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18

CHAPTER 3

One level of genetic variability
General recombination

Another level of genetic variability
Random distribution
2

23

 possible combinations

B

A

● Figure 3-1 (A) Diagram of chromosome 18. The diagram shows chromosome 18 in its “single chromosome’’ state
and “duplicated chromosome’’ state (that is formed by DNA replication during Meiosis I). It is important to understand
that both the “single chromosome” state and “duplicated chromosome” state will be counted as one chromosome 18.
As long as the additional DNA in the “duplicated chromosome” is bound at the centromere, the structure will be counted
as one chromosome 18 even though it has twice the amount of DNA. The “duplicated chromosome” is often referred
to as consisting of two sister chromatids (chromated 1 and chromatid 2). (B) Diagram of Meiosis I and Meiosis II. This
diagram emphasizes the changes in chromosome number and amount of DNA that occur during gametogenesis. Only
one pair of homologous chromosomes is shown (white 

 maternal origin and black  paternal origin). The point at

which DNA crosses over is called the chiasma. Segments of DNA are exchanged thereby introducing genetic variability
to the gametes. In addition, various cell types along with their appropriate designation of number of chromosomes and
amount of DNA is shown.

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19

MEIOSIS AND GENETIC RECOMBINATION

c.

There are 2

23

possible ways the maternal and paternal homologous duplicated

chromosomes can be combined. This random distribution of maternal and
paternal homologous duplicated chromosomes introduces another level of
genetic variability among the gametes.

6.

Cell division:

two secondary gametocytes (23 duplicated chromosomes, 2 N) are

formed.

B. MEIOSIS II. Events that occur during Meiosis II include

1.

Synapsis:

absent.

2.

Crossover:

absent.

3.

Alignment:

23 duplicated chromosomes align at the metaphase plate.

4.

Disjunction:

23 duplicated chromosomes separate to form 23 single chromosomes

when the centromeres split.

5.

Cell division:

gametes (23 single chromosomes, 1 N) are formed.

Genetic Recombination (Figure 3-2).

For genetic variability to occur, DNA has the

ability to undergo rearrangements by a process called genetic recombination. There are two
types of genetic recombination as indicated below:

A. GENERAL RECOMBINATION (Figure 3-2A)

1.

General recombination involves single-stranded DNA and requires DNA sequence
homology.  An important example of general recombination occurs during
crossover when 2 homologous chromosomes pair during the formation of the
gametes.

2.

Rec BCD

protein will make single-strand nicks in DNA to form single-stranded

“whiskers.”

3.

SSB

(single-strand binding) proteins stabilize the single-stranded DNA.

4.

Rec A

protein allows the single strand to invade and interact with the DNA dou-

ble helix of the other chromosome. This interaction requires DNA sequence
homology.

5.

A DNA strand on the homologous chromosome repeats the same process to form
an important intermediate structure called a crossover exchange (or Holliday junc-
tion) which consists of two crossing strands and two noncrossing strands.

6.

In a complex process called resolution that involves rotation, the DNA strands are
cut and DNA repair occurs to produce two homologous chromosomes with
exchanged segments of DNA.

B. SITE-SPECIFIC RECOMBINATION (Figure 3-2B)

1.

Site-specific recombination involves insertion of double-stranded DNA. An im-
portant example of site-specific recombination is the insertion of viral DNA into
host DNA.

2.

Many DNA viruses and other transposable elements encode for a recombination
enzyme called integrase or transposase.

3.

Integrase

recognizes specific nucleotide sequences (hence the name, site specific)

and cuts the viral DNA.

4.

The cut ends of the viral DNA attack and break the host double helix DNA.

5.

The viral DNA is inserted into the host DNA.

6.

Gaps are filled in by DNA repair.

II

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20

CHAPTER 3

Sister chromatid 1

Sister chromatid 2

Sister chromatid 1

A

B

Sister chromatid 2

Paternal

Step 1

Rec BCD

SSB

Integrase

DNA repair

cut or nick site

“Whisker”

Viral DNA

Host DNA

Crossover

exchange

(Holliday

Junction)

Step 2

Rec A

Repeat

Step 1

and

step 2

Cut DNA

DNA

repair

Maternal

Resolution

● Figure 3-2 Types of genetic recombination. (A) This diagrams shows general recombination that occurs during
meiosis. (B) This diagram shows site-specific recombination that occurs during DNA viral infection. (

) Cut or nick sites.

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21

MEIOSIS AND GENETIC RECOMBINATION

COMPARISON OF MEIOSIS AND MITOSIS

TABLE

3-1 

Meiosis

Mitosis

Occurs only in the testis and ovary

Occurs in a wide variety of tissues 

and organs

Produces haploid (23, 1N) gametes (sperm and secondary oocyte)

Produces diploid (46, 2N) somatic 

daughter cells

Involves two cell divisions and one round of DNA replication

Involves one cell division and one 

round of DNA replication

Stages of Meiosis

Stages of Mitosis

Meiosis I

Interphase

Meiosis S Phase (DNA Replication) 

G

0

Phase

Prophase

G

1

Phase

Leptotene (homologue pairing begins; long, thin DNA stands)

S Phase

Zygotene (synapsis occurs; synaptonemal complex is formed)

G

2

Phase

Pachytene (crossover occurs; short, thick DNA strands)

Prophase

Diplotene (chromosomes separate except at centromere)

Prometaphase

Prometaphase

Metaphase

Metaphase

Anaphase

Anaphase

Telophase

Telophase

Meiosis II (essentially identical to mitosis)

Prophase
Prometaphase
Metaphase
Anaphase
Telophase

Male: Prophase of Meiosis I lasts 

22 days and completes Meiosis II 

Interphase lasts 

15 hours 

in a few hours

M phase lasts 

1 hour

Female: Prophase of Meiosis I lasts 

14 years (until puberty) and 

completes Meiosis II when fertilization occurs

Pairing of homologous chromosomes occurs 

No pairing of homologous 

chromosomes

Genetic recombination occurs (exchange of large segments of 

Genetic recombination does not 

maternal and paternal DNA via crossover during Meiosis I)

occur

Maternal and paternal homologous chromosomes are randomly

Maternal and paternal homologous 

distributed among the gametes to ensure genetic variability

chromosomes are faithfully 
distributed among the daughter
cells to ensure genetic similarity

Gametes are genetically different

Daughter cells are genetically identical

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Chapter

4

The Human Nuclear Genome

22

General Features (Figure 4-1)

A. The human genome refers to the total DNA content in the cell which is divided into

two genomes: the very complex nuclear genome and the relatively simple mitochon-
drial genome.

B. The human nuclear genome consists of 24 different chromosomes (22 autosomes; X

and Y sex chromosomes).

I

C. The human nuclear genome codes for

30,000 genes (precise number is uncertain)
which make up 

2% of human nuclear

genome. Figure 4-1 shows the organization of
the human nuclear genome.

D. There are 

27,000 protein-coding genes (i.e.,

they follow the central dogma of molecular bi-
ology: DNA transcribes RNA S mRNA trans-
lates protein).

E.

There are 

3000 RNA-coding genes (i.e., they

do not follow the central dogma of molecular
biology: DNA transcribes RNA S RNA is NOT
translated into protein).

F.

The fact that the 

30,000 genes make up only

2% of the human nuclear genome means
that 

2% of the human nuclear genome con-

● Figure 4-1 Pie chart indicating the or-
ganization of the human nuclear genome.

~45%

Transposons

~44%
Other

~7%

Heterochromatin

~2%

~30,000 genes
    • ~27,000 protein-coding
      genes
    • ~3000 RNA-coding genes

Coding DNA 
Noncoding DNA

sists of coding DNA and 

98% of the human nuclear genome consists of noncoding

DNA.

G. There is no correspondence between biological complexity of a species and the num-

ber of protein-coding genes and RNA-coding genes (i.e., biological complexity 

amount of coding DNA).

H. There is correspondence between biological complexity of a species and the amount of

noncoding DNA (i.e., biological complexity 

 amount of noncoding DNA).

I.

To fully understand how heritable traits (both normal and disease related) are passed
down, it is important to understand three aspects of the human nuclear genome which
include the following:

1.

Protein-coding genes.

For decades, protein-coding genes were enshrined as the

sole repository of heritable traits. A mutation in a protein-coding gene caused the for-
mation of an abnormal protein and hence an altered trait or disease. Today, we

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