Human Biology (BIOL 104)

Talk Eleven:

Cell Reproduction

Chapter 18

A Quick look at DNA• DNA is organized into structures called chromosomes

in the nucleus of a cell.

• When a cell is not dividing, its threadlike chromosomes are dispersed in the nucleus.

• As a cell prepares to divide, however, each chromosome is copied and each copy is coiled and packed tightly.

• Each copy is condensed as it coils back on itself again and again

• The DNA loops around some proteins (called histones), forming beadlike structures. The “beads” then coil up into a long fiber.

Types of cells • In biology, reproduction is when a “parent” cell

produces a new generation of cells, or when parents produce a new individual.

• Reproduction is part of a life cycle, a recurring series of events in which individuals grow, develop, maintain themselves, and reproduce a new generation.

• The instructions for the human life cycle are encoded in our DNA, which we inherit from our parents.

• Two types of cells:– Somatic Cells– Gametes

Somatic Cells

• Non-sex Cells. • Contain a full

compliment of chromosomes

• Characteristic to their species.

• Referred to as the diploid number of chromosomes.

• Diploid

• Means double number.

• Designated 2n

• All somatic cells in an organism have the 2n or diploid number of chromosomes.

Somatic CellsHuman somatic cells have 46 chromosomes. A The presence of pairs of chromosomes (two of each type) tells you that they came from a diploid cell.

One member of each pair contains genetic instructions inherited from the father.

The other member contains instructions from the mother.

B Close-up of a pair of homologous chromosomes from an animal cell.

Gametes

• Sex Cells

• Contain one half the compliment of chromosomes characteristic to their species.

• Referred to as the haploid number of chromosomes.

• Haploid

• Means single number.

• Designated n

• All gametes formed by an organism have the n or haploid number of

• chromosomes.

The Human cell cycle• G0 phase• The term "post-mitotic" is

sometimes used to refer to both quiescent and senescent cells. Nonproliferative cells in multicellular eukaryotes generally enter the quiescent G0 state from G1 and may remain dormant for long periods of time, possibly indefinitely.

• This is very common for cells that are fully differentiated.

• Cellular senescence occurs in response to DNA damage or degradation that would make a cell's progeny nonviable

The Human cell cycle• G1 phase• It is also called the growth phase.

• During this phase the biosynthetic activities of the cell, which had been considerably slowed down during M phase, resume at a high rate.

• This phase is marked by the use of 20 amino acids to form millions of proteins and later on enzymes that are required in S phase, mainly those needed for DNA replication.

• Duration of G1 is highly variable, even among different cells of the

same species.

The Human cell cycle• S phase• Starts when DNA replication

commences; when it is complete, all of the chromosomes have been replicated, i.e., each chromosome has two (sister) chromatids.

• Thus, during this phase, the amount of DNA in the cell has effectively doubled, though the ploidy of the cell remains the same.

• During this phase, synthesis is completed as quickly as possible due to the exposed base pairs being sensitive to external factors– ie - U.V, Heat, drugs

The Human cell cycle• G2 phase• During the gap

between DNA synthesis and mitosis, the cell will continue to grow.

• The G2 checkpoint control mechanism ensures that everything is ready to enter the M (mitosis/ Mieosis) phase and divides

Mitosis or Meiosis• For much of the cell division process, each chromosome and its

copy stay together.

• During this time they are called sister chromatids.

– A sister chromatid has at least one “pinched in” region called a centromere.

• It will provide attachment sites for microtubules that move chromosomes when a cell nucleus is dividing.

Mitosis or Meiosis• Mitosis maintains the

chromosome number. 

• After mitosis is complete, the parent cell’s cytoplasm divides.

• Mitosis occurs in cells that are dividing as the body grows or repairs itself.

• It assures that each new “daughter” cell is diploid, with a full complement of 46 parental chromosomes.

Mitosis or Meiosis• Meiosis occurs in germ cells in the ovaries and

testes

• It halves the number of parental chromosomes

• Two rounds of division, called meiosis I and meiosis II produces gametes that are haploid cells

• half the diploid number of chromosomes.

Stages of Division- Mitosis

• Prophase--nuclear envelope breakdown, chromosome condensation, spindle formation.

• Metaphase--chromosomes are lined up precisely on the metaphase plate, or middle of the cell.

• Anaphase--spindle pulls sister chromatids apart.

• Telophase--chromatids begin

to decondense and become chromatin. Spindle disappears.

• Cytokinesis--divide cell and organelles. Actin ring, or cleavage furrow splits cell.

Mitosis Begins with Prophase

• During interphase each chromosome was duplicated, forming two sister chromatids joined at the centromere.

• The sister chromatids of each chromosome twist and fold into a more compact form.

• By the end of prophase, all chromosomes will be condensed into thick rod shapes.

• Chromosomes move into new positions with help of a spindle.

• This structure consists of microtubules that extend from centrioles

Moving from Prophase to Metaphase• A lot happens between

prophase and metaphase—so much that this transitional period has its own name, “prometaphase.”

• The nuclear envelope breaks apart, allowing the chromosomes in the nucleus to interact with microtubules extending toward them from the poles of the forming spindle.

• When all the duplicated chromosomes are lined up midway between the poles of a spindle, we say the cell is in metaphase (meta-means “midway between”). – This alignment sets the

stage for anaphase

Anaphase, then Telophase

• During anaphase, the sister chromatids of each chromosome separate from each other and move to opposite spindle poles.

• Once each chromatid is separated from its sister, it is an independent chromosome.

• Telophase begins as soon the two clusters of chromosomes arrive at a spindle pole.

• New nuclear envelope forms around each cluster, separating it from the cytoplasm.

• Once two nuclei form, telophase is over—and so is mitosis

How the Cytoplasm Divides• Cytokinesis, usually begins toward the end of anaphase.

• By this time, the two sister chromatids of each chromosome have been separated and are independent chromosomes. About midway between the cell’s two poles, a patch of plasma membrane sinks inward, forming a cleavage furrow.

• Now there are two new cells, each with a nucleus, cytoplasm, and a plasma membrane.

What is the Function of Meiosis?• Meiosis is the first step in

the formation of gametes—sperm and eggs—for sexual reproduction.

• Meiosis reduces the parental chromosome number by half, to the haploid number.

• In males, meiosis and gamete formation are called spermatogenesis.

• In females, these two processes are called oogenesis.

Meiosis – a two stage process

Pieces of Chromosomes may

be Exchanged• No one ever looks, or has a

body that operates, exactly like his or her parents.

• Most of the trait variations we take for granted result from changes to chromosomes.

• These occur during meiosis, when germ cells were forming sperm in a father’s testes or eggs in a mother’s ovaries.

Pieces of Chromosomes may

be Exchanged• Some genetic variations come

about during prophase I of meiosis.

• This is a time when parts of the duplicated chromosomes—the sister chromatids—are rearranged.

• These homologues therefore can be called “maternal” and “paternal” chromosomes and they carry genes for the same traits.

• During meiosis I, the two pairs of sister chromatids (one maternal, the other paternal) line up very closely.

Pieces of Chromosomes may

be Exchanged• Crossing Over (Synapsis):

• Leads to variation in the details of inherited traits because a gene may have several chemical forms.

• Often, particular forms of genes on one chromosome differ from corresponding ones on its homologue partner.

• When a crossover occurs between chromosomes in a germ cell, both then have a slightly different version of their genes than they had before.

Meiosis I• The first stage of meiosis I is prophase I. During this

phase, the chromosomes condense, align, and exchange segments. The centrosome is duplicated along with its two centrioles.– Crossing over (SYNAPSIS) occurs here!!!!!!

• Pairs of centrioles are positioned at the opposite sides of the nucleus as the nuclear envelope breaks up (step 1).

• Microtubules of the spindle link the chromosomes to the spindle poles so that each chromosome is attached to one spindle pole, and its homologous partner is attached to the other.

• At metaphase I, all the chromosomes are aligned midway between the spindle poles (step 2).

Meiosis I• In anaphase I, the spindle microtubules separate the

homologous chromosomes and pull them toward opposite spindle poles (step 3).

• During telophase I, the chromosomes reach the spindle poles (step 4). New nuclear envelopes form around the two clusters of chromosomes.

• Each of the two haploid nuclei that form contains one set of duplicated chromosomes. The cytoplasm divides at this point (by cytokinesis), forming two haploid cells, but the cells’ DNA is not copied before meiosis II begins.

First stage of Meiosis

Things to remember!!!!• The first meiotic division effectively ends when the

chromosomes arrive at the poles.

• Each daughter cell now has half the number of chromosomes but each chromosome consists of a pair of chromatids, .

• Remember, Cytokinesis completes the creation of two daughter cells. – Sister chromatids remain attached during telophase I

• So, as there is no replication of DNA before prophase II, the separation of chromosomes/chromatids result in four haploid, genetically different cells.

Meiosis II• In prophase II No more crossing over, and no

duplication of chromosomes (step 5)

• In metaphase II, the same events occur in each new nucleus as in metaphase I (step 6).

• In anaphase II, the spindle microtubules pull the sister chromatids apart (step 7). – Each chromosome now consists of one DNA molecule.

• During telophase II, the chromosomes cluster around each spindle pole. There are four clusters in all.

• Each one is a set of unduplicated chromosomes.

• New nuclear envelopes form around the four clusters, forming four haploid nuclei—two in each cell (step 8).– Cytokinesis then divides the cytoplasm of each cell,

producing four haploid cells.

Second stage of Meiosis

What are the Key Events of Meiosis?

• In meiosis the nucleus of a diploid germ cell divides twice. Ultimately four haploid nuclei form, each with 23 chromosomes—one of each type.

• Meiosis takes place in two steps, called meiosis I and meiosis II.

• Similar to mitosis, each step occurs in stages called prophase, metaphase, anaphase and telophase.

• Cytokinesis occurs at the close of both meiosis I and meiosis II. The result is four haploid cells.

Sexual life cycles• Haploid Gametes join to form a zygote• Somatic cells divide by Mitosis to produce adult

organism• Meiosis produces gametes in sex cells

Human variation

• A grand total of 223, or 8,388,608, combinations of maternal and paternal chromosomes are possible every time meiosis in a germ cell produces a gamete!

• This is why striking mixes of traits can show up even in the same family

Summary• Somatic Cells - Non-sex Cells. Contain

a full compliment of chromosomes. Characteristic to their species. Referred to as the diploid number of chromosomes.

• Gametes - Sex Cells. Cell which carry genetic information for sexual reproduction. Contain one half the compliment of chromosomes characteristic to their species .Referred to as the haploid number of chromosomes.

The End.

Any Questions?