chromosomes ….called the “Diploid” number (the symbol is 2n). Examples would be … skin cells, brain cells, etc.

2. Gametes are the “sex” cells and contain only ½ the normal number of chromosomes…. called the “Haploid” number (the symbol is n)….. Sperm cells and ova are gametes.

Fertilization

The fusion of a sperm and egg to form a zygote.

A zygote is a fertilized egg

n=23egg

sperm n=23

2n=46zygote

Chromosomes

If an organism has the Diploid number (2n) it has two matching homologues per set. One of the homologues comes from the mother.… the other homologue comes from the father.

Most organisms are diploid. Humans have 23 sets of chromosomes… therefore humans have 46 total chromosomes….. The diploid number for humans is 46 (46 chromosomes per cell).

Homologous Chromosomes(because a homologous pair consists of 4 chromatids it is called a “Tetrad”)

Paternal Maternal

eye color locus

eye color locus

hair color locus

hair color locus

Humans have 23 Sets of Homologous ChromosomesEach Homologous set is made up of 2 Homologues.

Homologue

Homologue

Sex ChromosomesThe Sex Chromosomes code for the sex of the offspring.** If the offspring has two “X ” chromosomes it will be a female. ** If the offspring has one “X ” chromosome and one “Y” chromosome it will be a male.

XX chromosome - female XY chromosome - male

In Humans the “Sex Chromosomes” are the 23rd set

Types of Chromosome

Chromosomes are placed into broad categories depending on the position of the centromere.

Metacentric: centromere in the middle, with arms of equal length. (A)

Telocentric: centromere at one end, with only 1 arm. (D) Acrocentric: centromere near one end, with arms of very

different lengths. (C) Sub-metacentric: centromere near the middle, with

arms of slightly different lengths. (B)

Cell Division

Mitosis & Meiosis

Phases of Mitosis

Mitosis

The Basic Phases of a Cell’s Life:

•Interphase

•Prophase

•Metaphase

•Anaphase

•Telophase

•Cytokinesis

Interphase – The Cell spends the majority of its life here, growing and functioning. During the S Phase of the Cell Cycle, the DNA replicates, in anticipation of Mitosis

InterphaseInterphase

ProphaseThe Cell begins

the division process

1.The nucleolus disappears,

2.The nuclear membrane breaks apart

ProphaseProphase

3. The chromosomes become visible

4. The spindle apparatus forms and attaches to the centromeres of the chromosomes

ProphaseProphase

During Metaphase the Chromosomes line up across center of the cell, also called the equator, or Metaphase plate.

Spindle Fibers

Chromosomes

Equator, or Metaphase Plate

MetaphaseMetaphase

In Anaphase the Chromatids that make up each Chromosome move apart and travel to opposite ends of cellular spindle

Daughter Chromosomes

Chromosome

Chromatid

AnaphaseAnaphase

In Telophase an envelope surrounds each set of Chromatids to form new Nucleus and the Cytoplasm starts to divide

Cleavage Furrow

TelophaseTelophase

Cytokinesis takes place when the Cytoplasm divides and two cells with identical genetic material are formed

Daughter Cells

CytokinesisCytokinesis

A B C

D E

Quick Review – Place Cells in Mitosis Order

MitosisMitosis

Quick Review: Identify What happens in each phase of Mitosis:

Prophase

Metaphase

Anaphase

Telophase

Interphase

Meiosis

Why We Are Who We Are

Meiosis

is the process by which ”gametes” (sex cells) , with half the number of chromosomes, are

produced.

During Meiosis diploid cells are reduced to haploid cells

Diploid (2n) Haploid (n)

If Meiosis did not occur the chromosome number in each new generation would

double…. The offspring would die.

Meiosis

Meiosis is Two cell divisions (called meiosis I and meiosis II)

with only one duplication of chromosomes.

Meiosis in males is called spermatogenesis and produces sperm.

Meiosis in females is called oogenesis and produces ova.

Spermatogenesis

2n=46

humansex cell

diploid (2n)

n=23

n=23

meiosis I

n=23

n=23

n=23

n=23

sperm

haploid (n)

meiosis II

4 sperm cells are produced from each

primary meiosis

Primary Spermatocyte

Secondary Spermatocyte

Secondary Spermatocyte

Oogenesis

*** The polar bodies die… only one ovum (egg) is produced from each

primary oocyte.

Phases of Meiosis

Interphase I

Similar to mitosis interphase.

Chromosomes replicate (S phase).

Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.

Centriole pairs also replicate.

Meiosis I (four phases)

Cell division that reduces the chromosome number by one-half.

four phases:a. prophase Ib. metaphase Ic. anaphase Id. telophase I

Prophase I

Longest and most complex phase. 90% of the meiotic process is spent in

Prophase I

Chromosomes condense.

Synapsis occurs: homologous chromosomes come together to form a tetrad.

Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I

centriolesspindle fiber

asterfibers

Question 1:

A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

10 chromosomes (haploid)

Question 2:

A cell containing 40 chromatids40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomeschromosomes?

10 chromosomes (haploid)

Metaphase IMetaphase I

Shortest phaseShortest phase

TetradsTetrads align on the metaphase platemetaphase plate.

INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS:1. Orientation of homologous pair to poles is random.2. Variation3. Formula: 2n

Example:Example: 2n = 42n = 4 then then n = 2 n = 2 thusthus 2 22 2 = 4 combinations= 4 combinations

Metaphase IMetaphase I

metaphase plate

OR

metaphase plate

Anaphase IAnaphase I

Homologous chromosomes separate and move towards the poles.

Sister chromatids remain attached at their centromeres.

Anaphase IAnaphase I

Telophase ITelophase I

Each pole now has haploidhaploid set of chromosomeschromosomes.

CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase ITelophase I

Meiosis IIMeiosis II

No interphase II (or very short - no more DNA replication)

Remember: Meiosis II is similar to mitosis

Prophase IIProphase II

same as prophase in mitosis

Metaphase IIMetaphase II

same as metaphase in mitosis

metaphase platemetaphase plate

Anaphase IIAnaphase II

same as anaphase in mitosis sister chromatids separate

Telophase IITelophase II

Original Gamete Metaphase 1 Anaphase 1 Telophase 1 Cytokinesis

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

MeiosisMeiosis

Mitosis vs. Meiosis

Mitosis Meiosis

Results in 2 diploid cells (2n) 4 haploid cells (1n)

Cells are Genetically identical Genetically different

Occurs in Somatic (body) Cells Sex Cells