DNA Structure, Replication, and Human Genome Overview

Main function of DNA

DNA stores genetic instructions for building proteins and can copy itself before cell division.

Human genome

The complete set of DNA in each cell.

Location of DNA in human cells

In the nucleus, coiled around histone proteins and packaged into chromosomes.

Number of chromosome pairs in humans

23 pairs (46 total).

Autosomes and sex chromosomes

Autosomes (pairs 1-22) control general body functions; sex chromosomes (X and Y) determine biological sex.

Biological sex determination in humans

XX = female, XY = male.

Parts of a nucleotide

Phosphate group, deoxyribose sugar, nitrogenous base.

Four nitrogenous bases in DNA

Adenine (A), Thymine (T), Cytosine (C), Guanine (G).

Purines and pyrimidines

Purines: A and G; Pyrimidines: C and T.

Bonds connecting nucleotides in DNA backbone

Phosphodiester bonds between phosphate and sugar.

Bonds connecting complementary bases

Hydrogen bonds between nitrogenous bases.

Hydrogen bonds between A-T and C-G

A-T: 2 bonds; C-G: 3 bonds.

Antiparallel in DNA

The two strands run in opposite directions (one 5'→3', the other 3'→5').

5' and 3' ends of a DNA strand

5' end: phosphate group; 3' end: hydroxyl (OH) group.

Importance of the 3' end

New nucleotides can only be added to the 3' end during DNA synthesis.

Reaction forming a phosphodiester bond

A dehydration reaction (removes water to form the bond).

Complementary strand to 5' A G T T C A G C T 3'

3' T C A A G T C G A 5'.

Semiconservative replication

Each new DNA double helix has one original (parental) strand and one new (daughter) strand.

Purpose of DNA replication

To copy DNA before mitosis so each daughter cell gets a full genome.

Where replication begins

At origins of replication (ORIs).

Replication bubble

An open section of DNA where replication occurs.

Replication forks

The Y-shaped ends of a replication bubble where the DNA is being unwound.

Leading strand

The strand made continuously.

Lagging strand

The strand made in short fragments (Okazaki fragments).

Enzyme that unwinds DNA double helix

DNA helicase.

Enzyme that relieves strain ahead of replication fork

Topoisomerase.

Enzyme that builds RNA primers

Primase.

Need for RNA primers

They give DNA Polymerase III a 3' hydroxyl starting point.

Enzyme that builds new DNA strand

DNA Polymerase III.

Direction DNA Polymerase III builds in

5' to 3' direction.

Why lagging strand is built in Okazaki fragments

Because DNA Polymerase III can only add nucleotides in the 5'→3' direction, opposite to the replication fork movement.

Enzyme that removes RNA primers

DNA Polymerase I.

Enzyme that fills in gaps between Okazaki fragments

DNA Ligase (forms new phosphodiester bonds).

DNA Polymerase III proofreading

It checks for incorrect bases and replaces them while synthesizing DNA.

Function of nucleotide excision enzymes

They remove incorrectly copied sections of DNA for repair.

Ensuring accurate DNA copying

Proofreading by DNA Polymerase III and correction by excision enzymes.

Importance of accurate DNA replication

To prevent mutations that can cause diseases or cell malfunction.

Okazaki fragments

Short segments of DNA made on the lagging strand.

Role of phosphodiester bonds in DNA structure

They form the sugar-phosphate backbone that holds nucleotides together.

Why hydrogen bonds are weaker than phosphodiester bonds

So DNA strands can easily separate during replication and transcription.

Effect of DNA packaging on gene expression

Tightly packed DNA (around histones) is less active in transcription.

What happens when replication bubbles meet

They fuse, completing the DNA molecule's replication.

Karyotype

A display showing an organism's complete set of chromosomes arranged by size.

Difference between bacterial and human DNA replication

Bacteria have one ORI; humans have many ORIs per chromosome.

Genes

Segments of DNA that code for proteins.

Function of proteins made from DNA instructions

They perform cell functions like glucose transport and hormone production (e.g., insulin).