dna and cell reproduction. structure of dna deoxyribonucleic acid dna is one of the two nucleic...

DNA and Cell ReproductionDNA and Cell Reproduction

Structure of DNADeoxyribonucleic Acid

• DNA is one of the two nucleic acids– DNA– RNA

• It is composed of repeating subunits (monomers) called NUCLEOTIDES.

• A nucleotide has three components– – deoxyribose sugar– A phosphate– A nitrogenous base

Nitrogenous Bases

• Adenine (A)• Guanine (G)• Cytosine (C)• Thymine (T)• These are grouped

into– Purines – two carbon

rings (A and G)– Pyrimidines – one

carbon ring (T and C)

The Double Helix

• A always pairs with T• C always pairs with G

DNA Replication

1. Helicase enzymes cause the strands to unwind and separate. (The point where the two chains separate is the replication fork.)

2. DNA polymerases bind to each chain of DNA.

3. The DNA polymerase moves along the chain and aids in the addition of nucleotides for the formation of the new chain.

• DNA replication does not begin at one end of the molecule and proceed to the other end. DNA replication occurs in many points simultaneously along the molecule.

Accuracy and Repair

• DNA replication is very accurate – about 1 error in every 10,000 paired nucleotides.

• DNA has the ability to proofread and repair mistakes.

• An error in replication is known as a MUTATION.

• DNA may also be damaged by a variety of agents which include chemicals and ultraviolet light.

What is an enzyme?What is an enzyme?• An enzyme is a catalyst that is usually

an protein.• Enzymes are substrate specific.• They have a lock and key fit.• Allow reaction in living organisms to

occur at lower temperatures by reducing the activation energy.

• Usually end in –ase and identify the substrate the work on. Example: proteases, nucleases

Chromosomes are Composed of DNA

• DNA in humans is about 6 billion pairs of nucleotides and is located in the nucleus as chromosomes.

• Each chromosome is a single DNA molecule associated with protein.

• DNA in eukaryotes wraps tightly around proteins called HISTONESHISTONES.

• A chromosome consists of two identical halves called chromatids.

• The consticted area of each chromosome is called a chromatid (a protein disk),

• Between cell divisions, the DNA is not so tightly wound and is called chromatin.

CentromereCentromere

chromatidchromatid

• Each species has a characteristic number of chromosomes – see pate 146.

• Human and animal chromosomes are categorized as sex chromosomes or autosomes.– Sex chromosomes determine the sex of

the organism– Autosomes are all other chromosomes

• Every cell has two copies of autosomes. These copies are called homologous chromosomes

HOMOLOGOUS CHROMOSOMES

• SAME SIZE• SAME SHAPE• SAME GENES FOR

THE SAME TRAITS

• Homologous chromosomes may be arranged in order to form a karyotype.

Karyotype

• Diploid – having two sets of chromosomes– Both chromosomes of the homologous

pair– Both of the sex chromosomes

• Haploid – having only one set of chromosomes – Half of the chromosome number– One chromosome of the homologous pair– Only one sex chromosome

Cell Division

• Prokaryote– No membrane

bound organelles

– Only one circular chromosome

– Reproduce by binary fission

• Eukaryotes– Membrane bound

organelles– DNA is in

chromosomes– Two processes

for cell division• Mitosis• Meiosis

The Cell Cycle