Unit 8

Chromosomes

Meiosis

Genetics

Review

• What is a chromosome?

• What is a gamete?

• When can chromosomes be seen in the nucleus of a cell?

• What is this process called?

• What are genes?

• Where are genes located?

Chromosomes

• DNA wrapped around protein cores (histones) and coiled further into a rod-shaped form

• Only seen during cell division (mitosis)

• Since mitosis is occurring, how much DNA is present?

Chromosomes

• Chromatid: one half of the chromosome

• Centromere: the constricted “middle” of the chromosome

• Chromatin: the DNA and histones in the nucleus of a non-dividing cell

Chromosome Number

• Each species has a characteristic number of chromosomes

• Adder’s tongue fern has 2524 chromosomes

• Does the complexity of the organism correspond to its chromosome number?

Chromosomes are categorized as either sex chromosomes or

autosomes.• Humans have two sex chromosomes:

XX = female

XY = male

• All other chromosomes are autosomes.

(22 of the 23 pairs)

Homologous chromosomes

• Every cell of an organism (except gametes) has two copies of each autosome—one from each parent.

• Homologous chromosomes: the two copies of each autosome; also known as homologues.

• Are the same size and shape, and carry genes for the same traits

Karyotype

• A karyotype is a photomicrograph of the chromosomes in a dividing cell

• The chromosomes are arranged by size, with the largest pair being designated pair 1, and with the sex chromosomes being designated the last pair.

What is the sex of this individual?

What about this one?

Diploid vs. Haploid Cells

• Diploid: cells having two sets of chromosomes (2n)

diploid # for humans = 46• Haploid: cells having only one set of

chromosomes (n)

haploid # for humans = 23

Which cells in our bodies would be haploid?

Why is it necessary for these cells to be haploid?

Meiosis

• Meiosis: the process of making gametes

• Reduces the number of chromosomes in new cells to half the number in the original cell

• Mitosis: cell division

• Meiosis: nuclear division

• Meiosis Animation

Interphase

• G1 (gap 1) phase: cell growth

• S (synthesis) phase: DNA is copied

• G2 (gap 2) phase: growth and preparation for cell division

Chromatids, Chromosomes, Eiyiyi…I can’t keep it all

straight!

• G1 – DNA has not been copied yet (46 chromosomes)

• G2 – DNA has been copied (still 46 chromosomes—made up of 92 sister chromatids)

Meiosis I

• Prophase I: DNA coils tightly into chromosomes; spindle fibers appear; nucleus dissolves; each chromosome lines up next to its homologue (synapsis)

• Each pair of homologous chromosomes is called a tetrad.

• Genes on one chromosome are adjacent to the corresponding genes on the other chromosome

Crossing-Over• During synapsis, the

chromatids within a homologous pair twist around one another.

• Portions of chromosomes may break off and attach to adjacent chromatids on the homologous chromosome—crossing-over

• This process permits the exchange of genetic material between maternal and paternal chromosomes.

Meiosis vs. Mitosis

Size of Genomes

• Does the # of genes have anything to do with the complexity of the organism?

Classical Genetics

The Legacy of Gregor Mendel

Or

The Monk with the Missing “Peas”

The Big Question• Why do children look like their parents?

Genetics

• The branch of biology which deals with heredity

-or-

• Why do children look like their parents?

The Father of Genetics

• Gregor Mendel was the 1st person to succeed in predicting how traits are transferred from one generation to the next

Why is he so important?• He studied one trait at a time

• He analyzed his data mathematically

• He looked at multiple traits

• He used multiple trials

The First Generation

• Mendel chose true-breeding pea plants as his parental generation (when self-pollinated, always produced the same type of offspring)

• He crossed a true-breeding tall plant with a true-breeding short plant

• All of the offspring were tall!

The Second Generation

• Next, he crossed two tall offspring plants with each other

• ¾ of the offspring in the second generation were tall; ¼ were short

How to self-pollinate a pea plant

The Second Generation, cont.

• Mendel did similar monohybrid crosses with the other traits as well.

• In every case, he found that one trait seemed to disappear in the F1 generation and reappear in ¼ of the F2 plants

• This is where “dominant” and “recessive” come from

The Law of Segregation

• Based on the results of his second generation crosses

• The two alleles for each trait must separate when gametes are formed

• A parent, therefore, passes on at random only one allele for each trait to each offspring

Mendel’s Dihybrid Crosses

• Performed another set of crosses where he used peas that differed from each other in two traits rather than just one

The first generation

• Took true-breeding pea plants that had round yellow seeds (RRYY) and crossed them with true-breeding pea plants that had wrinkled green seeds (rryy).

• The F1 plants all had round yellow seeds

The second generation

• F1 plants self-pollinated

• F2 plants:

9 round yellow

3 round green

3 wrinkled yellow

1 wrinkled green

The Law of Independent Assortment

• Genes for different traits are inherited independently of each other

• When a pea plant with the genotype RrYy produces gametes, the alleles R and r will separate from each other (the law of segregation) as well as from the alleles Y and y and vice versa

• These alleles can then recombine in four different ways

• We now know that this is only true if genes are located on different chromosomes or are far apart on the same chromosome

Testcross• A cross of an individual of unknown genotype

with an individual of a known genotype (usually homozygous recessive)

• Unknown R_ x rr

• If any offspring show the recessive phenotype, then the unknown parent must have been heterozygous

Pedigrees

• A graphic representation of an individual’s family tree, which permits patterns of inheritance to be recognized.

When are pedigrees used?

• When testcrosses cannot be made

• When number of offspring is too small

• Or if results of testcross would take too long