Copyright © 2012 Pearson Education, Inc. Chapter 19 Chromosomes and Cell Division Betty McGuire...

Chapter 19

Chromosomes and Cell Division

Betty McGuireCornell University

Lecture Presentation

Chromosomes and Cell Division

Two types of cell division Form of chromosomes The cell cycle Mitosis: Creation of genetically identical

diploid body cells Cytokinesis Karyotypes Meiosis: Creation of haploid gametes

Two Types of Cell Division

The human life cycle has two types of cell division Meiosis Mitosis

Meiosis Gives rise to gametes that have half the

number of chromosomes as the original cell In females

Occurs in ovaries Produces eggs

In males Occurs in testes Produces sperm

Mitosis Results in identical body cells Occurs during growth and repair

[INSERT FIGURE 19.1 ON THIS SLIDE]

Web Activity: The Human Life Cycle

Form of Chromosomes

Chromosomes Location: nucleus Structure and function: each is a tightly

coiled combination of a DNA molecule and specialized proteins called histones DNA contains genetic information, which

directs body development and maintenance

Histones help with support and control of gene activity

Form of Chromosomes

Gene Specific segment of the DNA

Directs synthesis of a protein, which plays a structural or functional role in the cell

Form of Chromosomes

Somatic cells All cells except eggs and sperm In humans, have 46 chromosomes

Two sets of 23 chromosomes One set of 23 from each parent

Form of Chromosomes

Each somatic cell contains two chromosomes with genes for the same traits Called homologous pairs of chromosomes

One chromosome of the pair is from the mother

One chromosome of the pair is from the father

Form of Chromosomes

Diploid A cell with two sets of chromosomes (2n)

Genes also occur in pairs in diploid cells Members of each gene pair are located at

the same position on homologous chromosomes

Form of Chromosomes

Of the 23 pairs of chromosomes Sex chromosomes make up one pair

Determine gender Two types: X and Y XX = genetic female XY = genetic male

Autosomes make up 22 pairs Determine expression of most of a

person’s inherited characteristics

The Cell Cycle

Mitosis The process of cell division for body cells When one nucleus divides into two daughter

nuclei with the same number and kinds of chromosomes

The Cell Cycle

Cell cycle Predictable set of events that a cell goes

through from its origin to its own division into two daughter cells

Two major phases Interphase Cell division

The Cell Cycle

Interphase Period of growth and preparation for cell

division; not a “resting period” Three phases

The Cell Cycle

Interphase G1

First “gap” Time of major growth before DNA

synthesis begins Chromosomes consist of a strand of DNA

and proteins

The Cell Cycle

Interphase S

DNA is replicated, and the two copies of the chromosome, called chromatids, remain attached at the centromere Two attached chromatids are

genetically identical Called sister chromatids

Growth continues

The Cell Cycle

Interphase G2

Second “gap” Period after DNA is synthesized and

before mitosis begins Growth continues

The Cell Cycle

Cell division consists of two processes Mitosis

Division of the nucleus Cytokinesis

Division of the cytoplasm

The Cell Cycle

Developing embryo and fetus Body cells divide continually

Children Body cells divide in growth and repair

Adults Some cells lose ability to divide (most neurons) Some cells stop dividing but retain ability to divide

should the need arise (liver cells) Some cells actively divide throughout life (skin cells)

Mitosis: Creation of Genetically Identical Diploid Body Cells

Mitosis occurs in four phases Prophase Metaphase Anaphase Telophase

Prophase Chromatin condenses and forms

chromosomes as DNA wraps around histones

Nuclear membrane begins to break down Mitotic spindle forms

Metaphase Chromosomes attach to mitotic spindles and

form a line at the center of the cell Alignment ensures each daughter cell

receives one chromatid from each of the 46 chromosomes

Anaphase Sister chromatids of each chromosome

begin to separate, splitting at centromere Now separate entities, the sister chromatids

are considered chromosomes

Telophase Nuclear envelope forms around each group

of chromosomes at each pole Mitotic spindle disassembles

Web Activity: Mitosis

Cytokinesis

Cytokinesis Division of the cytoplasm Begins during telophase Ring of microfilaments contracts at midline of

cell, eventually pinching cell in two

Karyotypes

Karyotype Constructed by arranging chromosomes

from photographs based on size and centromere location

Can be examined for defects in number and structure of chromosomes

Meiosis: Creation of Haploid Gametes

Gametes Differ from somatic cells

Haploid (n) Have only one member of each

homologous pair of chromosomes 23 chromosomes

Formed by meiosis Two divisions that result in up to four

haploid daughter cells

Functions of meiosis Keeps the number of chromosomes in a

body cell constant from generation to generation

Increases genetic variability in population

Meiosis involves two cell divisions Meiosis I – separates homologues Meiosis II – separates sister chromatids

Each division has its own Prophase Metaphase Anaphase Telophase

Meiosis I Preceded by interphase, during which each

chromosome was copied and now consists of two attached chromatids

Reduction division because it produces two haploid cells Each daughter cell has 23 chromosomes

One member of each homologous pair Each chromosome consists of two

sister chromatids

Prophase I Chromosomes condense Homologous chromosomes pair

Phenomenon called synapsis

Metaphase I Matched homologous pairs line up at the

midline of cell and attach to spindle fibers

Anaphase I Homologous pairs of chromosomes separate

and move to opposite ends of cell Each homologue still consists of two sister

chromatids

Telophase I Nuclear envelope forms around

chromosomes at each end of cell Cytokinesis occurs to form two haploid cells

Interkinesis Brief interphase-like period before meiosis II Differs from mitotic interphase in that there is

no replication of DNA

Meiosis II Prophase II

Chromosomes condense Metaphase II

Chromosomes line up along midline of cell

Meiosis II (cont.) Anaphase II

Centromere holding two sister chromatids separates

Chromatids of each pair now considered chromosomes

Chromosomes move to opposite poles of cell

Meiosis II (cont.) Telophase II

Nuclear membrane forms around chromosomes at each end of cell

Cytokinesis occurs in both daughter cells, forming four haploid daughter cells

The important feature of meiosis II It separates the two sister chromatids of

each chromosome

Web Activity: Comparing Mitosis and Meiosis

Following meiosis, changes in shape and function of the haploid cells result in functional gametes Spermatogenesis produces four sperm cells

specialized for transporting the male’s genetic information to the egg

Oogenesis produces up to three polar bodies and one ovum packed with nutrients to nourish the early embryo

During meiosis, genetic variation is created through Crossing over Independent assortment

Crossing over Occurs when corresponding pieces of

chromatids of maternal and paternal homologues are exchanged during synapsis Prophase I

Independent assortment The relative positioning of homologous

maternal and paternal chromosomes with respect to poles of the cell is random Members of each homologous pair orient

independently of other pairs Metaphase I

Nondisjunction Failure of homologous chromosomes to

separate during meiosis I or of sister chromatids to separate during meiosis II

Nondisjunction results in too many, or too few, chromosomes in a cell The imbalance of chromosome numbers

usually causes abnormalities in development and miscarriage

Trisomy Three representatives of a chromosome in

a cell

Monosomy Only one representative of a chromosome

in a cell

Down syndrome An infant born with three copies of

chromosome 21 Risk of having a baby with Down syndrome

increases with maternal age Multiple physical and mental abnormalities

Nondisjunction also can occur with sex chromosomes (X and Y) Nondisjunction of sex chromosomes during

sperm formation Resulting sperm will carry both X and Y or

no sex chromosome at all Nondisjunction of sex chromosomes during

egg formation Resulting egg will have two X

chromosomes or none at all

Turner syndrome XO

Klinefelter syndrome XXY

Copyright © 2012 Pearson Education, Inc. Chapter 19 Chromosomes and Cell Division Betty McGuire...

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