Human BiologyIntroduction to the Cell Cycle & DNA

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Cells reproduce by dividing in two. This is why we alternatively use the term “cell division.”

Cell reproduction is a defining feature of life.

Mechanisms of cell reproduction are the same for all eukaryotes although some variations occur due to specialization of the 200 types of specialized cells in humans.

Many familiar cell structures and regulation necessary

2.4 million new erythrocytes are produced per second!

Energy! Materials!

Cell Reproduction

Figure 17.1

Mitosis Nucleus divides

CytokinesisCell divides

Cell prepares for division.Growth continuesslowly.

Primary period of cell growth.

G2G1

S

DNA is duplicated.Growth continuesslowly.

M

G0

The Cell Cycle Creates New Cells Step by Step

• InterphaseLong growth period between cell divisions

• G1 (gap 1)– primary growth phase, very active growth

• S (synthesis)– synthesis of DNA for next cell division

• G2 (gap 2) final growth phase before cell division

• Mitotic phase (cell division phase)Mitosis

• Nuclear division– duplicated DNA is distributed between two daughter nuclei, nucleus

dividesCytokinesis– Cytoplasm divides, two new daughter cells are formed

DNA, Genes & Chromosomes

DNA replication • Process of copying DNA prior to cell division

Gene • Short segment of DNA that contains the code, or recipe, for a protein

or for enzymatic RNA• Smallest functional unit of DNA

Chromatin• Threadlike combination of DNA and histone proteins present in

interphase cells (preparing for division) or in resting cells (taking a break from division)

Chromosome• Rod-like structure of highly coiled chromatin in cells in the mitotic

phase of cell division

Double-stranded DNA

Histones

Chromatinmaterial:not visibleduringInterphase

One chromatid

Its sisterchromatidCentromere

Chromosome: visible during mitosis

DNA to Chromosome

Replication, Transcription & Translation: Overview

Transcription• Process of copying DNA information of a gene into mRNA

(messenger ribonucleic acid)• Occurs within the cell nucleus

Translation• Process of converting the mRNA code into one or more

proteins• Occurs in the cytoplasm at ribosomes

Figure 17.3

Replication

•Produces an exactcopy of DNA

•Occurs prior toevery cell division

Transcription

•Converts a portion ofdouble-stranded DNA to asingle-stranded mRNA

•Occurs within the nucleus

Translation

•Converts an mRNAcode into one ormore proteins

•Takes place in the cytoplasm

DNA

mRNA

Protein

Replication: Copying DNA Before Cell Division

1. DNA strands uncoil and “unzip”

2. Each single strand serves as a template for the creation of a new complementary strand

3. DNA nucleotides are positioned and linked by DNA polymerase

4. Precise base-pairing (A with T & C with G) assures that an exact copy is made (mistakes corrected?)

Keys:

= Cytosine

= Adenine

= Guanine

= Thymine

Parent strand New(daughter)strandsforming

Parent strand

The double-stranded DNA unwinds, and each single strand serves as a template for a new complementary strand.

Replicationbubble

Parent DNAmolecule

2 completedaughter DNAmolecules

Parent strands

Newcomplementarystrands

The unwinding and the formation of new strands occur simultaneously at many sites on the DNA molecule. The sites of replication expand outward until they join. For simplicity the two strands are shown as parallel in (b), but in actuality they form a helical shape.

Replication: Mutations

• Alterations or “mistakes” in the DNA code

• Occur most frequently during DNA replication

• Causes: Chemical and physical forces

• Mutations can be:• Silent—have no effect• Deleterious—harmful, may result in cell death or cancer• Beneficial—basis for evolution

• Some mutations are repaired by repair enzymes

• The higher the rate of replication, the higher the mutation rate.