🧬 DNA, Chromosomes & Forensics – Study Guide

Chromosome

A tightly coiled structure of DNA and proteins that carries genetic information.

Human chromosomes

Humans have 46 chromosomes (23 pairs).

Karyotype

A picture showing all chromosomes arranged in pairs.

What a karyotype shows

Number, size, and shape of chromosomes.

Normal human karyotype

46 chromosomes total.

Male karyotype

XY.

Female karyotype

XX.

Monomer of DNA

Nucleotide.

Nucleotide components

Phosphate group, deoxyribose sugar, nitrogenous base.

Adenine (A)

Pairs with Thymine (T).

Thymine (T)

Pairs with Adenine (A).

Cytosine (C)

Pairs with Guanine (G).

Guanine (G)

Pairs with Cytosine (C).

DNA structure

Double helix.

DNA strands

Two strands.

Complementary bases

A–T and C–G.

Hydrogen bonds

Hold base pairs together.

Sugar-phosphate backbone

The sides of the DNA ladder.

Antiparallel strands

DNA strands run in opposite directions (5′→3′ and 3′→5′).

Purpose of DNA replication

To copy DNA so each new cell gets identical genetic information.

Helicase

Unzips DNA by breaking hydrogen bonds.

Primase

Adds RNA primers to start replication.

DNA polymerase

Builds new DNA by adding complementary nucleotides and proofreading.

DNA ligase

Connects DNA fragments together.

Leading strand

Made continuously toward the replication fork.

Lagging strand

Made in short pieces called Okazaki fragments.

Why leading and lagging strands occur

DNA polymerase can only build DNA in the 5′→3′ direction.

How to identify leading strand

Smooth, continuous strand.

How to identify lagging strand

Fragmented strand with primers.

Semi-conservative replication

Each new DNA molecule has one original strand and one new strand.

Template strand

Original DNA strand used for copying.

Complementary strand

New strand built to match the template.

Roy Brown case

A man wrongfully convicted of rape in the 1990s.

Accusation

based on bite mark and hair evidence.

Bite mark evidence

Later shown to be unreliable and inaccurate.

Hair analysis evidence

Not scientifically reliable; led to wrongful conviction.

What freed Roy Brown

DNA testing proved his innocence.