How do cells reproduce?

• Cell division is at the heart of reproduction• Multicellular organisms originate from a rapidly

dividing fertilized egg (cell); eggs and sperm are themselves created from a special type of cell division

• Cell division replaces worn-out or damaged cells, keeping the total number of cells relatively constant

• There are two types of cellular division: mitosis and meiosis

Cell division and reproduction

• Asexual reproduction involves the creation of genetically-identical offspring from a single parent; no eggs or sperm are involved

• Involves replication of chromosomes, the structures containing the organism’s DNA

• Bacteria, yeast, protists, and certain plants and animals

Asexual Reproduction

• Asexual reproduction is a very efficient means of reproduction– Faster than sexual reproduction– Increases numbers of organisms quickly– Ability to reproduce in absence of mate (male

doesn’t need female and vice versa)

• Genetic diversity, however, is sacrificed

Sexual Reproduction

• The ability for an organism to form gametes, or sex cells (eggs and sperm), results in the formation of similar, but not identical, offspring

• In sexual reproduction, the resulting offspring are genetically similar, but not identical to either parent; offspring inherits a combination of genes from each parent

Cells arise from pre-existing cells

• Cell division allows an embryo to develop into an adult, and is the basis of egg and sperm formation

• It also ensures the continuity of life from one generation to the next

• In the case of unicellular organisms, cell division can reproduce an entire organism

Binary fission

• Prokaryotes (Bacteria and Archaea) reproduce by a type of cell division called binary fission; “dividing in half”

• These cells possess a single chromosome, which is replicated prior to the cell dividing into 2

What about eukaryotic cells?

• A bacteria contains ~3,000 genes; human cells contain ~25,000 which are grouped into multiple chromosomes located in the nucleus

• Each chromosome consists of 1 long DNA strand, with hundreds or thousands of genes

• Integrated into this chromosome are proteins!, which help maintain its structure and control the activity of its genes

Chromosomes

• Human cells have 46 chromosomes

• Before a eukaryotic cell can divide, it must replicate its chromosomes

• The DNA molecule of each chromosome is copied and new proteins attach as needed

A duplicated chromosome

Centromere

Chromosomeduplication

Sister chromatids

Chromosomedistribution

todaughter

cells

The cell cycle

• The process of cell division is a key component of the cell cycle, an ordered sequence of events beginning with the ‘birth’ of the cell from a dividing parent and ending with its own division into 2 cells

• The cell cycle consists of a growing stage called interphase, and the actual cell division, called the mitotic phase

The Cell Cycle

• Most of the cell cycle in spent in interphase• During this time, the cell performs its various

functions within the organism• Additionally, the cell acquires a rich supply of

proteins, creates more organelles such as mitochondria and ribosomes, and grows during this time

• Chromosomes are replicated during interphase

The Cell Cycle

• Interphase is divided into 3 stooges, er, stages…– The G1 phase: cell grows

– The S phase: cell grows, chromosomes replicated– The G2 phase: cell grows

G stands for “gap” (first and second gap)S stands for “synthesis” (DNA)

S(DNA synthesis)G1

G2

Cytokinesis

Mito

sis

INTERPHASE

MITOTICPHASE (M)

The Cell Cycle

• During interphase, the cell grows (G1), continues to grow while DNA is replicated (S), and then grows more as it completes preparations for cell division (G2)

• Cell division occurs in the mitotic phase (also called the M phase)– Accounts for only 10 of the total time required for

the cell cycle

The Cell Cycle

• Like interphase, the mitotic phase is divided into (2) stages– Mitosis: the nucleus (and all its contents,

including the duplicated chromomes) divide and are evenly distributed to the ‘daughter’ cells

– Cytokinesis: the cytoplasm is divided into 2

Mitosis and cytokinesis produces 2 genetically identical cells, each with a single nucleus, surrounding cytoplasm and plasma membrane

Mitosis

• Mitosis (the division of nuclear material) is subdivided into 5 main stages:

• Prophase• Prometaphase• Metaphase• Anaphase• Telophase

Mitosis

• During mitosis, chromosome movement is dependent on the mitotic spindle, a football (go Giants! Go Jets!) shaped structure of microtubules that guides the separation of the 2 sets of separating chromosomes

• During interphase, chromosomes are not distinguishable because they exist as loose fibers of chromatin; chromatin becomes more tightly packed and visible as mitosis ensues, allowing easy tracking of each step of mitosis

Prophase

• The mitotic spindle forms during the first stage, prophase

• The chromatin fibers containing DNA become more tightly coiled and folded forming discrete chromosomes that can be seen with a light microscope

• Remember, there are 2 pairs of chromosomes at this stage as they were replicated during the S phase of interphase

Prophase

Visible chromosomes; nuclear envelope still present

Early mitotic spindle present

Prometaphase

• During the second stage of mitosis, prometaphase, the nuclear envelope breaks away

• Proteins embedded in the chromatin attach to microtubules of the spindle, and move the chromosomes towards the center of the cell

Prometaphase

Dissolution of nuclear envelope; chromosomes moved towards the center of the cell

Mitotic spindle extend ‘pole’ to ‘pole’

Metaphase and Anaphase

• During metaphase, the mitotic spindle spreads across the entire cell, with the chromosomes aligned perpendicularly at its center (remember each chromosome has been replicated into 2 prior to mitosis)

• In Anaphase, the sister chromatids of each chromosome separate and move away from each other (toward opposing poles)

Metaphaseplate

METAPHASE ANAPHASE

DaughterchromosomesSpindle

During mitosis, each chromosome has been replicated consisting of 2 sister chromatids; these chromosomes align and separate during metaphase and anaphase, respectively

Telophase

• During the fifth (and final) stage of mitosis called telophase, nuclear envelopes form around the 2 copies of separated chromosomes; the chromatin fiber uncoils and the mitotic spindle disappears

• Sort of a reverse prophase!• Cytokinesis follows this final stage of mitosis,

pinching the cell into 2

Telophase and Cytokinesis

Got all that?• The eukaryotic cell cycle consists of:– Interphase (G1, S, G2) – growth & DNA replication– Mitosis

• Prophase – mitotic spindle forms, chromatin condenses• Prometaphase – nuclear envelope dissolves, chromosomes

attach to spindle• Metaphase – mitotic spindle spreads pole to pole with

chromosomes aligned at center• Anaphase – each sister chromatid of replicatec

chromosome separates • Telophase – nuclear envelope reforms, chromatin uncoils

– Cytokinesis – cell divides into 2

Cell Division

• The timing of cell division must be regulated in order to grow and develop normally

• Skin cells and stomach cells are replaced regularly as they are constantly abraded and sloughed off

• Other cells, such as liver cells, do not divide unless damaged; In this way, cell division repairs wounds and heals

Cell Division

• Proteins regulate cell division by stimulating cells to divide in their presence

• For example, injury to the skin causes blood platelets to release a protein which promotes rapid growth of connective tissue cells that help seal the wound

• Proteins control each cycle of mitosis and each stage does not occur until triggered to do so by these proteins

Cell Division

• Proteins serve as a control system for each stage of the cell cycle

• Want a job? Research on controls over the cell cycle is one of the hottest areas in biology today. Why?

• Without check points, cells will continue to divide unregulated….. = cancer

Cancer

• Cancer is a disease of the cell cycle• Cancer cells divide uncontrollably and

do not respond normally to the cell cycle control system

• Cancer begins when a single cell undergoes transformation from a normal cell to a cancer cell

• Cancer cells may proliferate into a tumor, an abnormally growing mass of body cells

Cancer

• Benign tumors remain at the site and can usually be removed easily with surgery

• Malignant tumors spread into neighboring tissues and other parts of the body, interrupting organ function as it goes

• Cancer cells may secrete molecules that cause blood vessels to spread toward the tumor, and allow proliferation of the cancer cells via the circulatory system (metastasis)

Cancer

• Radiation damages DNA in cancer cells moreso than it does in normal cells and can be used as a cancer treatment

• Chemotherapy is used to treat metastatic or widespread tumors; involves the use of drugs that disrupt cell division (some drugs prevent the mitotic spindle from forming in the first place); however side effects are seen in normal, rapidly-dividing cells

Meiosis

• Meiosis is the process of cell division in which the number of chromosome is cut in half

• Unlike mitosis, which results in a ‘daughter’ cell containing the exact number of chromosomes as the ‘parent’ cell

• Meiosis takes place in reproductive organs and produces gametes, sex cells, such as eggs, sperm, and pollen (plants)

Meiosis

Sister chromatids One duplicatedchromosome

Homologous pair ofchromosomes • Human cells have 46

chromosomes, made up of 23 pairs of homologous chromosomes

• Cells with 2 sets of chromosomes are considered diploid

Meiosis

• The two chromosomes composing a pair are called homologous because they both carry genes controlling the same inherited characteristics

• One exception are the sex chromosomes, X and Y– Females have a homologous pair (XX), while males

have 1 X and 1 Y– The other 22 chromosomes are called autosomes

Meiosis

From Mother From Father

Homologous pair ofchromosomes • For both sex

chromosomes and autosomes, we inherit one chromosome of each pair from our mother and the other from our father

Meiosis• The 46 chromosomes in the human cell consists

of 23 pairs of homologous chromosomes• Homologous chromosomes are similar, but not

identical; they may carry different versions of the same genetic information

• For example, one chromosome may code for blond hair, while the other codes for dark hair; or both may contain the same gene (ex. Blue eyes)

Meiosis

• Human cells contain 22 pairs of autosomes, and 1 pair of sex chromosomes (X and/or Y)

http://www.sciencemuseum.org.uk/exhibitions/genes/153.asp

Chromosome 1 is the largest;

containing 8000 genes

Chromosome 21 is the smallest;

containing only 300

genes

Sex chromosomes

Meiosis

• Meiosis is a special type of cell division that will produce cells containing half the number of chromosomes

• Cells containing half the number of chromosomes are sex cells, or gametes

• Gametes contain a single set of chromosomes and are considered haploid (half)

• All other cells containing 2 homologous sets of chromosomes is said to be diploid

Meiosis

• For humans, the diploid number is 46• Nearly all of our cells are diploid; the exceptions

are the gametes!• Sexual reproduction allows a haploid sperm cell

to fuse with a haploid egg cell during the process of fertilization producing a zygote

• The resulting zygote is diploid; it has 2 sets of homologous chromosomes: 1 from Mom, and 1 from Dad

Meiosis

• Meiosis occurs only in reproductive organs• During meiosis, a ‘mother’ cell divides and

produces 4 genetically distinct ‘daughter’ cells which contain half the number of chromosomes as the ‘other cell

• Why 4? This is because meiosis begins with mitosis! (insert “UGH!!!”s here….)

Meiosis reduces the chromosome number from diploid to haploid

• Just as a cell entering mitosis has duplicated its chromosomes, so too, does a cell entering meiosis (resulting in 92 chromatids)

• During prophase 1 (so called because it is the first cycle occuring during Meiosis 1 (out of 2)) the process of crossing over occurs

Crossing over

• Crossing over is the process by which aligned chromatids of homologous chromosomes exchange genetic segments resulting in a genetically-new chromatid

• The driving force of genetic diversity and evolution!

• Independent orientation of chromosomes in meiosis and random fertilization lead to varied offspring

Crossing over

Cell division

• In both mitosis and meiosis, the chromosomes duplicate only once, in the preceding interphase

• Mitosis replicates cells for growth, tissue repair and asexual reproduction and produces daughter cells genetically identical to the parent cell (diploid)

• Meiosis produces haploid cells that are genetically distinct from the parent cell

Genetic diversity

• Changes in an organism’s DNA create different versions of genes (and resulting characteristics)

• Reshuffling of these different versions during sexual reproduction produces genetic variation

http://www.duggarfamily.com/

Genetic diversity• For a human there are 23 chromosomes and 223

combinations of chromosomes that meiosis can package into gametes

• 223 equals 8 million (possible combinations)!!!• Each gamete you produce contains 1 of ~8 million

possible combinations inherited from your father and your mother

• The random fusion of egg and sperm will produce a zygote with any of 64 trillion (8 mil x 8 mil) combinations of chromosomes!!!

http://www.accessexcellence.org/AB/GG/crossing.html

• Points contact each other

• DNA is exchanged

• Occurs at 1 or more points along adjacent chromatids

Homologous chromosomes carry different versions of genes

• A pair of homologous chromosomes can bear 2 different kinds of genetic information for the same characteristic

Coat-colorgenes

Chromosomes ofthe four gametes

Meiosis

PinkWhite

BlackBrown

Eye-colorgenes

C

e

E

c

C

e

E

c

C

e

E

c

White coat (c); pink eyes (e)

Brown coat (C); black eyes (E)

Alterations of chromosome number and structure

• With 64 trillion possible combinations of chromosomes, what could possibly go wrong??!!??

• Chromosome number abnormalities do occur and are often fatal

• An additional copy of chromosome 21 (the short one) results in Down’s syndrome (also known as trisomy 21)

Alterations of chromosome number and structure

• Individuals with Down’s syndrome exhibit distinctive features: flattened nose bridge, short stature, heart defects, and a shortened life span

• The additional chromosome usually comes from the mother (with a risk of 1% with pregnancy after age 40)

• Why?

Alterations of chromosome number and structure

Abnormal numbers of sex chromosomes

• Alteration in the number of copies of sex chromosomes are not lethal

Abnormal numbers of sex chromosomes

• The Y chromosome is very small and carries relatively few genes

• What about the X chromosome? It’s big and it does carry a lot of genes

• In females, the extra X chromosome is inactivated since the presence of this additional chromosome would otherwise be fatal!

Abnormal numbers of sex chromosomes

• The inactivation of X chromosomes is random; results in a random expression of genes!

www.flickr.com/photos/mayason/3194660130/

Alterations of chromosome structure can cause birth defects

• Other errors can occur involving deletion or duplication of chromosome structure

• Such chromosomal changes present in sperm and egg can cause congenital disorders

• Such changes in a somatic cell may contribute to cancer (not inheritable); why damage to our DNA may cause cancer (radiation, UV, etc.)