•Cell Cycle –Series of stages that takes place during cell division and forms two new daughter cells

•Consist of two phases

• Interphase – includes no. of stages

• Cell Division stage or Mitotic stage – Mitosis and Cytokinesis

• Interphase

• Cell cycle spent most of the time (90%)• Can be divided into subphases

• G1 phase• S phase• G2 phase

• G1 Phase (First gap):G1 Phase (First gap):

- Recovery from previous divisionRecovery from previous division

- Cell doubles its organelles, eg. Cell doubles its organelles, eg. mitochondria, ribosomesmitochondria, ribosomes

-Accumulates raw materials for Accumulates raw materials for DNA synthesisDNA synthesis

• S Phase (Synthesis):S Phase (Synthesis):- - DNA replication (synthesis)DNA replication (synthesis)

G2 Phase (Second gap):G2 Phase (Second gap):•Cell synthesizes proteins Cell synthesizes proteins necessary for cell divisionnecessary for cell division

– In DNA replication, two new strands of DNA are made

– The parent molecule unwinds

– Each strand of parent molecule acts as a template for building a new strand in replication

– Two new daughter strands are built based on base-pairing rules

(a) The parent molecule has two complementary strands of DNA. Each base is paired by hydrogen bonding with its specific partner, A with T and G with C.

(b) The first step in replication is separation of the two DNA strands.

(c) Each parental strand now serves as a template that determines the order of nucleotides along a new, complementary strand.

(d) The nucleotides are connected to form the sugar-phosphate backbones of the new strands. Each “daughter” DNA molecule consists of one parental strand and one new strand.

A

C

T

A

G

A

C

T

A

G

A

C

T

A

G

A

C

T

A

G

T

G

A

T

C

T

G

A

T

C

A

C

T

A

G

A

C

T

A

G

T

G

A

T

C

T

G

A

T

C

T

G

A

T

C

T

G

A

T

C

• DNA replication is semiconservative– Each of the two new daughter molecules will have one

old strand, derived from the parent molecule, and one newly made strand

• DNA`s sugar-phosphate backbone runs in opposite direction

• Each strand has 3` (three –prime) end and 5` end

• Prime no. refers to the carbon atoms of nucleotide sugar

• At one end of DNA strand , sugar`s 3` carbon atom is attached to an – OH group

• At other end, the sugar`s carbon has a phosphate group

• Enzyme helicases unwind the double helix• DNA polymerases add nucleotides – help of RNA Primer

– RNA polymerase adds short amount of RNA – RNA Primer– That is complementary to parent strand of DNA– Then DNA polymerase add nucleotide to the 5`3end of a

growing strand and synthesize complimentary strand– Leading strand

• For formation of other strand of DNA - Lagging strand– DNA polymerase work away from the replication fork

Primase

Helicase

Helicase

Primase

• The lagging strand– Is synthesized as a series of small segments (100 to 200

nucleotides) called Okazaki fragments

• Only one primer is needed for synthesis of the leading strand– But for synthesis of the lagging strand, each Okazaki

fragment must be primed separately

– DNA polymerase removes old RNA primer – replaces

with DNA adjacent to the

new Okazaki segment

– Gaps between them are

joined together by DNA ligase

– Two identical DNA molecules

are produced

• M (Mitotic Stage):M (Mitotic Stage):

• MitosisMitosis• CytokinesisCytokinesis

• Mitosis:Mitosis:

-Division of the nucleus Division of the nucleus into two daughter nucleiinto two daughter nuclei- Has same amount and Has same amount and type of DNA as parent or type of DNA as parent or original nucleusoriginal nucleus

•Cytokinesis:Cytokinesis:

- Cytoplasm division- Cytoplasm division- Results in two genetically - Results in two genetically identical daughter cellsidentical daughter cells

Mitosisnuclear division of a cell

produces 2 identical nuclei

Each nucleus has the same amount and type of DNA as the original nucleus

Is preceded by Interphase

4 stages: ProphaseMetaphaseAnaphaseTelophase

Interphase Time between cell division

period of cell growth

chromosomes not clearly visible in nucleus

Found as thin threads of chromatin in the nucleus

cell may contain pair of centrioles

Prophasechromatin in nucleus condenses into chromosomes,

becomes visible

Each chromosome consists of two chromatids joined at centromere

nucleolus disappears

centrioles begin moving to opposite ends of cell

fibers extend from centrosomes

some fibers cross cell to form mitotic spindle

In late prophase, nuclear membrane dissolves chromosomes begin moving

Metaphase

Centrosomes are at opposite end of the cell

spindle fibers align chromosomes along middle

of nucleus (metaphase plate)

Spindle fibers are attached to kinetochores

in the centromere of each chromosome

Anaphase

Chromosomes separate at kinetochores

separated chromatids move to opposite sides of cell

chromatids arrive at opposite poles of cell

Nuclear envelops around the chromosomes, nucleoli reappear

chromosomes disperse, no longer visible

spindle fibers diappear

• Typically occurs during telophase.

• Is the division of the cytoplasm

• forming a cleavage furrow or an indention of plasma membrane

• mother cell becomes two daughter cell

• each daughter cell has one nucleus

• Sperm cell development and oocyte development (gamete formation) involve meiosis

• this cell division occurs only in testes and ovaries

• consists of two nuclear division

• Four daughter cells are produced

• Each has half no. of chromosomes as parent cell

• In humans– Each somatic cell has 46 chromosomes– called diploid no. or 2n (2n=46)– made up of two pairs of 23 chromosomes – homologous pair– One set of chromosomes comes from each parent

• Gametes– Gametes (sperm and oocyte cells) contain only one set of

chromosomes– called haploid cells (n)– For humans, haploid no. is 23 (n=23)

• Fertilized egg (zygote)– Fusion of haploid gametes in fertilization forms a diploid

zygote– reestablishes 2n no. of chromosomes

• Set of 23 chromosomes consists of

- 22 autosomes

- 1 sex chromosome– In oocyte cell sex chromosome is –

X chromosome– In sperm cell sex chromosome is –

X or Y chromosome• Meiosis: • cell division leading to production of

gametes (sperm & oocyte)• Meiosis reduces the number of

chromosome sets from diploid to haploid

Human Karyotype

– Like mitosis,Meiosis is preceded by chromosome duplication

– Takes place in two sets of divisions, meiosis I and meiosis II– End with 4 haploid daughter cells instead of

2 diploid daughter cells like in mitosis• Meiosis I

– Reduces the number of chromosomes from diploid to haploid• Meiosis II

– Produces four haploid daughter cells

• Gene– Gene is a segment of DNA molecule– Functional unit of heredity– Heredity is the transfer of characteristics from parent to

offspring through their genes

• Gene expression– Genetic information in DNA is transcribed into RNA and

then translated into proteins– Production of proteins from the

information stored in DNA is called

Gene expression– Gene expression involves two steps:– Transcription– Translation

• Transcription– Synthesis of RNA under the direction of DNA in nucleus– Information is copied from

DNA – mRNA

• Translation– Is the actual synthesis of a

polypeptide (protein), which occurs under the

direction of mRNA in cytoplasm– Occurs on ribosomes

Transcription Translation

• Transcription– transfer of genetic information from DNA into

RNA

• RNA: uses U instead of T– And ribose instead of deoxyribose

• RNA: 3 types– mRNA = messenger RNA

• Has protein-building instructions

– rRNA = ribosomal RNA• Major component of ribosomes

– tRNA = transfer RNA• Delivers amino acid to ribosome

phosphate group

sugar (ribose)

uracil (base)

• During transcription, a segment of DNA serves as template for RNA production

• RNA Polymerase binds to Promotor (DNA nucleotide sequence) with the help of proteins – Transcription factors and unwind DNA strand

•Complementary RNA molecule synthesizes from DNA by following the rules of complimentary base pairing, A with U, C with G

• When RNA polymerase encounter stop signal, it detaches from DNA and release newly formed mRNA

• RNA transcribed in the nucleus is modified before moving to the cytoplasm for translation

• The RNA that encodes an amino acid sequence is messenger RNA (mRNA)

– RNA Splicing• Non protein coding segments

called introns are cut out• Remaining exons, protein

coding sections are joined to form a continuous coding sequence

• A 7-methyl guanosine cap and a adenine nucleotides called tail are added to the ends

• Genetic Information– Is contained in mRNA as a sequence of three

nucleotides units, or codons– And translated into amino acid sequences– Each word codes for one amino acid in a polypeptide– Nearly all organisms use exactly the same genetic code

• 64 codons of life = GENETIC CODE– Usually more than 1

codon for same amino acid

• Eg. Glutamate = GAA or GAG

• All new polypeptide chains start with AUG– =methionine– =first point of mRNA

transcript• UAA, UAG, UGA =

STOP SIGNALS– Don’t code for amino acid– Instructions to stop

adding amino acids to polypeptide chain

Second mRNA baseU C A G

U

C

A

G

UUUUUCUUAUUG

CUUCUCCUACUG

AUUAUCAUAAUG

GUUGUCGUAGUG

Met orstart

Phe

Leu

Leu

lle

Val

UCUUCCUCAUCG

CCUCCCCCACCG

ACUACCACAACG

GCUGCCGCAGCG

Ser

Pro

Thr

Ala

UAUUAC

UGUUGC

Tyr Cys

CAUCACCAACAG

CGUCGCCGACGG

AAUAACAAAAAG

AGUAGCAGAAGG

GAUGACGAAGAG

GGUGGCGGAGGG

UGGUAAUAG Stop

Stop UGA StopTrp

His

Gln

Asn

Lys

Asp

Arg

Ser

Arg

Gly

U

CA

GUCAG

UCAG

UCAG

Fir

st m

RN

A b

ase

(5

end

)

Th

ird

mR

NA

bas

e (3

en

d)

Glu

– Translation• Is the conversion from the nucleic

acid language to the protein language

– Messenger RNA (mRNA)• Is the first ingredient for

translation

– Transfer RNA (tRNA)• Acts as a molecular interpreter• Carries amino acids• Matches amino acids with codons

in mRNA using anticodons

– Ribosomes• Are organelles that actually make polypeptides.• Are made up of two protein subunits.• Contain ribosomal RNA (rRNA)• Ribosome has three binding sites to facilitate pairing

between tRNA and mRNA

– The E (for exit) site

– The P (for peptide) site

– The A (for amino acid) site

– Translation is divided into three phases:• Initiation• Elongation• Termination

• mRNA enters ribosome• Initiatior tRNA binds to

“small unit” of ribosome• tRNA scans mRNA for

“start codon”– =AUG

– UAC anticodon of tRNA binds to AUG of mRNA

– Forms Methionine amino acid

– Small unit of ribosome combines with large unit and create functional ribosomes

– Initiator tRNA occupies the P site

Elongation– Codon recognition• The anticodon of an incoming tRNA pairs with the mRNA codon

at A site– Peptide bond formation

• The peptide bond formation takes place between amino acids• And attaches the polypeptide to the tRNA in the A site

– Translocation• mRNA moves forward, and peptide-bearing tRNA moves to P

site, empty tRNA exits from E site• Then at A site new codon is ready to receive another tRNA

Termination– Elongation continues until the

ribosome reaches a stop codon, UGA

– Termination occurs at a stop codon

– Termination is the final step in protein synthesis

– mRNA and polypeptide chain releases from ribosome

– Ribosome subunits dissociate

Summary of Gene Expression