The Cellular Basis of Inheritance

Chapter 8

All cells come from cells

• “Where a cell exists, there must have been a preexisting cell…” (Rudolf Virchow 1855)

• Repair and growth– Replacement of lost or damaged cells– Increasing in size

All cells come from cells

• Reproduction– Asexual reproduction= process in which a

single cell or group of cells each duplicates its genetic material then splits into two new genetically identical cells• Paramecium• Sea stars• Geranium

All cells come from cells

• Sexual reproduction= the process in which, the genetic material from each of two parents combines, producing offspring that differ genetically from either parent– Involves the union of sex cells (egg and sperm)

The cell cycle multiplies cells

• Nucleus – Contains almost all of an organisms genes– Chromatin= long, thin fibers made up of DNA

and proteins, located in the nucleus of eukaryotic organisms

– Chromosomes= condensed threads of genetic material formed from chromatin as a cell prepares to divide

The cell cycle multiplies cells• Before division a cell duplicates all of its

chromosomes…each chromosome now consists of two identical joined copies called sister chromatids

• Centromere= the region where the two chromatids are joined together

• Humans have 46 chromosomes

The cell cycle multiplies cells

• Eukaryotic cells that divide undergo an organized sequence of events called the cell cycle

• Interphase= stage during which a cell carries out its metabolic processes and performs its functions– G1= growth phase– S= synthesis phase…duplication of DNA– G2= cell gets ready for division– M= mitotic phase

The cell cycle multiplies cells

• Mitotic phase (2 stages)1. Mitosis= nucleus and duplicated

chromosomes divide and are evenly distributed

2. Cytokinesis= cytoplasm divides

Cells divide during the mitotic phase

• Movement of chromosomes is guided by a football shaped framework of microtubules called the spindle

• The spindle microtubules grow from two centrosomes= in animal cells, area of cytoplasmic material that contain structures called centrioles

• Centriole– Role unknown– Plant cells lack them– If a cell does not have them, the spindle will still form

normally

Mitosis

Prophase

Metaphase

Anaphase

Telophase

• Prophase– 1st stage– Chromosomes appear, seen as a pair of sister chromatids– Spindle forms– Chromatids attach to microtubules in spindle

• Metaphase– 2nd stage– Chromosomes line up in the center of the cell– Spindle is completely formed

• Anaphase– Sister chromatids separate…each chromatid

is called a daughter chromosome– Proteins in the centromere help the daughter

chromosomes travel along the microtubules toward the poles

– At the same time the spindle fibers are shortening, bringing the chromosomes closer to the poles

– The spindle fibers that are not attached to the centromeres grow longer, pushing the poles farther apart

• Telophase– Begins when the chromosomes reach the

poles of the spindle– Spindle disappears– 2 nuclear envelopes reform around each set

of daughter chromosomes– Chromosomes uncoil and lengthen– Nucleoli reappear

• Mitosis is now finished• Cytokinesis completes the division

process

CytokinesisAnimal Cells

• Indentation around the middle of the cell• Caused by a ring of microfilaments in the

cytoplasm, underneath the plasma membrane

• Like a drawstring

Plant cells• Disk containing cell wall material, called a

cell plate, forms inside the cell and grows outward

plant cell frog cell

What would happen if cells did not have mechanisms that

controlled cell division?

Licentious division - prostate cancer cells during division.

• Benign tumor= abnormal mass of “normal” cells

• Malignant tumor= masses of cells that result from the reproduction of cancer cells

• Cancer= a disease caused by the severe disruption of the mechanisms that control the cell cycle…leading to uncontrolled cell division

• Metastasis= spread of cancer

• Cancer treament– Radiation– Chemotherapy

Meiosis• Occurs in the sex organs

– Ovaries– Testes

• Typical human body cell contains 46 chromosomes…we get 23 chromosomes from our mother and 23 chromosomes from out father

• Karyotyping matches up our chromosomes in pairs, according to size, location of centromere, and staining bands

• Each pair consists of one maternal chromosome and one paternal chromosome

• Each pair of matching chromosomes are called homologous chromosomes

• Each homologous chromosome in a pair carries the same sequence of genes controlling the same inherited characteristics– Ex: eye color

• How do homologous chromosomes differ from sister chromatids?

• We have 23 pairs of chromosomes– 1 pair is called the sex chromosomes– Females have 23 homologous pairs

• The sex chromosomes look alike (X X)

– Males have 22 homologous pairs• The sex chromosomes are different (X Y)• Most of the genes carried on the X chromosome

do not have counterparts on the tiny Y chromosome and vice versa

X chromosomeY chromosome

• Diploid cell– Typical human body cell– Two sets of chromosomes– 46 chromosomes...for humans (will differ

depending on the species)

• Haploid cell– One set of chromosomes– (Half the number of chromosomes)

– Produced through the process of meiosis

• Gametes= sex cells (haploid)…eggs and sperm

• Fertilization= the fusion of the nuclei and cytoplasm of the sex cells

• Zygote= fertilized egg (diploid)– Has two homologous sets of chromosomes

• The zygote eventually develops into a sexually mature adult with trillions of cells produced by mitosis

• Interphase• Cell duplicates its DNA• Each chromosome consists of 2 identical sister chromatids

• Meiosis I– Prophase I

• Proteins cause the homologous chromosomes to stick together along their length

• Paired chromosomes now consist of 4 chromatids, called a tetrad

– Metaphase I• Tetrads line up in middle of cell

– Anaphase I• Homologous chromosomes separate and move to opposite

poles– Telophase I

• Chromosomes arrive at the poles• Nucleus forms around chromosomes• Each pole now has a haploid daughter nucleus (it has only

one set of chromosomes even though each chromosome consists of two sister chromatids)

– Cytokinesis

• Meiosis II– Prophase II

• Within each haploid cell, a spindle forms

– Metaphase II• Chromosomes line up in the middle of the cell

– Anaphase II• Sister chromatids separate and move to opposite

poles

– Telophase II• Chromatids are now considered individual

chromosomes• Chromosomes arrive at the poles

– Cytokinesis

• Result is 4 daughter cells

Genetic Variation

• Offspring that result from sexual reproduction are genetically different from their parents and from their siblings

• Genetic variation depends on1. How the homologous chromosomes line up and

separate at meiosis I• The assortment of chromosomes occurs randomly• For humans there are about 8 million possible

chromosome combinations (223)

2. Crossing over= the exchange of genetic material between homologous chromosomes• Occurs during prophase I

Crossing Over• Homologous chromosomes are stuck together along

their length• There is a precise gene-by-gene alignment between

adjacent chromatids of the two chromosomes• Segments of chromosomes can be exchanged at one or

more sites• Genetic recombination= a new combination of genetic

material as a result of crossing over• A single chromosome contains many hundred genes• More than one crossing over event can occur in each

tetrad• No wonder offspring can be so varied

• A karyotype is a display of chromosomes paired according to their size, location of the centromere, and staining patterns.

• A karyotype reveals abnormalities in chromosome number or structure.

• Humans have 23 pairs of chromosomes; 22 pairs of autosomes and one pair of sex chromosomes.

• Autosome= a non-sex chromosome• Females are XX and males are XY.

Amniocentesis

• Amniocentesis uses a needle to extract amniotic fluid from the uterus of a pregnant woman from the 14th to 17th week of pregnancy.

• Up to 400 chromosome and biochemical problems can be detected by culturing fetal cells that are in the amniotic fluid.

• There is a slight risk of spontaneous abortion with this procedure.

Amniocentesis

Karyotyping

• Sampled fetal cells are stimulated to divide in culture medium and another chemical stops division during metaphase when chromosomes are highly condensed.

• The stained cells are photographed and can be paired based on stained cross-bands, and size and shape.

Downs Syndrome

Asexual Reproduction vs. Sexual Reproduction

• When you think about the genetic information:– Is it the same for asexual reproduction? – Is it the same for sexual reproduction?

Binary Fission

– Binary fission means “dividing in half”– Occurs in prokaryotic cells– Two identical cells arise from one cell– Steps in the process

– A single circular chromosome duplicates, and the copies begin to separate from each other

– The cell elongates, and the chromosomal copies separate further

– The plasma membrane grows inward at the midpoint to divide the cells

Prokaryoticchromosome

Duplication of chromosomeand separation of copies

Cell wall

Plasmamembrane

1

Continued elongation of thecell and movement of copies

2

Division intotwo daughter cells

3

Asexual Reproduction vs. Binary Fission

• How are they the same?

• How are they different?